Method of moving magnetic skyrmions in a hosting material

Abstract

A hosting material 20 may be a stepped lamella of a chiral magnetic material (e.g. iron germanide, FeGe, CoZnMn, Cu2OSeO3). By application of a magnetic field to the hosting material, held at a predetermined temperature range (by heating or colling), skyrmions are formed in a thinner first part 22 of the hosting material but not induced in a second thicker part 24 of the hosting material (due to unfavourable skyrmion generating conditions). Passing an electric current pulse 26 through the hosting material, across contacts 32, may move skyrmions from the first part (existing as an equilibrium state) into the second part (persisting as a metastable state), were the skyrmions may persist at approaching room temperature. Reversing polarity of the electric current may move skyrmions from the thicker to thinner part. Application may be for data storage in low energy memory devices, racetrack memory or reservoir computing.

Description

The present disclosure relates to a method of moving magnetic skyrmions in a hosting material and relates particularly, but not exclusively, to a method of moving magnetic skyrmions into a region of a material in which such magnetic skyrmions would not normally exist under applied magnetic field and temperature conditions but can persist as a metastable state. Magnetic skyrmions are swirling vortex-like patterns of magnetisation, which exist in chiral magnetic materials. They are typically 5 to 100 nm in size and can be moved with ultra-low current densities (10s A / m2) , making them prime candidates for elements in next generation low energy memory devices . Referring to Figure 1, a magnetic Bloch skyrmion 10 is formed from magnetic spins (spin magnetic moments) 12 in a hosting material. Magnetic skyrmions 10 have unique topological properties. Note that the central spin 12a points in the opposite direction to the outer spins 12b. The topology of the spins is in some way analogous to a knot or a mobius strip. The magnetic skyrmion 10 can move independently through the crystal lattice of the hosting material with minimal interaction, and can be moved with ultra-low current density, which can therefore offer a huge energy saving in data storage devices. Referring to Figure 2, a general phase diagram for a magnetic Bloch skyrmion hosting material shows that the magnetic skyrmions are only stable over a small range of temperature and applied magnetic field. The magnetic skyrmions form in a small region of magnetic field and temperature below the Curie temperature (Tc) . This region is bounded by the conical phase, where the magnetic spins form cones. In bulk crystals, magnetic skyrmions typically form a hexagonal lattice in a small temperature and magnetic field region near the critical temperature of certain non-centrosymmetric crystals such as FeGe, MnSi and C^OSeCy. When bulk crystals of these materials are thinned down to lamella a couple of hundred nm thick, the stability region of the magnetic skyrmions dramatically increases in size. For example, in FeGe the skyrmion stability region changes from a 5 K range in bulk, to a 100 K range in a 120 nm thick sample. It is also known that for lamella, the extent of the skyrmion stability region can easily be tuned by adjusting the thickness of the lamella. Furthermore, outside of this equilibrium stability region, skyrmions can exist as a metastable state in FeGe and other skyrmion hosting materials. This metastable state is usually produced by field cooling through the equilibrium state, and, once formed, is long lived with lifetimes expected to exceed a week for temperatures more than 6 K below the equilibrium pocket. Or alternatively, for longer than 10 years if cooled to below 30 K below the equilibrium pocket. Such skyrmion metastability has been demonstrated both in bulk crystals, and in thin lamella of both Cu2OSeO3 and FeGe, where metastability has previously been exploited to bring skyrmions down 100 K below their equilibrium stability pocket. Skyrmion motion, driven by applied electric currents, has been demonstrated in thin lamella of FeGe with currents of less than 1 mA, which is a small enough current that resistive heating of the sample is minimal. This previously demonstrated skyrmion motion has been of the whole lattice of skyrmions moving together in a sample of constant thickness. For magnetic skyrmions to be effectively implemented in new device architectures, for example room temperature memory storage devices, it is necessary to be able to inject and remove magnetic skyrmions from hosting materials that can be used in memory devices and other applications. J. Zhu et al, "Current-driven transformations of a skyrmion tube and a bobber in stepped nanostructures of chiral magnets", Sci. China-Phys. Meeh. Astron. 