Optical magnetic writing system

Abstract

A system (1) is described including a magnetic medium (2). The magnetic medium (2) includes a thin film magnet structure (6). The thin film magnet structure (6) includes one or more ferromagnetic domains (7), each ferromagnetic domain (7) having a magnetization that is switchable between two or more states. The magnetic medium (2) also includes one or more optical structures (8). Each optical structure has an interface (9) with the thin film magnet structure (6) and is configured such that light of an operating wavelength incident on the magnetic medium interferes destructively at the interface (9). The system is configured to write states to the ferromagnetic domains (7) within a target area (Atarg) of the magnetic medium (2) by illuminating the target area (Atarg) of the magnetic medium (2) using laser light (10) spatially structured into a plurality of pixels (Pn,m). Each pixel (Pn,m) illuminates a corresponding sub-area (An,m) of the magnetic medium (2). All-optical magnetic switching of ferromagnetic domains (7) within each sub-area (An,m) of the magnetic medium (2) is controlled by the system (1) by the relative intensity and / or polarisation of the corresponding pixel (Pn,m) of the laser light (10).

Description

[0001] Optical magnetic writing system

[0002] Field

[0003] The present invention relates to system and methods of writing information to magnetic media.

[0004] Background

[0005] Seventy-five percent of global data is stored magnetically, with magnetic hard disks representing a $62 billion per year industry projected to grow at a 6% compound annual growth rate (CAGR). The vast majority of this market comprises large data centres and cloud computing, with major industry operators primarily concerned with several critical factors:

[0006] 1. Energy Consumption: Data centres are highly power-intensive, and energy efficiency is crucial.

[0007] 2. Cost per Storage Device: Market leaders operate magnetic storage centres with capacities in the thousands of petabytes. Reducing the cost of storage devices is highly advantageous.

[0008] 3. Storage Device Lifetime and Maintenance: Future data centre designs aim for human-free autonomy. Replacing the current complex magnetic write-units reduces the demand for human maintenance staff.

[0009] 4. Materials availability and supply chain security: Beyond the high costs associated with devices reliant on rare Earth elements, the overall low abundance and low number of potential sources are not are not aligned with the storage volumes.

[0010] While lower power consumption is an immediate economic benefit, the long-term ecological and societal implications are substantial. Data processing and storage are forecasted to consume 21% of global energy production by 2030. Energy-efficient computing and storage will remain a significant focus as society transitions away from a fossil fuel energy.

[0011] WO 2023 / 099887 Al describes new types of magnetic medium for all-optical magnetic switching at lower irradiances then previously reported, and using continuous wave (CW) or relatively long pulses (compared to high power, sub ps lasers used previously)."Deterministic all-optical magnetization writing facilitated by non-local transfer of spin angular momentum", Youri L. W. van Hees, Paul van de Meugheuvel, Bert Koopmans & Reinoud Lavrijsen Nature Communications volume 11, Article number: 3835 (2020), describes an experimentally demonstration of conversion from toggle switching to a deterministic mechanism by biasing all-optical magnetic switching in a Co / Gd bilayer with a spin polarized current which is optically generated in an adjacent ferromagnetic reference layer. The authors describe deterministic writing of an 'up' and 'down' state using a sequence of one or two pulses, respectively, and demonstrate the non-local origin by varying the magnitude of the generated spin current.

[0012] "Field-Free All-Optical Switching and Electrical Readout of Tb / Co-Based Magnetic Tunnel Junctions", D. Salomoni, Y. Peng, L. Farcis, S. Auffret, M. Hehn, G. Malinowski, S. Mangin, B. Dieny, L.D. Buda-Prejbeanu et al, Phys. Rev. Applied 20, 034070 -Published 28 September, 2023, describes a field-free 50-fs single-laser-pulse-driven magnetization reversal of a [Tb / Co]-based storage layer in a perpendicular magnetic tunnel junction (MTJ) with a estimated absorbed energy of 68.6 fJ / bit. The nanofabricated MTJ devices have a bottom reference electrode and show tunnel-magnetoresistance-ratio (TMR) values up to 74% after patterning down to sub-100 nm lateral dimensions.

[0013] "Plasmonic layer-selective all-optical switching of magnetization with nanometer resolution", D. O. Ignatyeva, C. S. Davies, D. A. Sylgacheva, A. Tsukamoto, H.

[0014] Yoshikawa, P. O. Kapralov, A. Kirilyuk, V. I. Belotelov & A. V. Kimel, Nature Communications volume 10, Article number: 4786 (2019), describes that a single femtosecond laser pulse of wavelength 800 nm can be used to toggle the magnetization exclusively within one of two 10-nm thick magnetic nanolayers, separated by 80 nm, without affecting the other one. The choice of the addressed layer is hypothesized to be enabled by the excitation of a plasmon-polariton at a targeted interface of the nanostructure, and realized by rotating the polarization-axis of the linearly-polarized ultrashort optical pulse by 90°.Summary

[0015] According to a first aspect of the invention there is provided a system including a magnetic medium. The magnetic medium includes a thin film magnet structure. The thin film magnet structure includes one or more ferromagnetic domains, each ferromagnetic domain having a magnetization that is switchable between two or more states. The magnetic medium also includes one or more optical structures. Each optical structure has an interface with the thin film magnet structure and is configured such that light of an operating wavelength incident on the magnetic medium interferes destructively at the interface. The system is configured to write states to the ferromagnetic domains within a target area of the magnetic medium by illuminating the target area of the magnetic medium using laser light spatially structured into a plurality of pixels. Each pixel illuminates a corresponding sub-area of the magnetic medium. All-optical magnetic switching of ferromagnetic domains within each subarea of the magnetic medium is controlled by the system by the relative intensity and / or polarisation of the corresponding pixel of the laser light.

[0016] The thin film magnet structure may include a plurality of ferromagnetic domains.

[0017] Spatially structuring the laser light may correspond to controlling the intensity (equivalently irradiance) of each pixel. Spatially structuring the laser light may correspond to controlling the polarisation of each pixel. Each pixel may be linearly polarised in one of two or more distinct orientations. Spatially structuring the laser light may correspond to controlling the intensity (equivalently irradiance) and the polarisation of each pixel.

[0018] When the polarisation of the laser light is not spatially structured, the laser light may be polarised linearly, circularly, or elliptically. In other words, even if the spatial structuring is only with respect to intensity of pixels, the laser light may have a global (or overall) polarisation.

[0019] Transitional regions may exist between adjacent pixels. Transitional regions need not be limited to diffraction limited dimensions.

[0020] The magnet structure may be formed of a ferromagnetic element, alloy or compound.

[0021] The system may include a laser source which is no larger than benchtop scale.

[0022] Preferably a laser source for the system takes the form of a small, self-contained and portable laser, for example, easily picked up and held in a single hand. For example a peak output power level of less than or equal to 50 kW. When shorter pulse lengthsare used, for example on the order of 100 ps or shorter, it may be convenient to refer to the energy of a laser pulse instead of peak power (which will be very high). For example, a laser source having a pulse energy of 1 mJ or less may be regarded as "benchtop" scale.

[0023] The laser light does not correspond to a femtosecond laser pulse.

[0024] The system may be configured to illuminate the target area of the magnetic medium using spatially structured laser light in the form of a pulse. The pulsed laser light is spatially structured as defined hereinbefore.

[0025] The system may be configured such that a peak irradiance of the pulse is less than or equal to IkW W.pnr2. The system may be configured such that a peak irradiance of the pulse is less than or equal to 500 W.pnr2. The system may be configured such that a peak irradiance of the pulse is less than or equal to 100 W.pnr2. The peak irradiance may refer to the value at the magnetic medium. The peak irradiance may refer to the value incident on an optical component of the system which provides the spatial structuring.

[0026] The peak irradiance may be less than or equal to 50 W.pnr2. The peak irradiance may be less than or equal to 10 W.pnr2. The peak irradiance may be less than or equal to 5 W.pnr2. The peak irradiance may be less than or equal to 3 W.pnr2.

[0027] The system may be configured such that a pulse width of the pulse is greater than or equal to 1 ps. The pulse width may be greater than or equal to 5 ps. The pulse width may be greater than or equal to 10 ps. The pulse width may be greater than or equal to 50 ps. The pulse width may be greater than or equal to 100 ps. The pulse width may be greater than or equal to 1 ns. The pulse width may be greater than or equal to 10 ns. The pulse width may be greater than or equal to 100 ns. The pulse width may be greater than or equal to 500 ns. The pulse width may be less than or equal to 1 ps.

[0028] The system may be configured to illuminate the target area of the magnetic medium using continuous wave, CW, laser light. The CW laser light is spatially structured as defined hereinbefore.

[0029] The CW laser light may be modulated. The CW laser light may be gated. In other words, the CW laser light may be switched on (or modulated high) for a duration, thenswitched off (or modulated low). Herein, the threshold between laser light being considered "pulsed" and "gated CW" may be regarded as a duration of 1 ps. Gated CW laser light may be applied for periods having a duration greater than or equal to 1 ps. CW laser light may be applied for periods having a duration greater than or equal to 10 ps. CW laser light may be applied for periods having a duration greater than or equal to 100 ps.

[0030] The irradiance of the CW laser light may be less than or equal to 10 W.pm-2. The irradiance of the CW laser light may be less than or equal to 5 W.pm-2. The irradiance of the CW laser light may be less than or equal to 1 W.pm-2.

[0031] The system may be configured such that each pixel corresponds to a sub-area including a single ferromagnetic domain.

[0032] The system may be configured such that each pixel corresponds to a sub-area including two or more ferromagnetic domains.

[0033] The thin film magnet structure may include, or take the form of, a plurality of discrete nanomagnets formed from the ferromagnetic element, alloy or compound.

[0034] Each nanomagnet may provide a respective ferromagnetic domain. Each nanomagnet may provide a single ferromagnetic domain. Each nanomagnet may provide a two or more ferromagnetic domains. Each nanomagnet may be coupled to (or form part of) a corresponding optical structure and / or second optical structure. Two or more (or all) of the nanomagnets may be coupled to (or form part of) a single optical structure and / or a single second optical structure.

[0035] The plurality of nanomagnets may be arranged in an array. The array may be a square array. The array may be a rectangular array. The array may be a triangular array. The array may be arranged according to any one of the five 2D Bravais lattices, with a motif including one or more nanomagnets. The nanomagnets may be arranged along a path. The path may include, or take the form of, a spiral. The path may include, or take the form of, a serpentine path. The nanomagnets may be disordered. The nanomagnets may possess only short range order.

[0036] The nanomagnets may be arranged to form an artificial spin ice geometry. The nanomagnets may be arranged to form a square artificial spin ice geometry. Thenanomagnets may be arranged in an artificial spin ice geometry according to any one of the five 2D Bravais lattices, with a motif including one or more nanomagnets.

[0037] One, some, or all of the nanomagnets may be bar shaped. One, some, or all of the nanomagnets may be T-shaped. One, some, or all of the nanomagnets may be X-shaped. The shape of each nanomagnet may be square, triangular, circular, elliptical, stadium shaped, or oval. All of the nanomagnets may have the same shape. The nanomagnets may each have one of two or more shapes. When the nanomagnets each have one of two or more shapes, the two or more shapes may be arranged in a repeating pattern.

[0038] The thin film magnet structure may include, or take the form of, a mesh formed from the ferromagnetic element, alloy or compound. The mesh may be a square mesh. The mesh may be a rectangular mesh. The mesh may be a triangular mesh. The mesh may be a disordered mesh. The mesh may take the form of a union of first magnetic elements extending in a first direction and second magnetic elements extending in a second, different, direction. The first and second directions may be perpendicular.

[0039] The thin film magnet structure may include, or take the form of, one or more nanowires formed from the ferromagnetic element, alloy or compound. The one or more nanowires may take the form of a plurality of parallel nanowires spaced apart to form an array in the direction perpendicular to the axes of the nanowires. The one or more nanowires may be disposed along a serpentine or spiral path.

[0040] The thin film magnet structure may take the form of a uniform film supporting a plurality of ferromagnetic domains.

[0041] The system may be configured to spatially structure the laser light as an image. The image may have a resolution equal to, or less than, the resolution of the pixels.

[0042] The system may be configured to encode a bit using the states written to the ferromagnetic domains by a single pixel. In other words, the magnetization states written to the ferromagnetic domains within the corresponding sub-area may determine the value of the bit. The bit may be binary. However, when the ferromagnetic domains are switchable between N distinguishable states, the stored bit may be N-ary.The system may be configured to encode a bit using the states written to the ferromagnetic domains by a group of pixels. In other words, the pattern of magnetization states written to the ferromagnetic domains within the union of the corresponding sub-areas may determine the value of the bit. The bit may be binary. The bit may be trinary, or in general N-ary, depending upon both the number of distinguishable states and the number of pixels used per bit.

[0043] In this way, the robustness of date written to the magnetic medium may be increased against errors / failed switching.

[0044] The one or more ferromagnetic domains may include a number of first ferromagnetic domains configured to undergo all-optical magnetic switching in response to a first polarisation state of the laser light. The one or more ferromagnetic domains may also include a number of second ferromagnetic domains configured to undergo all-optical magnetic switching in response to a second, different, polarisation state of the laser light. The first and second ferromagnetic domains may be intermixed. For example, interdigitated, or in an interpenetrating lattice arrangement.

[0045] The first polarisation may correspond to linear polarisation in a first direction and the second polarisation may correspond to linear polarisation in a second direction different to the first direction. The second direction may be perpendicular to the first direction.

[0046] The first polarisation may correspond to circular or elliptical polarisation in a first sense and the second polarisation may correspond to circular or elliptical polarisation in the opposite sense.

[0047] The system may be configured to switch the first ferromagnetic domains using first laser light uniformly polarised with the first polarisation state and having spatially structured intensity, and to switch the second ferromagnetic domains using second laser light uniformly polarised with the second polarisation state and having spatially structured intensity. The first and second laser light may be provided by separate sources. The system may be configured to supply the first and second laser light at different times, or concurrently (for example superposed).

[0048] Alternatively, the first and second laser light may be provided be a single source including a polarisation controlling component in the optical path (for example a spatial light modulator).The system may be configured to switch the first and second ferromagnetic domains in the same target area concurrently, using laser light spatially structured in both polarisation and intensity.

[0049] The target area may be smaller than a total area of the thin film magnet structure. The system may be configured to move the target area relative to the magnetic medium.

[0050] For example, the system may be configured to scan or raster the target area across the total area of the thin film magnet structure.

[0051] In another example, the magnetic medium may take the form of a spinning disc, and the system may be configured to supply the spatially structure laser light from a write-head radially movable relative to the magnetic medium.

[0052] Additionally or alternatively, one or more moveable mirrors may be used to scan the spatially structured laser light across the magnetic medium. For example, analogously to conventional laser scanning systems. The magnetic medium may be fixed and the spatially structured light may be scanned / rastered in two-dimensions. When the magnetic medium takes the form of a spinning disc, the spatially structured light may be scanned in the radial direction.

[0053] Depending on the variability in the incident angles of laser light across the magnetic medium, the thin film magnet structure may be configured with a geometry to compensate for variations in the shape and area of the projection onto the magnetic medium. Additionally or alternatively, the magnetic medium may have a surface curved to compensate for variations in incident angles of the laser light due to scanning using a moving mirror.

[0054] The target area may be substantially equal to a total area of the thin film magnet structure. Substantially equal may correspond to between 60% and 100% of the thin film magnet structure.

[0055] A configuration in which substantially the entire thin film magnet structure is addressable in parallel may be useful in applications where a comparatively low overall storage capacity may be a secondary consideration to parallel addressability. Forexample, for use as a register in a computing system, as a computing substrate in neuromorphic computing, and so forth.

[0056] Additionally or alternatively, the system may include a plurality of magnetic media, all configured as described herein (optionally identically configured). The system may be configured using multiple sources, each arranged to supply spatially structured laser light to a corresponding target area which is substantially equal to a respective one of the plurality of magnetic media. In this way, a larger storage capacity may be obtained whilst maintaining parallel addressability by "tiling" magnetic media and the corresponding elements for supplying spatially structured laser light.

[0057] The system may be configured to write states to the ferromagnetic domains within two or more target areas of the magnetic medium concurrently by illuminating each target area of the magnetic medium using laser light spatially structured into a plurality of pixels. Each such pixel illuminates a corresponding sub-area of the magnetic medium. In this way, the total time taken to write any given data to the magnetic medium may be reduced.

[0058] For example, if the magnetic medium takes the form of a spinning disc, then the system may include two or more write heads. Each write head may be moveable to span a different range of the radial direction. Alternatively, each write head may be moveable to span an overlapping range (or the entire range) in the radial direction, and be arranged at different angles spaced around the disc axis.

[0059] The system may be configured for readout of the states of ferromagnetic domains of the magnetic medium.

[0060] The system may be configured for optical readout of the magnetic medium. The system may include one or more optical readout units. Each of the one or more optical readout units may include, or take the form of, an analyser polarising filter and a detector for detecting the intensity of light as a function of polarisation angle. The optical readout of the magnetic medium may be based on the magneto-optical Kerr effect.

[0061] The system may be configured to readout information stored on the magnetic medium using a magnetic readout mechanism. The magnetic readout mechanism may include a magnetic read head sensor.The system may be configured to readout information stored on the magnetic medium using a spin polarised current. For example, using a spin-torque ferromagnetic resonance mechanism. A substrate supporting the magnetic medium may include one or more layers used for electrical readout of the states of ferromagnetic domains using spin-torque ferromagnetic resonance.

[0062] The system may include a laser source configured to provide input laser light at the operating wavelength, and a patterner. The patterner may be configured to receive the input laser light, and to output the laser light spatially structured into the plurality of pixels. The patterner may be controllable by the system to set the relative intensity and / or polarisation of each pixel of the laser light.

[0063] When the system is configured to illuminate two or more target areas, the system may include at least a patterner corresponding to each target area. The system may include a laser source corresponding to each patterner. Alternatively, the system may include fewer laser sources than patterners, in combination with beamsplitters arranged to divide input laser light from each laser source amongst the corresponding patterners.

[0064] When the system includes more than one laser source, each laser source may have a different operating wavelength. In such examples, the magnetic medium may include multiple types of optical structure, each configured to correspond to the operating wavelength and / or polarisation state of one of the laser sources. In this way, a density of ferromagnetic domains may be increased below the diffraction limit by wavelength multiplexing.

[0065] Each operating wavelength may be sufficiently different to each other operating wavelength to avoid cross-talk, and may not be harmonics of one another.