64, 227511 (2021), https : / / doi . org / 10.1007 / sll.4 33-020-1619-8 reports micromagnetic simulations of skyrmions and chiral bobbers in a B20 material with parameters similar to that found in FeGe. Chiral bobbers are effectively half skyrmions and they have been experimentally reported as occurring occasionally in B20 alloys, particularly around defects and at surfaces. A simulated nanostripe with a stepped shape having thicknesses 75 nm and 150 nm is disclosed which finds that skyrmion tubes are transformed into bobbers when the skyrmion is driven, by an electric current, from a thick region into a thinner region and vice versa. This transformation is suggested to be caused by the sharp step edge causing deformed topological magnetic structures which are not equilibrium but high-energy states . A racetrack is simulated in which skyrmions and bobbers can be sent down and encoded as ones and zeroes. However, the simulations and predictions made in this disclosure do not appear to have been experimentally observed and only relate to converting single chiral bobbers into skyrmions and vice versa across a step. The step is used to provide a nucleation site for the chiral bobbers. This disclosure does not address the movement, or injection, of a large number of skyrmions from one area to another in unfavourable magnetic field and temperature conditions. Ran et al, "Creation of a Chiral Bobber Lattice in Helimagnet-Multilayer Heterostructures", PHYSICAL REVIEW LETTERS 126, 017204 (2021), describes experimental observation of chiral bobbers in a complex sample of Cu2OSeO3 / [Ta / CoFeB / MgO]4 hetrostructure in which a chiral bobber lattice is induced in the surface of Cu2OSeO3. JP2014175417A discloses a method of generating magnetic skyrmions, an erasing method and a magnetic element capable of generating and / or erasing skyrmions. It is disclosed that generating a skyrmion may be easier if the thin film sample contains a corner or notch feature, which is a well-known experimental observation. Often skyrmions once formed will be stabilised by existing close to a local interface or structural feature which modifies the local magnetic environment, to the benefit of skyrmion stability. This disclosure, whilst addressing the formation and erasing of skyrmions, does not address the movement or injection of a large number of skyrmions from one area to another in unfavourable magnetic field and temperature conditions. CN109949842A discloses track memory using skyrmions in a nanotube. The disclosed apparatus suffers from the drawback that application of an electric current in the direction of the track tends to cause skyrmions to bend off the track due to the skyrmion Hall effect and suggests the track be replaced by closed tubular structure. Extremely large current densities are used to drive single skyrmions, which are generated individually. This disclosure therefore does not address the movement or injection of a large number of skyrmions from one area to another in unfavourable magnetic field and temperature conditions. US2022 / 0366955 discloses the formation and use of skyrmions in centrosymmetric materials such as Fe3Sn2. It is noted that the magnetic objects formed in this material are magnetic bubbles and target skyrmions (skyrmonium). Graded thickness of material are used to move magnetic bubbles in a conventional racetrack geometry. This disclosure therefore does not address the movement or injection of a large number of skyrmions from one area to another in unfavourable magnetic field and temperature conditions. A preferred embodiment of the present disclosure seeks to overcome the above disadvantages of the prior art. According to an aspect of the present disclosure, there is provided a method of moving magnetic skyrmions in a hosting material, the method comprising: providing a hosting material having a first part having a first thickness a second part having a second thickness which is greater than said first thickness applying a magnetic field to and holding said hosting material in a predetermined temperature range to form magnetic skyrmions in said first part of said hosting material; passing an electric current through said hosting material to move said magnetic skyrmions to said second part of said hosting material, wherein the temperature of the hosting material and magnetic field applied to the hosting material are of values which cannot induce skyrmions into the thickness of said second part of the hosting material. This provides the advantage of a method of injecting magnetic skyrmions from a thin region of a hosting material where magnetic skyrmions exist as an equilibrium state, into a thicker region, where they can persist as a metastable state. This method can therefore be used for injecting or removing large numbers of magnetic skyrmions into a defined region of a magnetic skyrmion hosting material under applied field and temperature conditions unfavourable to skyrmion generation for further manipulation in devices such as a skyrmion racetrack memory or reservoir computing, which therefore paves the way for potential future skyrmion applications. The step of passing an electric current through said hosting material to move said magnetic skyrmions to said second part of said hosting material may comprise passing current pulses through the hosting material. The current density of said current pulses may be approximately 3 x 108 A / m2. In a preferred embodiment, the step of providing a hosting material having variable thickness comprises preparing a stepped lamella of a chiral magnetic material. The method may further comprise reversing the polarity of the electric current to move said magnetic skyrmions from said a second part of said hosting material to said a first part of said hosting material. Providing a hosting material having a first part having a first thickness a second part having a second thickness which is greater than said first thickness may comprise providing a stepped lamella of FeGe in which said first thickness is 300 nm and said second thickness is 500 nm; and wherein applying a magnetic field to and holding said hosting material in a predetermined temperature range to form magnetic skyrmions in said first part of said hosting material may comprise applying a magnetic field of 75 mT and holding said hosting material at a temperature of 255 K. Providing a hosting material having a first part having a first thickness a second part having a second thickness which is greater than said first thickness may comprise providing a stepped lamella of beta-Mn type CoZnMn in which said first thickness is 300 nm and said second thickness is 500 nm; and wherein applying a magnetic field to and holding said hosting material in a predetermined temperature range to form magnetic skyrmions in said first part of said hosting material may comprise applying a magnetic field of 80 mT and holding said hosting material at a temperature of 285 K. This provides the advantage of a method of injecting magnetic skyrmions from a thin region of a hosting material where magnetic skyrmions exist as an equilibrium state, into a thicker region, where they can persist as a metastable state at temperatures approaching room temperature. This method can therefore be used for injecting or removing large numbers of magnetic skyrmions into a defined region of a magnetic skyrmion hosting material under applied field and temperature conditions unfavourable to skyrmion generation for further manipulation in devices such as a skyrmion racetrack memory or reservoir computing, which therefore paves the way for potential future skyrmion applications. A preferred embodiment of the present disclosure will now be described, by way of example only, and not in any limitative sense, with reference to the accompanying drawings in which: Figure 1 is a schematic of a Bloch skyrmion; Figure 2 is a phase diagram for a skyrmion hosting material such as a chiral magnetic material; Figure 3 is a perspective view of a hosting material for moving magnetic skyrmions in accordance with a the present disclosure showing the hosting material in a state before and after an electric current has been applied to move magnetic skyrmions from a first part of the hosting material to a second part of the hosting material having a greater thickness than the first part; Figure 4 is a perspective view schematic of a hosting material of Figure 3 showing an additional step of reversing the current polarity; Figure 5 is a perspective view of the hosting material of Figure 3; Figure 6 is a perspective view of an experimental set up of a first experimental example embodying the present disclosure for detecting skyrmions in the hosting material in which an incident polarised resonant X-ray beam on a sample is scattered through the thick region of the sample, the scattering pattern being measured on a charge coupled display (CCD); Figure 7 is a schematic perspective view of a skyrmion racetrack device using the hosting material of Figure 3 to add or remove skyrmions which injection current density Jin and current flow JT to move skyrmions around the track; Figures 8a to 8f show example scattering patterns for the hosting material in the first experimental example embodying the present disclosure Figure 8g shows phase diagrams for both the thick and thin regions of the hosting material in the first experimental example embodying the present disclosure; Figures 9a to 9h show scattering patterns of skyrmion lattice rotation under the application of repeated current pulses in the first experimental example embodying the present disclosure; Figures 10 show scattering patterns and a schematic of movement of skyrmions from the thin to the thick part and back to the thin part of the hosting material as a result of positive and negative current pulses in the first experimental example embodying the present disclosure; Figure 11 shows scattering patterns of skyrmion movements between the thick and thin part of the hosting materials with varying current densities in the first experimental example embodying the present disclosure; Figure 12a shows integrated intensities before and after magnetic skyrmion injection in an FeGe sample at 255k; Figure 12b shows difference in intensities, comparable to the number of magnetic skyrmions injected in the sample of Figure 12b; Figure 12c shows threshold pulse width required for