[0066] Alternatively, such restrictions may not be necessary for a pair of laser sources having different polarisations.

[0067] Each laser source may include or take the form of a pulsed laser. Each laser source may include, or take the form of, a continuous wave (CW) laser in series with a pulsed modulator. Each laser source may include, or take the form of, two or more CW sources having output wavelengths configured to generate beating at a desired pulse width and pulse frequency. Each laser source may be included in a corresponding write head of the system.The patterner may include, or take the form of, a digital micromirror device. When two or more patterners are included, each may include, or take the form of, a digital micromirror device.

[0068] Each patterner may include, or take the form of, a spatial light modulator. The spatial light modulator may produce spatial patterning of intensity. The spatial light modulator may produce spatial patterning of polarisation. The spatial light modulator may produce spatial patterning of intensity and polarisation.

[0069] Each patterner may be configured to generate the corresponding spatially structured laser light using holography.

[0070] The system may include a number of laser emitters disposed in an array and arranged such that each laser emitter provides the pixel illuminating a corresponding sub-area of the magnetic medium. The plurality of laser emitters may be addressable such that that system controls the relative intensity of each pixel of the laser light by controlling an emission intensity of the corresponding laser emitter.

[0071] The array of laser emitters may be movable relative to the magnetic medium. For example, the laser emitters may be installed in a writing head of the system. In other examples, the array of the plurality of laser emitters may be fixed relative to the magnetic medium.

[0072] The array of laser emitters may be supported on a substrate opposed to the magnetic medium across a gap. The gap may take the form of an air bearing. The gap may take the form of a substantially transparent (i.e. 50% or greater at the operating wavelength) material.

[0073] The array of laser emitters may be integrated with the magnetic medium. For example, the one or more optical structures may be disposed between an underlying substrate supporting the array of laser emitters and the thin-film magnet structure.

[0074] Each laser emitter may include, or take the form of, a surface emitting laser. Each laser emitter may include, or take the form of, a vertical-cavity surface emitting laser. Each laser emitter may include, or take the form of, a quantum dot.

[0075] The thin film magnet structure may form a part of one, some, or all, of the optical structures. For example, the thin film magnet structure may provide a reflector of oneor more of the optical structures. Each optical structure may be coupled to a portion or region of the thin film magnet structure, so as to increase absorbance of light at the target wavelength in that portion or region.

[0076] Each optical structure may include, or take the form of, a reflective surface and a dielectric layer interposed between the reflective surface and the thin film magnet structure. The dielectric layer may be substantially transparent at the operating wavelength. Substantially transparent may correspond to a transmission of 50% or more, 60% or more, 70% or more, 80% or more or 90% or more of incident light at the operating wavelength through the thickness of the dielectric layer.

[0077] The ferromagnetic alloy or compound may include two or more elements. One or more of the elements included in the ferromagnetic alloy or compound may be transition metal elements. For example, the ferromagnetic alloy or compound may take the form of an alloy comprising nickel (Ni) and iron (Fe). For example, the ferromagnetic alloy or compound may be NisiFeig or NisoFeso.

[0078] The ferromagnetic alloy or compound may include no rare earth elements. A ferromagnetic alloy or compound includes no rare earth elements if no rare earth elements are intentionally added, doped, and so forth to that alloy or compound. In other words, the ferromagnetic alloy or compound may include no rare earth elements beyond background or trace quantities. Herein, rare earth elements may refer to elements in the lanthanide series (elements with an atomic number between 57 and 71 inclusive, i.e., lanthanum through lutetium).

[0079] The ferromagnetic alloy or compound may have a magnetic anisotropy that is sufficiently low that the available magnetization states in a given structure are determined primarily by shape anisotropy, as opposed to another type of magnetic anisotropy such as magnetocrystalline anisotropy. The ferromagnetic alloy or compound may take the form of materials with low magnetic anisotropy (KI <500 kJ / m3). This is not limited to nickel-iron alloys such as Permalloy, and may also include, for example, a ferromagnetic alloy or compound including cobalt and iron, such as cobalt iron alloy (CoFe) or cobalt iron boride (CoFeB).

[0080] The ferromagnetic alloy or compound may have a magnetic anisotropy

[0081] (KI > 500 kJ. rm3) that is sufficiently high that the available magnetization states in a given structure are determined primarily by magnetocrystalline anisotropy, as opposed to shape anisotropy. The ferromagnetic alloy or compound may take theform of alloys including cobalt and platinum (CoPt), alloys including iron and palladium (FePd), and / or lanthanide alloys (such as NdFeB).

[0082] The ferromagnetic alloy or compound may take the form of compounds including chromium and oxygen. The ferromagnetic alloy or compound may take the form of chromium dioxide.

[0083] The magnet structure may include a first subset of the one or more ferromagnetic domains having a magnetization oriented substantially parallel to a first direction. The magnet structure may include a second subset of the one or more ferromagnetic domains having a magnetization oriented substantially parallel to a second direction different from the first direction. For example, the second direction may be perpendicular to the first direction.

[0084] The magnetic medium may be supported on a substrate. The magnetic medium may form part of a hard disc. The magnetic medium may form part of an optical disc.

[0085] The magnetic medium may be configured in any way described in WO 2023 / 099887 Al. In particular, the magnetic medium of the present specification may include any features of the magnetic medium described in WO 2023 / 099887 Al from page 4, line 2 to page 9, line 11. Similarly, definitions applicable to the magnetic medium described in WO 2023 / 099887 Al from page 4, line 2 to page 9, line 11 may be equally applicable to the magnetic medium of the present specification. The magnetic medium may be configured in accordance with the specific examples illustrated in any one of Figures 1 to 6 and 8 to 15 of WO 2023 / 099887 Al, and the corresponding portions of the description of WO 2023 / 099887 Al.

[0086] The system of the present specification may additionally include any features of the systems described in WO 2023 / 099887 Al from page 9, line 13 to page 11, line 25, adapted as necessary to incorporate the spatial structuring of laser light described herein.

[0087] A data storage device may include the system. The data storage device may take the form of a memory. The data storage device may take the form of a hard disc. The data storage device may be configured for long-term data storage and / or backup.

[0088] A neuromorphic computing device may include the system. The neuromorphic computing system may include features corresponding to any features of devices,methods and / or systems described in WO 2024 / 074799 Al, the entire contents of which are hereby incorporated by reference. In particular, the neuromorphic computing system may include features corresponding to any features of devices described from page 3, line 8 to page 4, line 24 of WO 2024 / 074799 Al. The neuromorphic computing system may include features corresponding to any features of systems described from page 4, line 26 to page 6, line 2 of WO 2024 / 074799 Al. The neuromorphic computing system may include features corresponding to any features of methods described from page 6, line 4 to page 7, line 7 of WO 2024 / 074799 Al.

[0089] A random number generator may include the system.

[0090] A quantum computing device may include the system.

[0091] The system may be used as part of a security or anti-counterfeiting feature. For example, after writing the magnetic medium may be removable from the system for application to an object. Alternatively, the magnetic medium may be integrated with an object.

[0092] The system may be used for medical applications.

[0093] According to a second aspect of the invention, there is provided a system including a magnetic medium. The magnetic medium includes a thin film magnet structure. The thin film magnet structure includes one or more ferromagnetic domains. Each ferromagnetic domain has a magnetization that is switchable between two or more states. The system is configured to write states to the ferromagnetic domains within a target area of the magnetic medium by illuminating the target area of the magnetic medium using laser light spatially structured into a plurality of pixels. Each pixel illuminates a corresponding sub-area of the magnetic medium. The system is configured such that all-optical magnetic switching of ferromagnetic domains within each sub-area of the magnetic medium is controlled by the relative intensity and / or polarisation of the corresponding pixel of the laser light.

[0094] The system of the second aspect may operate using any magnetic medium described in relation to the system of the first aspect, but is not limited to such media.

[0095] Alternative types of magnetic medium may require the system of the second aspect to operate at higher irradiances as compared to the system of the first aspect. For example, in the range from greater than 100 W.pm-2up to and including10 kW. pnr2.

[0096] The system of the second aspect may include features corresponding to any features of the system of the first aspect. Definitions applicable to the system of the first aspect (and / or features thereof) may be equally applicable to the system of the second aspect (and / or features thereof).

[0097] According to a third aspect of the invention, there is provided a method of writing data to a magnetic medium. The magnetic medium includes a thin film magnet structure. The thin film magnet structure includes one or more ferromagnetic domains, each ferromagnetic domain has a magnetization that is switchable between two or more states. The magnetic medium also includes one or more optical structures. Each optical structure has an interface with the thin film magnet structure and is configured such that light of an operating wavelength incident on the magnetic medium interferes destructively at the interface. The method includes writing states to the ferromagnetic domains within a target area of the magnetic medium by illuminating the target area of the magnetic medium using laser light spatially structured into a plurality of pixels. Each pixel illuminates a corresponding sub-area of the magnetic medium. All-optical magnetic switching of ferromagnetic domains within each sub-area of the magnetic medium is controlled by the relative intensity and / or polarisation of the corresponding pixel of the laser light.