injection in the sample of Figure 12a; Figure 12d shows threshold current density required for injection in the sample of Figure 12a; Figure 13a is a schematic of a micromagnetic simulation of a hosting material of a second experimental example embodying the present disclosure before a current pulse is applied; Figure 13b is a schematic of a micromagnetic simulation of the hosting material of Figure 12a a second experimental example embodying the present disclosure after a current pulse is applied; Figure 14a is an image taken using X-ray holography showing an experimental hosting material of said second experimental example embodying the present disclosure before a current pulse is applied; and Figure 14b is an image taken using X-ray holography showing the experimental hosting material of said second experimental example embodying the present disclosure after a current pulse is applied. Referring to Figures 3 and 5, a hosting material 20 comprises a first part 22 having a first thickness for formation of magnetic skyrmions. This may be accomplished by application of a magnetic field in a predetermined temperature range to the first part 22 of hosting material 20. Hosting material 20 also has a second part 24 of a thickness greater than the first part. The hosting material may be formed from Cu2OSe03, iron germanide (FeGe), CoZnMn or other chiral magnetic materials . It has been found that by first heating or cooling the first part of the hosting material 22 in magnetic field conditions to form magnetic skyrmions 28, the application of electric current 26 across contacts 32 is able to move magnetic skyrmions from the thin part 22 of the hosting material 20 where they exist as an equilibrium state into the second part 24 where they can only persist as a metastable state, in other words where the temperature of the hosting material and magnetic field applied to the hosting material are of values which cannot induce skyrmions into the thickness of the second part of the hosting material Furthermore, referring to Figure 4, it has been found that applying a negative current pulse 30 across contacts 32 moves the magnetic skyrmions 28 back out of the second part 24 of the hosting material into the first part 22. Referring to Figure 7, an application of hosting material 20 is magnetic skyrmion race-track device 40 having a first thickness injector step 22a to add or remove magnetic skyrmions with injection current density J^i and current flow J^T to move magnetic skyrmions around the thicker track 24a across read element 42. First Experimental Example Magnetic skyrmions have attracted significant research interest over the past decade, both for their intrinsic emergent physical properties, and for their potential as information carriers in novel computing devices. FeGe is one example of a material that hosts skyrmions, and is particularly useful as a test case for skyrmionic devices as the skyrmions are found near room temperature. For any skyrmionic device to prove useful, a reliable method of injecting and removing magnetic skyrmions from a defined region of the material is needed. A demonstration of such a skyrmion injector device utilizing electric currents applied across a thickness step in FeGe lamella is disclosed here. Measurements show that in certain temperature and field conditions, skyrmions can be reversibly injected from a thin region of an FeGe lamella where they exist as an eguilibrium state, into a thicker region, where they can only persist as a metastable state. This injection is achieved with a current density of 3 * 108 A / m2 which is nearly three orders of magnitude lower than the currents required to move magnetic domain walls in existing memory devices, and hence viable for low energy electronic applications. This injector could be used for injecting or removing small numbers of skyrmions into a defined region of a skyrmion hosting material for further manipulation in devices such as a skyrmion racetrack memory or reservoir computing, and therefore paves the way for potential future skyrmion applications . Resonant small angle X-ray scattering (SAXS) has been used to demonstrate a method of and apparatus for moving magnetic skyrmions into a hosting material 20, also referred to in the experimental results as a skyrmion injector. Hosting material 20 in the disclosed experiment comprises a lamella of FeGe where one half 22 is 300 nm thick, and the other half 24 is 500 nm thick. This results in certain regions of temperature-magnetic field phase space where skyrmions exist as an equilibrium state in the thin region, but only as a metastable state in the thick region. By measuring the X-ray scattering patterns of skyrmions in the thick region of the sample, the ability to reversibly move skyrmions into and out of this region with a current density of 3 * 108 A / m2 is demonstrated. A thin lamella of FeGe was lifted out of a single crystal using a focussed ion beam instrument, oriented with the

[111] direction out of plane,

[112] and

[110] along the two perpendicular edges of the lamella. This lamella was thinned down to a stepped sample with one region 300 nm thick and one 500 nm thick, and placed on a substrate with a 4.5 pm aperture under the thicker region, and attached to electrodes to allow current injection along the

[112] direction. Small angle X-ray scattering (SAXS) measurements were then performed using circularly polarized X-rays at the Fe L3 absorption edge. For the measurements in Figures 8 and 11 an energy of 705.8 eV was used, while for Figures 9 and 10 conducted in a second experimental session several months later on the same sample, an energy of 704.6 eV was used. Referring to Figure 6, the experimental setup comprises an incident X-ray beam 50, scattering through the thick region 24 of the sample, and the scattering pattern being measured on the charge coupled display (CCD) 54. Referring to Figure 8, phase diagrams of the lamellae 20 measured by zero field cooling are shown. Figures 8a to 8c show example scattering patterns for the stepped sample at positions shown by the dots 60, 62 and 64: - Dot 60 is 255 K for the helical state at 0 mT (Fig 8a) - Dot 62 is an intermediate (likely conical) state at 70 mT (Fig 8b) - Dot 64 is a skyrmion state at 120 mT (Horizontal peaks hidden under beam stop) (Fig 8c) Figures 8d to 8f show scattering patterns measured at 250 K, 70 mT at star 66 for Figure 8d, the 300 nm sample 22 after Zero Field cooling (ZFC). Figure 8e shows the 500 nm sample after ZFC, and Figure 8f the 500 nm sample after field cooling (FC) in 70 mT from 290 K. Figure 8d shows the phase diagrams for both the thick and thin regions with measurements at 705.8 eV. The colour bar 68 refers to the intensity of the measured X-ray scattering, and is shared for all plots provided. The phase diagrams for both 300 nm thin 22 and 500 nm thick 24 FeGe lamellae in Figure 8 were used to determine an appropriate temperature and magnetic field region for magnetic skyrmion injection. These were measured using SAXS, with the 300 nm measurements using a second piece of the lamella that was placed without electrical contacts, and the 500 nm measurements using the stepped sample (where the aperture is such that only the 500 nm region is sampled). Both phase diagrams were measured by zero field cooling from 290 K to the desired temperature, and then measuring scattering patterns as a function of increasing field. Example patterns measured in the thick region are shown in Figure 8a for multi-domain helices (two-fold scattering pattern), Figure 8b for no measured scattering (either conical or field polarized), and Figure 8c for skyrmion states (six-fold scattering), with the scattering angle corresponding to a skyrmion diameter of 72 nm. Figure 8c therefore demonstrates that the thin sample (first part) 22 of hosting material transitions directly from a helical state to a magnetic skyrmion state as a function of field, while the thick sample (second part) 24 passes through an intermediate, presumably conical, state where no magnetic scattering was observed in the imaging plane perpendicular to the magnetic field. Further, for a region of temperature between 250 K and 260 K and magnetic fields from about 70 mT to 90 mT, magnetic skyrmions are found as an equilibrium state in the thin region 22 of the sample 20, but not the thick region 24. A measurement of the 500 nm sample at 250 K, 70 mT, after field cooling from 270 K in 70 mT, was also performed and obtained the resulting scattering pattern in Figure 8f. This shows a clear six-fold scattering pattern indicating a magnetic skyrmion lattice state, with the leftmost peak partially blocked by the beam spot, and the rightmost peak shadowed by the magnet. This demonstrates that in this region magnetic skyrmions can exist in the 500 nm part 24 as a metastable state after field cooling, but not as an equilibrium state (see Fig. 8e), whereas magnetic skyrmions do exist as an equilibrium state in the 300 nm part 22 (see Fig. 8d) . Referring to Figure 9, magnetic skyrmion lattice rotations under the application of repeated current pulses are shown measured at 260 K, 75 mT with 10 ms 5 x 107 A / m2 pulses. Figures 9a to 9d show the behaviour with positive current, with sample images after 1, 5, 10, and 15 pulses. Figures 9e to 9h show the behaviour with negative current, sample images after 1, 15, 30, and 45 pulses. Figure 9i shows the skyrmion lattice orientation as a function of pulse number for negative 72 and positive 74 pulses, measured with respect to the beam stop. Measurements were taken at 704.6 eV X-ray energy. Measurements of the magnetic skyrmion state under an applied current were therefore investigated. First, the behaviour of the equilibrium skyrmion state was investigated in hosting material 20 under applied currents by ZFC to 260 K and applying a 75 mT magnetic field. Referring to Figure 9, the evolution of the skyrmion lattice is shown after repeated 10 ms 5 x 107 A / m2 current pulses of both positive polarity (from thin region 22 towards the thick region 24) and negative polarity. Figure 3 demonstrates continuous rotation of the skyrmion