[0098] The method of the third aspect may include features corresponding to any features of the system of the first aspect. Definitions applicable to the system of the first aspect (and / or features thereof) may be equally applicable to the method of the third aspect (and / or features thereof).Brief Description of the Drawings

[0099] Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

[0100] Figure 1 schematically illustrates a system for all-optical magnetic writing;

[0101] Figures 2A and 2B schematically illustrate an example of writing states to an array of ferromagnetic domains in the form of nanomagnets;

[0102] Figure 3 schematically illustrates a first write head for the system of Figure 1;

[0103] Figure 4 schematically illustrates a second write head for the system of Figure 1; Figure 5 schematically illustrates a laser system used for experimentally writing patterns imaged in Figures 6B, 7B and 8B;

[0104] Figure 6A shows a first example of a binary image written to a magnetic medium; Figure 6B shows a magnetic force microscopy image corresponding to the result of all-optical magnetic writing using the pattern shown in Figure 6A with the laser system shown in Figure 5;

[0105] Figure 7A shows a second example of a binary image written to a magnetic medium; Figure 7B shows a magnetic force microscopy image corresponding to the result of all-optical magnetic writing using the pattern shown in Figure 7A with the laser system shown in Figure 5;

[0106] Figure 8A shows a third example of a binary image written to a magnetic medium; Figure 8B shows a magnetic force microscopy image corresponding to the result of all-optical magnetic writing using the pattern shown in Figure 8A with the laser system shown in Figure 5;

[0107] Figure 9 shows a magnetic force microscopy image of a nanomagnet array initialised to a saturated magnetic state;

[0108] Figure 10 shows a magnetic force microscopy image of the nanomagnet array shown in Figure 9 following application of a single 100 ps laser pulse;

[0109] Figure 11A shows a magnetic force microscopy image corresponding to a nanomagnet in a macrospin magnetic state;

[0110] Figure 11B shows a magnetic force microscopy image corresponding to a nanomagnet in a single-vortex magnetic state; and

[0111] Figure 11C shows a magnetic force microscopy image corresponding to a nanomagnet in a double-vortex magnetic state.

[0112] Detailed description

[0113] In the following, like parts are denoted by like reference numerals.

[0114] In a prior application, published as WO 2023 / 099887 Al, the entire contents of which are herein incorporated by reference, new types of magnetic media were describedwhich include optical structures coupled to magnetic structures to increase absorption of light therein. WO 2023 / 099887 Al also describes systems and methods for changing the state of magnetic nanoparticles within a dense array formed on such magnetic media, using low-power, continuous wave (CW) light from lasers. It was also described in WO 2023 / 099887 Al that the CW light could be modulated or pulsed, on timescales relatively long compared to high powered femtosecond lasers. Where any disclosure of WO 2023 / 099887 Al contradicts the present specification, the present specification takes precedence, reflecting the inventors most current understanding.

[0115] In the present specification, systems and methods are described for massively parallel writing to magnetic media using all-optical switching. The systems and methods described herein may be applied to any magnetic medium capable of all-optical magnetic switching, though they may be particularly advantageous in the context of writing data to examples of magnetic media as described in WO 2023 / 0099887 Al. In particular, the systems and methods of the present specification can be applied to any magnetic medium as described in WO 2023 / 099887 Al from page 4, line 2 to page 9, line 11. Similarly, definitions applicable to the magnetic medium described in WO 2023 / 099887 Al from page 4, line 2 to page 9, line 11 may be equally applicable to a magnetic medium used in conjunction with the systems and methods described herein. Moreover, the specific examples illustrated in any one of Figures 1 to 6 and 8 to 15 of WO 2023 / 099887 Al, and described in the corresponding portions of the description of WO 2023 / 099887 Al, may be used in the systems and methods of the present specification.

[0116] The present specification applies wavefront shaping (in the spatial domain) for controlled, parallelised addressing of target areas. The systems and methods of the present specification may obtain switching using low power in the pJ range, and subns switching speeds. The performance obtainable may be comparable to, or even exceed, state-of-the-art technologies such as optically switchable magnetic tunnel junction memory devices, electrically switchable spin-transfer and spin-orbit torque devices and so forth. Moreover, the presently described systems and methods may be used with magnetic media formed from alloys of nickel and iron (permalloy), an inexpensive and non-toxic alloy, eliminating the need for rare and geopolitica lly sensitive materials. Although it is advantageous to use abundant and relatively cheaper materials, the presently described systems and methods are applicable to other types of media capable of use for all-optical magnetic switching. Magnetic media as described hereinafter in relation to Figure 1 are preferred (in accordancewith WO 2023 / 099887 Al), since the presently known alternative media are understood to require several orders of magnitude higher irradiance.

[0117] The present specification describes improvements over the disclosure of WO 2023 / 099887 Al, including but not limited to:

[0118] • Achieving faster, sub-ns switching - a continuous wave laser as described in WO 2023 / 099887 Al is attractive because such devices are cheap, but for fast magnetic switching a pulsed-laser and sub-ns switching times are desirable. Surprisingly, even when applying spatial structuring as described herein, sufficient irradiance for all-optical switching could also be obtained, localised and patternable to a degree sufficient to write images with a resolution approaching that of the ferromagnetic domains of the magnetic medium (see Figures 6A to 8B hereinafter).

[0119] • Reducing, and in some examples entirely removing, the need to mechanically move the laser or magnetic medium between writing. Much of the time and energy cost in existing magnetic writing schemes is typically related to moving the laser to different positions relative to the magnetic medium.

[0120] Experimental data presented hereinafter (see Figure 5 onwards) demonstrates the applicability for a scheme using 400 ps laser pulses supplying just ~3.63 pJ per nanomagnet (210 nm x 80 nm nanomagnet), which uses a digital micromirror device to address an area of 150 um X 150 um. This area is able to contain 1.4 million 210 x 80 nm magnets, or a larger number of smaller magnets or other types of ferromagnetic domain. The magnetic medium can still be moved relative to the laser illumination, but for a given relative position of laser and magnetic medium, it is possible to address many bit-patterned nanomagnets (or alternatively structured ferromagnetic domains) for concurrent all-optical magnetic writing.

[0121] Further experimental data presented hereinafter (see Figure 9 onwards) demonstrates the applicability for a scheme using shorter, 100 ps laser pulses applied to nanomagnets with dimensions of 470 nm x 120 nm. Although higher intensities were used for the 100 ps studies, this is not believed to represent a lower bound on the energy required at such timescales (5.83 W.pm'2, corresponding to 9.79 pJ per 210 nm x 80 nm sized nanomagnet).Existing technology functions with a single magnetic write element and must address separate writing operations sequentially. In comparison, the presently described systems and methods allow addressing any number of ferromagnetic domains within a target area in parallel by spatially structuring the laser illumination in terms of intensity (equivalently irradiance), polarisation, or both. Moreover, light from a single laser can be split using a beam splitter, or directed using a galvanometer scanner, on to multiple parallel DMDs to increase the writing area by multiple times to cover potentially millions of ferromagnetic domains in parallel.

[0122] Referring to Figure 1, a system 1 for all-optical magnetic writing is shown.

[0123] The system 1 includes a magnetic medium 2, a write head 3 and a controller 4.

[0124] Optionally, the system 1 also includes a read head 5. In some applications, such as a hard drive, the read head 5 would be included. In other applications, for example security labelling, the magnetic medium 2 may be read by an entirely separate reader device (not shown), meaning that a read head 5 may not be required.

[0125] The magnetic medium 2 includes a thin film magnet structure 6 which includes one or more ferromagnetic domains 7. Only a single ferromagnetic domain 7 is labelled in Figure 1 for visual clarity, although in practice the thin film magnet structure 6 will in practice include at least one ferromagnetic domain for each pixel Pn,m of the spatially structured laser light 10. Each ferromagnetic domain 7 has a magnetization that is switchable between two or more states. In many example, a pair of states, i.e. binary states, may be used. However, in other systems, each ferromagnetic domain may support three (trinary) or more distinguishable and stable magnetisation states. For example, a ferromagnetic domain may accept a pair of macrospins, a pair of single vortices, and / or four double vortices.

[0126] Further details and optional features of the magnetic medium 2 have been described in the "Summary" section hereinbefore. Still further examples of suitable magnetic media 2 are described in WO 2023 / 099887 Al from page 4, line 2 to page 9, line 11. Examples of further, specific implementations of suitable magnetic media 2 are illustrated in Figures 1 to 6 and 8 to 15 of WO 2023 / 099887 Al, and described in the corresponding portions of the description of WO 2023 / 099887 Al.

[0127] The magnetic medium also includes one or more optical structures 8. Each optical structure 8 has an interface 9 with the thin film magnet structure 6 and is configuredsuch that laser light 10 of an operating wavelength Ao incident on the magnetic medium 2 interferes destructively at the interface 9.

[0128] In the example shown in Figure 1, the optical structure 8 has the form of a reflective layer 11 separated from the thin film magnet structure 6 by a transparent dielectric layer 12 having a thickness d. The dielectric layer 12 has a transmission of at least 50% through the thickness d, and preferably as high as possible. The laser light 10 is at least partially transmitted through the thin film magnet structure 6 (for example due to patterning, low thickness and so forth), and reflects from the reflective interface 13 between the dielectric layer 12 and the reflective layer 11. The thickness d is tuned relative to the operating wavelength Ao such that the light reflected from reflective interface 13 interferes destructively with incident laser light 10. Without wishing to be bound by theory, it is believed that the suppression of both transmission and reflection of the laser light 10 is responsible for the increased absorbance by the thin film magnet structure 6.