lattice when current is applied, with rotation of the scattering pattern proceeding clockwise for negative current (Figures 8e to 8h), and anticlockwise (with lower rotation rate) for positive currents (Figures 8a to 8d). It can be seen in the negative current measurements that the rate of rotation varies significantly as the angle changes, with minimum rotation speeds 60 degrees apart, indicated by the dashed lines 70 in Figure 9i corresponding to where the scattering spots are parallel to the real (110) crystal axis. This indicates a significant preferred direction for the magnetic skyrmions. Such continuous rotation of the magnetic skyrmions, and reversible direction of rotation by flipping polarity, demonstrates that we are seeing current-induced motion of the magnetic skyrmions, not ohmic heating effects. When the same measurements are performed with high pulse amplitudes, the rotation rate dramatically increases, becoming difficult to distinguish from random re-orientation when the rotation per pulse exceeds 60 degrees. Referring to Figure 10 which demonstrates skyrmion injection, sequential 100 ms current pulses of ±3 * 108 A / m2 were applied to hosting material 20 alternating between negative and positive polarity. The Figures show measurements after each pulse, starting with a negative pulse 30a before Figure 10a, and measuring after each successive pulse. All images in the top row are measured after a negative pulse (indicated by pulse 30 preceding the images, and all images in the bottom row after the sequential positive pulse 26. The entire image represents one continuous pulse sequence. This demonstrates magnetic skyrmions appearing when a positive current pulse is applied, and vanishing when a negative current pulse is applied. Figure 9 is a schematic of the injection process resulting in a partial-volume magnetic skyrmion state with some disorder. Measurements were taken at 704.6 eV X-ray energy. To demonstrate magnetic skyrmion injection, the sample of hosting material 20 was zero field cooled from 290 K to 255 K, set the magnetic field to 75 mT (where no magnetic skyrmions are initially present in the thick region 24), and then applied single 100 ms square pulses of ±3><108 A / m2 to the sample 20, where the positive direction is defined as being from the thin region 22 into the thick region 24. Starting with the reversed with a negative pulse, repeated pulses were applied magnetic scattering measured and the polarity between each successive pulse. The results of this are shown in Figures 10a to lOp. Figure 10a shows that after an initial negative pulse is applied, no scattering However, once a positive pulse is applied, magnetic appear in the thick part 24 (Figure 10b), which are appears . skyrmions then removed by the application of a further negative pulse at Figure 10c. Figures lOd of magnetic Further repetitions of this pulse sequence in to lOp show reproducible injection and ejection skyrmions from the thick part 24 of hosting material 20. Figure 10 therefore demonstrates that the thickness-step skyrmion injector concept functions. We see controlled injection of skyrmions into the thick region of the sample, which can then be easily ejected by reversing the polarity of the current. We can also see that this process is repeatable after multiple pulse sequences. Furthermore, this figure demonstrates that the production of magnetic skyrmions is not merely a heating effect, as the heating produced by pulses of opposite polarity in this regime would yield the same effect. Hence if magnetic skyrmions were only showing up because of the sample being heated into the equilibrium state by the current pulse and then FC back down, magnetic skyrmions should have appeared with the negative pulse just as they appeared with a positive pulse. That this did not happen, combined with the ability to reproducibly inject and eject magnetic skyrmions by flipping the polarity, shows that this is indeed the current induced effect that is demonstrated. Examining the scattering patterns in Figure 10 in more detail, some specific features of the injected skyrmion state can be inferred. First, by comparison with the metastable skyrmions shown in Figure 8f, the peaks are more diffuse and slightly less intense. This indicates that some disorder is present in the skyrmion lattice after injection, and likely that the injected skyrmions hence do not completely fill the area of the aperture. Hence, we can conclude that the skyrmion motion between the thin 22 and thick 24 regions does not proceed as one continuous unit, but rather as smaller rafts of skyrmions lattice being translated independently (as per the schematic shown in Figure lOo, resulting in the disordered state. The final image Figure lOp particularly demonstrates this, where regions of skyrmion lattice orientated 30 degrees to each other clearly coexist in the sample after injection. Finally, it can be noted that the orientation of the magnetic skyrmions is not completely consistent after each injection pulse. This feature likely arises from the rotation observed