[0129] Further details and optional features of the magnetic medium 2 have been described in the "Summary" section hereinbefore. Still further examples of possible optical structures 8 are described in WO 2023 / 099887 Al from page 4, line 2 to page 9, line 11. Examples of further, specific implementations of suitable optical structures 8 are illustrated in Figures 1 to 6 and 8 to 19 of WO 2023 / 099887 Al, and described in the corresponding portions of the description of WO 2023 / 099887 Al.

[0130] The functional layers 6, 8 of the magnetic medium 2 may be supported on a substrate 14. In some examples, the substrate 14 may also provide the reflective interface 13, so that a separate reflective layer 11 may be omitted. In some examples, the substrate 14 may also include one or more additional layers (not shown) used for electrical readout of the magnetisation states of overlying ferromagnetic domains 7 of the thin film magnet structure 2.

[0131] The system 1 is configured to write states to all of the ferromagnetic domains 7 within a target area Atarg of the magnetic medium 2 by illuminating the target area Atarg using laser light 10 spatially structured into a plurality of pixels P. In the example shown in Figure 1, the spatially structured laser light 10 is provided by the write head 3 under control of the controller 4. The controller 4 also receives and processes data 15 for writing to the magnetic medium 2 (and when a read head 5 is included, also processes and outputs read data).For the sake of definiteness of description, if the laser light 10 is spatially structured into an N by M array of pixels P, the pixel P in the nthof N rows and mthof M columns is denoted Pn,m hereinafter. Each pixel Pn,m illuminates a corresponding sub-area An,m of the target area Atarg of the magnetic medium 2. All-optical magnetic switching of ferromagnetic domains 7 within each sub-area An,mOf the magnetic medium 2 is controlled by the relative intensity and / or polarisation of the corresponding pixel Pn,m of the laser light 10. Without wishing to be bound by theory, the mechanisms of all-optical magnetic switching in the magnetic medium 2 were discussed in WO 2023 / 099887 Al on page 15, lines 23 to 26, on page 17, lines 12 to 15, on page 17, lines 20 to 28, on page 1, lines 16 to 20, on page 27, lines 1 to 3, on page 28, lines 16 to 28, on page 30, lines 1 to 19, on page 31, lines 14 to 22, on page 31, line 32 to page 32, line 12, on page 32, line 36 to page 33, line 7, on page 38, lines 7 to 13, and on page 39, lines 18 to 27, and in the Figures referred to in these passages.

[0132] Spatially structuring the laser light 10 may correspond to controlling one or both of:

[0133] • the intensity (equivalently irradiance) of each pixel Pn,m,' and

[0134] • the polarisation of each pixel Pn,m. For example, each pixel may be linearly polarised in one of two or more distinct orientations.

[0135] Experimental evidence of all-optical magnetic switching using spatially structured laser light 10 is described hereinafter in relation to Figure 5 onwards.

[0136] In examples when the polarisation of the laser light 10 is not spatially structured, the laser light 10 may still have a global, uniform polarisation which may be linearly, circularly, or elliptically. Polarisation selectivity was demonstrated and discussed in WO 2023 / 099887 Al, see for example Figures 9B and 11 and the corresponding description.

[0137] Although illustrated in Figure 1 as ideally tessellating to fill the target area Atarg, in practice transitional regions (or boundaries) (not shown) may exist between adjacent pixels Pn,m, Pn+i,m etc. Such transitional regions need not be limited to diffraction limited dimensions. In some examples, the thin film magnet structure 2 may be patterned so that any transitional regions coincide with gaps between ferromagnetic domains 7. For example, a nanomagnet (or cluster of nanomagnets) may be patterned to correspond to the central region of each sub-area An,m.In this way, data in the form of an "image" may be written to the ferromagnetic domains 7 within the target area Atarg (see also the example described in relation to Figures 2A and 2B). An image written to the ferromagnetic domains 7 in this way will have a resolution equal to, or less than, the resolution of the pixels Pn,m.

[0138] Writing the state of every ferromagnetic domains 7 within a target area Atarg including a potentially very large (up to millions) of ferromagnetic domains 7 in a single parallel operation has advantages for both writing speed and energy requirements, the latter because even when the write head 3 needs moving relative to the magnetic medium 2, the frequency of movement can be reduced greatly compared to scanning over every ferromagnetic domain.

[0139] The magnet structure may be formed of a ferromagnetic element, alloy or compound. Examples are described in the "Summary" section hereinbefore. Further examples of suitable materials are described in WO 2023 / 099887 Al, for example on page 7, line 30 to page 8, line 31, on page 17, line 20 to page 18, line 14, on page 23, lines 13 to 31, on page 24, lines 4 to 14, and in the specific magnetic media for which experimental and / or modelling results are presented in WO 2023 / 099887 Al.

[0140] Importantly, even when a beam of laser light 10 is expanded and spatially structured, it is still possible to perform all-optical magnetic switching of a magnetic medium 2 including optical structures (as described in WO 2023 / 0099887 Al) using a relatively compact and low powered laser source. The laser light 10 does not correspond to a femtosecond laser pulse, and can instead be provided by a laser source which is no larger than benchtop scale. For example a peak output power level of less than or equal to 50 kW.

[0141] Although CW laser light 10 can be spatially structured into pixels Pn,m and used to control all-optical switching as described herein, in the interests of switching speed it is preferable that the system 1 be configured to illuminate the target area of the magnetic medium using spatially structured laser light 10 in the form of a pulse. The system 1 is preferably configured such that a peak irradiance of the pulse is less than or equal to 100 W.pnr2, and more preferably less than 10 W.pm'2, so as to reduce power dissipated as heat and improve scalability. Using the experimental setup and magnetic medium 2 described hereinafter in relation to Figure 5, the peak irradiance of the pulse of spatially structured laser light 10 was less than 2.5 W.pm'2with a pulse width of 400 ps.In some examples, some power dissipation as heat may be desired, for example, to assist with switching in a heat-assisted magnetic recording (HAMR) application. In such examples, the power dissipation should be optimised to balance between enough local heating to facilitate switching, whilst minimising an overall heat load to the medium and system overall (to reduce cooling requirements).

[0142] The system 1 is preferably configured such that a pulse width of the pulse is greater than or equal to 1 ps. Using shorter pulses would begin to approach the regime of femto-second pulsed lasers and associated high peak power levels, requiring increasingly bulky and expensive laser sources. In general, the pulse width may be adjusted above 1 ps, up to around 1 ps, depending on the required switching speed and on the material and patterning of the thin film magnet structure 2.

[0143] For pulses longer than about 1 ps, it may be preferable to provide the spatially structured laser light 10 using a "gated" CW source. In other words, the CW laser light may be switched on (or modulated high) for a duration, then switched off (or modulated low). Maximum irradiances for CW or gated / modulated CW sources may be relatively lower than for short pulses, for example less than or equal to 10 W.pnrr2, down to less than 1 W.pm-2.

[0144] The system 1 may be configured such that each pixel Pn,m corresponds to a sub-area An,m including a single ferromagnetic domain 7. For example, the thin film magnet structure 6 may be patterned to include a single nanomagnet 16 within each sub-area An,m (see Figures 2A and 2B). As a modification, using polarisation selective effects described hereinbefore, each pixel Pn,m may corresponds to a sub-area An,m including a two or more ferromagnetic domains 7, each of which will undergo all-optical magnetic switching in response to a different polarisation (for example, interpenetrating arrays of differently oriented nanomagnets). Alternatively, the system 1 may be configured such that each pixel Pn,m corresponds to a sub-area An,m including two or more ferromagnetic domains.

[0145] When a read head 5 is included, this may be configured for optical readout of the magnetic medium 2 (more specifically, the ferromagnetic domains 7 of the thin film magnet structure 2). For example, for optical readout the read head 5 may include an analyser polarising filter (not shown) and a detector (not shown) for detecting the intensity of light as a function of polarisation angle. In some examples the optical readout of the magnetic medium may be based on the Magneto-Optical Kerr Effect. Alternatively, the system 1 may be configured to readout information stored on themagnetic medium 2 using a magnetic readout mechanism, for example a magnetic read head sensor such as a magnetoresistance sensor. As mentioned hereinbefore, the substrate 14 may additionally include one or more layers (not shown) used for electrical readout of the states of ferromagnetic domains 7. For example, using spin polarised currents, spin-torque ferromagnetic resonance, and so forth.

[0146] Referring also to Figures 2A and 2B, an example of writing states to an array of ferromagnetic domains 7 is schematically illustrated. Figure 2A schematically illustrates the pattern of a pulse of spatially structured laser light 10 applied to an uniformly oriented array of nanomagnets 16, and Figure 2B schematically shows the magnetisation states of the nanomagnets 16 following the pulse of spatially structured laser light 10.

[0147] The example shown in Figures 2A and 2B is one in which the thin film magnet structure 2 is patterned as a plurality of discrete nanomagnets 16 formed from the ferromagnetic alloy or compound. In the illustrated example, each nanomagnet 16 provides a single ferromagnetic domain 7, but in other examples each nanomagnet 16 could provide two or more ferromagnetic domains 7. In the illustrated example, each nanomagnet 16 is magnetised in-plane along a long axis, either "up" (illustrated with white fill and black arrow) or "down" (illustrated with black fill and white arrow). This can be accomplished in a number of ways such as, for example, using shape induced anisotropy, material anisotropy, or a combination. Although not shown in Figures 2A and 2B, each nanomagnet 16 is be coupled to (or forms part of) a corresponding optical structure 8.

[0148] In the illustrated example, the nanomagnets 16 are arranged in an array, specifically a 6 by 6 array having a square lattice. In other examples, the array may be a rectangular array, hexagonal, or according to any one of the five 2D Bravais lattices, with a motif including one or more nanomagnets. In some examples, an array of nanomagnets may be arranged and configured to form an artificial spin ice (ASI) geometry. Further examples of suitable ASI arrays are shown in Figures 8, 9A, 9B, 10, 11, 15, and 20G of WO 2023 / 0099887 Al, and described in the corresponding sections of the description thereof.

[0149] Referring in particular to Figure 2A, an example of spatial structuring by intensity is shown. Each pixel Pi,i, Pn,m, Pe,e is illustrated as either white (high) or black (low / zero), representing binary patterning of an input last pulse of substantially uniform irradiance by the binary mask illustrated in Figure 2A. Before the pulse ofspatially structured laser light 10, all of the nanomagnets 16 were set to the "up" state (for example, due to pre-formatting of the magnetic medium).

[0150] Referring now to in particular to Figure 2B, the nanomagnets 16 corresponding to illuminated white pixels Pn,m undergo all-optical switching to the "down" state. The nanomagnets 16 corresponding to non-illuminated, black pixels Pn,m remain in the original "up" state.

[0151] Although for visual clarity the example illustrated in Figures 2A and 2B only includes thirty six nanomagnets 16 and uses an arbitrary input image, in general the number of pixels Pn,m will be significantly larger, and the input image patterned into the laser light 10 may encode data using any suitable encoding scheme.

[0152] The spatial patterning of intensity is not limited to binary, though this is an efficient implementation for ferromagnetic domains 7 having a pair of magnetisation states. In other examples, multiple intensity levels, or even grayscale, could be used instead for spatial patterning. For example, if each sub-area An,m corresponded to a large number of ferromagnetic domains having a switching fidelity (probability) which varied proportionately to intensity, then spatial patterning with greyscale intensity would result in switching a fraction of ferromagnetic domains 7 in each sub-area An,m in proportion to the intensity applied via the pixel Pn,m.

[0153] The system 1 utilising all-optical magnetic switching (AOMS), and preferably using ultrafast (1 ps to 1 ns) lasers presents may provide several advantages for magnetic data storage technology:

[0154] 1. Write Speed: Ultrafast (1 ps to 1 ns) laser pulses can switch 10,000 magnetic nanoparticle states on picosecond timescales, which is orders of magnitude faster than conventional magnetic switching methods. This could significantly enhance data writing speeds, directly addressing the need for faster data processing in large data centres.

[0155] 2. Higher Energy Efficiency: As described herein, massively parallelised laser- induced switching can be achieved with lower energy consumption compared to traditional current-based methods, potentially reducing the power requirements for data storage devices. This is critical for data centres with a view to minimising climate impacts and reducing energy costs.3. Non-Contact Writing: Utilising light to switch magnetic states can, in some examples (for example using a laser scanning system) eliminate the need for the write head 3 to be in close proximity to the magnetic medium (compared to direct application of magnetic field. This may reduce wear and improving reliability. This aligns with the goal of reducing maintenance needs and enhancing the lifetime of storage devices to further improve sustainability.

[0156] 4. Multiplexing Potential: Systems 1 and methods described herein are suitable to address the ferromagnetic domains of many sub-areas An,m in parallel using the spatially structured laser light 10, enabling massively parallel writing operations.

[0157] 5. No requirement of rare earth elements: Systems 1 and methods described herein are applicable to magnetic mediums 2 as described in WO 2023 / 099887 Al, enabling implementation of suitable thin film magnet structures using permalloy and other alloys based on comparatively abundant materials such as iron, nickel, cobalt and so forth.

[0158] Patterning an input laser beam

[0159] Referring also to Figure 3, a first write head 3a for the system 1 is schematically illustrated.

[0160] The first write head 3a includes a laser source 17 configured to provide input laser light 18 at the operating wavelength Ao to a patterner 19. Although in some examples the laser source 17 may provide the input laser light 18 directly to the patterner, in practical one or more beam-shaping optical components 20 will be provided in the optical path prior to the patterner 19 in order to:

[0161] • Match the shape and size of the beam of input laser light 18 to an active area of the patterner; and

[0162] • Provide substantial uniformity to the intensity profile of the beam of input laser light 18. For example, to ensure that a difference between the maximum and minimum intensity incident on the patterner 19 is less than or equal to 25% of the maximum intensity.

[0163] Optical components 20 may also be provided to perform additional functions, for example to divide the initial beam into two or more beams for delivery to respectivepatterners 19, to provide neutral density filtering to reduce the irradiance of the input laser light 18, and so forth. In the illustrated example, the optical components 20 include a diverging and converging lens pair to expand the beam width of the input laser light 18.

[0164] The patterner 19 is configured to receive the input laser light 18 and to output the laser light 10 spatially structured into the plurality of pixels Pn,m. The patterner is controllable by the system 1 (for example using controller 4) to set the relative intensity and / or polarisation of each pixel Pn,m of the spatially structured laser light 10.

[0165] The laser source 17 may take the form of a pulsed laser, or a continuous wave (CW) laser in series with a pulsed modulator. In other examples, the laser source 17 may include two or more CW laser sources having individual output frequencies fi, f2 configured to generate beating at a desired pulse width and a pulse frequency corresponding to the operating wavelength Ao. For example, the operating wavelength Ao may correspond to frequency fi + f2) / 2 modulated at frequency fi-f2 / 2.

[0166] The patterner 19 may take the form of any device suitable to imprint a spatial pattern of intensity and / or polarisation to the input laser light 18. For example, when a binary image is used, a digital micromirror device (DMD) may be particularly suitable. A spatial light modulator (SLM) may be used when structuring of polarisation is required, and / or when more than two intensity levels are desired. SLMs are optical devices that manipulate the phase, amplitude, and or polarisation of light in a spatially resolved manner. Unlike DMDs, which use micromirrors to reflect light, SLMs typically use liquid crystal arrays or other materials that modulate light as it passes through.

[0167] The patterner 19 may include multiple components in series, for example, a DMD may apply a binary intensity mask, and the output may then pass through a spatial light modulator to control the polarisation state of each pixel Pn,m.

[0168] Alternatively, each patterner 19 may be configured to generate the corresponding spatially structured laser light using holography techniques. For example, the pattern for the spatially structured laser light 10 may be encoded as an interference hologram. Holographic can provide high efficiency in light utilisation and the ability to produce 3D or highly complex 2D patterns. Such benefits may be particularly relevant for magnetic media 2 patterned with a high density of ferromagnetic domains 7 (for example nanomagnets 16), where fine control over pattern depth may be useful.Patterned laser array

[0169] Whilst DMDs and SLMs are commercially available mature technologies which continue to grow smaller, cheaper and capable of processing greater fluence - in some applications these options may still be relatively bulky.

[0170] However, due to the possibility to induce all-optical magnetic switching using much lower irradiances then have been reported previously, the present systems 1 and methods could also be implemented using on-chip package laser emitters. For example, instead of using a laser source 17 to generate a beam 18 which is subsequently patterned, the laser light 10 may instead be generated in a spatially structured pattern using an array of miniaturised laser emitters.

[0171] Referring also to Figure 4, a second write head 3b is shown, along with a corresponding section of a magnetic medium 2 in which the thin film magnet structure 6 takes the form of an array of nanomagnets 16.

[0172] The second write head 3b includes a number of laser emitters 21 disposed in an array and arranged such that each laser emitter 21 provides a respective pixel Pn,m to illuminate the corresponding sub-area An,m of the magnetic medium 2. The array of laser emitters are addressable such that that system 1 can control (for example using controller 4) the relative intensity of each pixel Pn,m of the laser light by controlling an emission intensity of the corresponding laser emitter 21.

[0173] In the illustrated example, the array of laser emitters 21 is supported on a substrate 22 opposed to the magnetic medium 2 across a gap 23. The gap 23 may take the form of, for example, an air bearing or a larger air gap. The substrate 22 (and hence the writing head 3) may be movable relative to the magnetic medium 2 to allow writing across a larger total area. The substrate 22 also supports the circuitry for addressing each laser emitter 21.

[0174] Although illustrated in Figure 4 as illuminating the magnetic medium 2 from above the substrate 14, in other examples the laser emitters 21 may illuminate the optical structure 8 and thin film magnet structure 6 through a rear surface 24 of the substrate 14.

[0175] In other examples, the array laser emitters 21 may be fixed relative to the magnetic medium. In such examples, the gap 23 may remain an air gap, but equally could befilled with a substantially transparent material (i.e. 50% or greater at the operating wavelength).

[0176] In still other examples (not shown), the array of laser emitters 21 may be integrated into the magnetic medium 2. For example, the laser emitters 21 may be supported on or integrated into the substrate 14, and the one or more optical structures 8 may be disposed between the laser emitters 21 and the thin-film magnet structure 6. In such examples, the system 1 will not include a separate write head 3, since the capability to provide and pattern the laser light 10 will be integrated directly into the magnetic medium 2 itself.

[0177] Each laser emitter 21 may take the form of, for example, a surface emitting laser, a vertical-cavity surface emitting laser, a quantum dot laser and so forth.

[0178] Experimental results for massively parallel all-optical writing.

[0179] Referring also to Figure 5, a schematic diagram of the experimental equipment used to obtain the results described hereinafter is shown.

[0180] The optical path 25 included, in order pulsed laser source 26, neutral density filter 27, mirror 28, diverging lens 29, converging lens 30, a patterner 19 in the form of DMD 31, converging lens 32, dichroic mirror 33, beam splitter 34, microscope objective lens 35 and magnetic medium 2.

[0181] Imaging of the magnetic medium was provided using light source 36 coupled in via beam splitter 34. Light source 36 used a different wavelength to the operating wavelength Ao. Dichroic mirror 33 reflected the operating wavelength Ao, whilst allowing the imaging light from light source 36 to pass to camera 38 via converging lends 37. In the experimental equipment illustrated in Figure 5, the camera 38 arrangement was used for alignment of the target area Atarg. Manual alignment using the camera 38 is convenient for experimental work, but is not required. In other systems, positional sensors and / or similar automated measures may be used instead. For example, when the magnetic medium is formed as a disc, existing systems already used to determine the relative position of a hard disc read / write head may be employed.

[0182] The results described hereinafter relate to a scheme using 400 ps laser pulses supplying ~3.63 pJ per nanomagnet 16 (210 nm x 80 nm nanomagnets), in combination with a DMD 31 to address a target area Atarg of 150 um X 150 um. Thistarget area Atarg would be able to contain 1.4 million 210 x 80 nm nanomagnets 16, or a larger number of smaller nanomagnets 16. The magnetic medium 2 could be moved relative to the laser illumination to allow shifting the target area Atarg. Further details of the magnetic medium 2 and optical system used are provided hereinafter.

[0183] Fabrication of the magnetic media

[0184] Magnetic media 2 were fabricated on silicon (Si) chips with a 300 nm buffer layer of SiO2. The substrate 14 was overlaid to form the optical structure 8 with a sputtered metallic backplane at a base pressure of 10'6mbar, comprising either Au (250 nm) reflective layer 11 and SiC>2 (300 nm) dielectric layer 12, or Al (250 nm) reflective layer 11 and SiC>2 (90 nm) dielectric layer 12. These thicknesses were optimised through finite element simulations to maximise light absorption in the thin film magnet structure 6.

[0185] Electron beam lithography (EBL) was performed on a bilayer resist composed of PMMA 495K and 950K to define the thin film magnet structure 6. Following the lithography step, 20 nm of permalloy (Py) was thermally evaporated at a base pressure of 10'6mbar through the patterned mask and subsequently capped with a 4 nm layer of AI2O3. Lift-off was carried out to remove excess material, yielding magnetic media patterns with feature sizes as small as 210 nm x 80 nm. The patterned surfaces were then plasma ashed using O2 to eliminate resist residues and ensure clean interfaces.

[0186] The thin film magnet structure 6 was patterned with the 210 nm x 80 nm nanomagnets 16 arranged in a square lattice having a motif of one nanomagnet oriented along a first direction and a second oriented perpendicular to form an artificial spin-ice structure. The structure and arrangement of nanomagnets 16 was similar to that shown in Figure 8 of WO 2023 / 0099887 Al and described in the corresponding portion of the description.

[0187] Optical Setup and all-optical magnetic data writing

[0188] All-optical magnetic data writing was conducted using the optical microscopy setup schematically illustrated in Figure 5. The pulsed laser 26 was a single-shot pulsed laser (Powerchip, PNG-M02010-lx0, Teem Photonics) with an operating wavelength of Ao = 632 nm, pulse width of 400 ps, and maximum single-pulse energy of 20 pJ (the minimum applied energy was ~luJ after the neutral density filtering 27). The laser pulse repetition rate was tunable up to 1 kHz.The laser pulse energy was modulated using a motorised, continuously variable neutral density (ND) filter 27. The beam was then expanded using a telescope comprising a diverging 29 and a converging lens 30, ensuring uniform Gaussian intensity distribution across the beam. The expanded beam was directed onto a digital micromirror device 31 (DMD; AJD-4500 Standalone DMD Controller), consisting of 912 x 1140 programmable mirror pixels - each corresponding to a pixel Pn,m of the spatially structured laser light 10. The DMD 31 shaped the incident beam into pixelated "black and white" images corresponding to the desired patterns for writing.

[0189] The spatially structured laser light 10 was focused onto the sample using a 20x objective lens 35, producing an illumination area of approximately 150 pm x 150 pm on the magnetic medium 2. Magnetic patterns were written by setting the DMD 31 mirrors to produce spatial structuring corresponding to the binary images, and exposing the target area Atarg to a single laser pulse. The process was repeated as necessary, with higher laser repetition rates and DMD refresh rates (up to 6,500 fps) allowing for efficient patterning.

[0190] Referring also to Figures 6A, 7A and 8A, three examples of binary images used for spatial structuring of the laser light 10 are shown. Referring also to Figures 6B, 7B and 8B, magnetic force microscopy (MFM) images are shown corresponding to the target area Atarg after all-optical magnetic writing using the patterns shown respectively in Figures 6A, 7A and 8A. Scale bars in Figures 6B, 7B and 8B all correspond to 5 pm.

[0191] Referring in particular to Figures 6A and 6B, the "I" pattern exemplifies the present method's versatility in creating intricate patterns with varying geometrical complexities. The ability to reproduce sharp corners and areas with different thicknesses highlights the method's adaptability to designs, patterns and encoding schemes which require fine detail. This result indicates the system's potential for producing complex and heterogeneous magnetic patterns, which may be important in applications requiring high spatial resolution and / or dense data storage.

[0192] Referring in particular to Figures 7A and 7B, the diamond-shaped pattern illustrates the presently disclosed method's ability to reproduce sharp-edged, geometrically defined structures. The observed clarity of the diamond's boundaries and fidelity to the intended design demonstrate a robust capacity to target and modify larger areas with precision, only limited by the size of the individual nanomagnet 16 size and / ordiffraction limit of light. This capability may be important for scaling the presently disclosed method to applications requiring larger, uniform features.

[0193] Referring in particular to Figures 8A and 8B, the spiral pattern demonstrates the ability to imprint curved and continuous structures within magnetic media 2. The pattern begins and ends within the magnetic medium, highlighting precise control over the spatial boundaries of the written features. Achieving such intricate patterns without the use of spatially structured laser light 10 would be exceptionally challenging. This result underscores the potential of this method for replicating real-life images, precision data encoding schemes and so forth, as it effectively handles curved and non-linear geometries.

[0194] From Figures 6A to 8B, the presently disclosed system's capability to generate a diverse range of curved, geometric, and intricate patterns may be observed, thereby demonstrating its potential for broad applications for writing to magnetic media 2 and applications thereof.

[0195] Experimental results for 100 ps pulses

[0196] Further experiments were conducted using 100 ps pulses instead of 400 ps, and data is presented in Figures 9, 10 and 11A to 11C.

[0197] The 100 ps data was obtained using an optical path 25 similar to that illustrated in Figure 5 in relation to the 400 ps data, except that previously used laser was replaced by a different pulsed laser 26 (Picophotonics CP32 laser) operating at a wavelength of Ao =532 nm with a pulse duration of 100 ps, operated in single pulse mode. The maximum average power of the pulsed laser, when pulses are repeated at the maximum rate of 200 KHz, was 200mW. The pulsed laser 26 used for 100 ps pulse experiments incorporated integrated power control, allowing the neutral density filter 27 to be omitted from the optical path 25. Optical writing was demonstrated over an irradiance range of approximately 5.83 to 13.1 W.pm-2per pulse. Good switching was observed for single pulses with energies between 1.46 to 3.27 pj.

[0198] The results described hereinafter relate to a scheme using 100 ps laser pulses supplying ~38 pJ per nanomagnet 16, with 470 nm x 120 nm nanomagnets 16, in combination with a DMD 31 to address a target area Atarg of 50 um X 50 um.

[0199] The nanomagnetic arrays used in the new experiments differed from those used to obtain the 400 ps data (Figures 6A to 8B) in their lateral dimensions of nanomagnets16, which were 470 nm x 120 nm, and in that nanomagnets 16 were arranged in a rectangular array instead of square (see for example Figure 9). The material composition, thickness, fabrication methods and backplane were unchanged.

[0200] Referring also to Figure 9, a MFM image is shown of a nanomagnetic array with 470 nm x 120 nm nanomagnets in a saturated magnetic state. The scalebar is 5 pm.

[0201] Figure 9 shows the nanomagnetic array following preparation in a uniformly magnetised reference state. All nanomagnets 16 are observed to exhibit a consistent magnetic contrast corresponding to saturation along a common direction of 45°. Figure 9 may provide a baseline reference against which subsequent optically induced magnetic state changes can be compared.

[0202] Referring also to Figure 10, a MFM image is shown of the same nanomagnetic array shown in Figure 9 following application of a single 100 ps laser pulse as described hereinbefore. The scalebar is 5 pm.

[0203] It may be observed that following irradiation with a single 100 ps laser pulse, the array of nanomagnets 16 exhibits a spatially heterogeneous magnetic contrast, indicating that individual nanomagnets 16 have transitioned into distinct, stable magnetic configurations. Compared to the saturated reference state (Figure 9), multiple discrete magnetic states may be observed simultaneously across the array.

[0204] Referring also to Figures 11A to 11C, representative magnetic configurations accessible within individual nanomagnets 16 of the array shown in Figures 9 and 10 are illustrated. Figures 11A to 11C represent details of regions extracted from the MFM image of Figure 10.

[0205] Referring in particular to Figure 11A, a macrospin magnetic state having a two-fold degeneracy is shown. Figure 11B shows a single-vortex magnetic state, also possessing a two-fold degeneracy. Figure 11C shows a double-vortex magnetic state having a four-fold degeneracy.

[0206] These states represent examples of distinct, stable magnetic configurations that may be accessed within a single nanomagnet 16 following optical excitation. The MFM images presented in Figures llA to 11C illustrate that multiple internal magnetic textures, each associated with a different degeneracy, can be realised within the same physical structure of a 470 nm x 120 nm nanomagnet.The following examples of application areas for the presently described systems 1 and methods are intended as examples, and are not intended to provide an exhaustive list.

[0207] Neuromorphic Computation Hardware

[0208] The semiconductor industry has widely acknowledged the end of Moore's Law, shifting focus towards specialised chips optimised for specific tasks like machine learning and neuromorphic computing.

[0209] These emerging architectures emphasise massively parallel processing and interconnected element. This approach aligns well with nanomagnetic arrays, which inherently exhibit high degrees of coupling and interaction between processing elements without additional energy consumption or complex engineering. The new magneto-optical writing technology described herein has high relevance for neuromorphic computing technologies.

[0210] However, writing information into these strongly-coupled arrays previously presented a significant challenge. Traditional magnetic switching techniques using electromagnets are inadequate due to their inability to confine magnetic fields spatially, leading to unintended switching of multiple elements.

[0211] All-optical magnetic switching offers a solution to this problem, but has been limited by the need for expensive, high-powered lasers and rare, toxic materials. The methods described herein, in combination with the magnetic media 2 including optical structures 8, can address these issues, enabling quick, cost-effective, and accurate writing of magnetic information into arrays ideally suited for neuromorphic computation hardware.

[0212] Quantum Computing and Quantum Simulation

[0213] Magnetic media 8 in the form of nanomagnetic arrays could serve as platforms for quantum computing or quantum simulation, with patterns of magnetisation encoding qubits or representing complex quantum systems. The precise control over nanoscale features and magnetic properties could enable robust and scalable quantum devices.

[0214] Spintronics and Spin-Wave Devices

[0215] Magnetic media 8 could be used in spintronics, where information is carried by spin waves (magnons) rather than electrons. Structured patterns could be configured to control the propagation and interference of spin waves for logic and signal processing.Anti-Counterfeiting and Security Features

[0216] Nanomagnetic patterns written to almost invisible small magnetic media 2 could be used for secure identification and anti-counterfeiting, such as magnetic barcodes or watermarks. The ability to create intricate, high-resolution, and difficult-to-replicate patterns makes this a potential candidate for security applications.

[0217] Biomedical Applications

[0218] Nanomagnetic patterns written to magnetic media 2 could be employed for targeted drug delivery, magnetic hyperthermia, or biosensing. The high resolution and customisation of patterns make them suitable for addressing specific biomedical challenges.

[0219] Random Number Generator

[0220] Through control of the characteristics of nanomagnet 16 of a magnetic medium 2, characteristics— such as shape anisotropy, thermal stability, and coercivity — and carefully tuned laser energy, the system 1 could be configured exploit the stochastic nature of magnetisation switching.

[0221] When a laser pulse is applied at a specific energy near the switching threshold, the outcome of whether a nanomagnet flips or remains in its initial state can be influenced by thermal fluctuations, resulting in inherently random magnetic states.

[0222] This represents a configuration differing from the configurations useful for magnetic information storage, which would instead prioritise switching fidelity and / or operate sufficiently above the switching threshold to be insensitive to thermal fluctuations.

[0223] Modifications

[0224] It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design and use of systems for writing to magnetic media, and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment.

[0225] The system 1 has been described in relation to magnetic media 2 including optical structures 8 to enhance absorbance. This combination is particularly preferred due to the surprising finding that the switching could be accomplished across a large area(compared to the ferromagnetic domains) in parallel, at low irradiances. However, the spatial patterning of laser light 10 described herein may equally be applied to other types of media which may undergo all-optical magnetic switching systems. Such all-optical magnetic switching media 2 as the inventors are aware of, prior to

[0226] WO 2023 / 0099887 Al, would require irradiances several orders of magnitude higher, and the scalability of the target area Atar may need to be balanced against the need to avoid damage to a patterner 19 and / or excessive heating of the magnetic medium 2.

[0227] Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and / or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

1. Claims1. A system comprising:a magnetic medium comprising:a thin film magnet structure comprising one or more ferromagnetic domains, each ferromagnetic domain having a magnetization that is switchable between two or more states;one or more optical structures, each optical structure having an interface with the thin film magnet structure and configured such that light of an operating wavelength incident on the magnetic medium interferes destructively at the interface;wherein the system is configured:to write states to the ferromagnetic domains within a target area of the magnetic medium by illuminating the target area of the magnetic medium using laser light spatially structured into a plurality of pixels, each pixel illuminating a corresponding sub-area of the magnetic medium;wherein all-optical magnetic switching of ferromagnetic domains within each sub-area of the magnetic medium is controlled by the relative intensity and / or polarisation of the corresponding pixel of the laser light.

2. The system of claim 1, wherein the system is configured to illuminate the target area of the magnetic medium using spatially structured laser light in the form of a pulse.

3. The system of claim 2, configured such that a peak irradiance of the pulse is less than or equal to 100 W.pm-2.

4. The system of claims 2 or 3, wherein the system is configured such that a pulse width of the pulse is greater than or equal to 1 ps.

5. The system of claim 1, wherein the system is configured to illuminate the target area of the magnetic medium using continuous wave, CW, laser light.

6. The system of any one of claims 1 to 5, configured such that each pixel corresponds to a sub-area including a single ferromagnetic domain.

7. The system of any one of claims 1 to 5, configured such that each pixel corresponds to a sub-area including two or more ferromagnetic domains.

8. The system of any one of claims 1 to 7, wherein the thin film magnet structure comprises a plurality of discrete nanomagnets formed from the ferromagnetic alloy or compound.

9. The system of claim 8, wherein the plurality of nanomagnets are arranged in an array.

10. The system of any one of claims 1 to 9, wherein the thin film magnet structure comprises a mesh formed from the ferromagnetic alloy or compound.

11. The system of any one of claims 1 to 10, wherein the thin film magnet structure comprises one or more nanowires formed from the ferromagnetic alloy or compound.

12. The system of any one of claims 1 to 11, configured to spatially structure the laser light as an image.

13. The system of any one of claims 1 to 12, configured to encode a bit using the states written to the ferromagnetic domains by a single pixel.

14. The system of any one of claims 1 to 12, configured to encode a bit using the states written to the ferromagnetic domains by a group of pixels.

15. The system of any one of claims 1 to 14, wherein the one or more ferromagnetic domains comprise:a plurality of first ferromagnetic domains configured to undergo all-optical magnetic switching in response to a first polarisation state of the laser light; and a plurality of second ferromagnetic domains configured to undergo all-optical magnetic switching in response to a second, different, polarisation state of the laser light.

16. The system of any one of claims 1 to 15, configured such that the target area is smaller than a total area of the thin film magnet structure;wherein the system is configured to move the target area relative to the magnetic medium.

17. The system of any one of claims 1 to 15, configured such that the target area is substantially equal to a total area of the thin film magnet structure.

18. The system of any one of claims 1 to 16, configured to write states to the ferromagnetic domains within two or more target areas of the magnetic medium concurrently by illuminating each target area of the magnetic medium using laser light spatially structured into a plurality of pixels, each pixel illuminating a corresponding sub-area of the magnetic medium.

19. The system of any one of claims 1 to 18, configured for readout of the states of ferromagnetic domains of the magnetic medium.

20. The system of any one of claims 1 to 19, comprising:a laser source configured to provide input laser light at the operating wavelength; anda patterner configured to receive the input laser light and to output the laser light spatially structured into the plurality of pixels;wherein the patterner is controllable by the system to set the relative intensity and / or polarisation of each pixel of the laser light.

21. The system of claim 20, wherein the patterner comprises a digital micromirror device.

22. The system of any one of claims 1 to 19, comprising a plurality of laser emitters disposed in an array and arranged such that each laser emitter provides the pixel illuminating a corresponding sub-area of the magnetic medium;wherein the plurality of laser emitters are addressable such that that system controls the relative intensity of each pixel of the laser light by controlling an emission intensity of the corresponding laser emitter.

23. A data storage device comprising the system of any one of claims 1 to 22.

24. A neuromorphic computing device comprising the system of any one of claims 1 to 23.

25. A method of writing data to a magnetic medium comprising:a thin film magnet structure comprising one or more ferromagnetic domains, each ferromagnetic domain having a magnetization that is switchable between two or more states;one or more optical structures, each optical structure having an interface with the thin film magnet structure and configured such that light of an operating wavelength incident on the magnetic medium interferes destructively at the interface;the method comprising:writing states to the ferromagnetic domains within a target area of the magnetic medium by illuminating the target area of the magnetic medium using laser light spatially structured into a plurality of pixels, each pixel illuminating a corresponding sub-area of the magnetic medium;wherein all-optical magnetic switching of ferromagnetic domains within each sub-area of the magnetic medium is controlled by the relative intensity and / or polarisation of the corresponding pixel of the laser light.

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