in Figure 10. Differing skyrmion angles would arise from the current pulses both injecting skyrmions and rotating the skyrmion lattice that exists in the sample, hence resulting in variable skyrmion orientation after each current pulse. To determine the effect of varying pulse amplitude on skyrmion injection, a state with no skyrmions in the thick region 24 was prepared by zero field cooling to 255 K and setting the magnetic field to 75 mT. A series of positive pulses were then applied with increasing pulse amplitude. The results of this are shown in Figure 11 where ten 1-second-long current pulses were applied at each given pulse amplitude before measurement. This was done for current densities of: Figure Ila: 1 X 107 A / m2 Figure lib: 2 x 107 A / m2 Figure 11c: 4 x 107 A / m2 Figure lid: 8 x 107 A / m2 Figure lie: 1.6 X 1 08 A / m2 Figure Ilf: 2 x 108 A / m2 Figure 11g: 2.5 X 1 08 A / Figure llh: 3 x 108 A / m2 Measurements were taken at 705.8 eV X-ray energy These results demonstrate that no injected skyrmions appear for current densities between 1 x 107 A / m2 and 2.5 x 108 A / m2, they only appear in the last frame for a pulse amplitude of 3 x 108 A / m2, indicating that the critical current density required to inject skyrmions into this sample is between 2.5 x 108 A / m2 and 3 x 108 A / m2. This is about 2 orders of magnitude larger than the current density of 106 A / m2 required to move skyrmions on their own suggesting that there may be some significant energy barrier for the skyrmion state to overcome when being pushed over the thickness step. However, it can also be noted that this critical current is still nearly 3 orders of magnitude lower than the 1011 A / m2 needed to drive magnetic domain walls, which means that such a skyrmion injector element could still find productive use in low energy device applications. These results relate to the collective injection of a region of numerous skyrmions at once. Within a 4.5 pm imaging aperture there would be approximately 1000 skyrmions, and hence it is shown that several hundred skyrmions are being injected / ejected at once. This sort of operation may be useful for applications such as skyrmion reservoir computing or reshuffler devices that deal with collective skyrmion motion . In conclusion, these results demonstrated the ability to inject and eject metastable skyrmions across a thickness step in a 300 nm / 500 nm thick FeGe lamella. This injection is achieved by a current density of 3 x 108 A / m2, which is viable for applications. It is further demonstrated that such injection is accompanied by current induced magnetic skyrmion rotation, whose direction depends on the polarity of current applied to the sample. Some further characterisation of device properties in FeGe at 255k are shown in Figure 12a to Figure 12d obtained from a resonant soft x-ray small angle diffraction measurements. Referring to Figures 12a and 12b, by integrating the intensity of the x-ray diffraction peaks, details can be inferred about the number of magnetic skyrmions injected into the thicker region 24. Intensities before and after an injection pulse as a function of magnetic field are shown. The differences between these two numbers gives an indication of the number of magnetic skyrmions injected and indicates that there is an optimal region in which a significant number of skyrmions 28 can be transported across the barrier before further manipulation. Referring to Figures 12c and 12d, critical current densities and threshold pulse widths required to observe this mechanism are derived. Pulse widths on the order of 0.5 ms were shown to be the threshold for magnetic skyrmion injection at a constant current density. However, increasing this current density is expected to reduce the operating time down to something more comparable to current electronic devices The critical current density is found to be of the order of 108 A m-2, two orders of magnitude greater than that shown for skyrmion motion. This is indicative that there is an energy barrier to overcome the pinning effects from the thickness barrier itself. In addition, this provides many operational benefits since large current pulses can be applied to inject skyrmions and then manipulate the injected skyrmions with smaller current pulses, before re-initialising the state with a larger current pulse. Second Experimental Example The concept of skyrmion injection focuses on the ability to move skyrmions from one region of a device, across some form of barrier, into another region in a controllable manner. Within bulk single crystal skyrmion materials, skyrmions are only stable within a 1-2 K, and ~20 mT range in temperature and magnetic field respectively just below the magnetic transition temperature, which is often too cool to be utilised within modern computing devices. However, one material series, the beta-Mn type CoZnMn series, has a tuneable transition temperature dependent on the exact composition, which is often much higher than room temperature. In addition, as the sample thickness is reduced onto a scale of ~100 nm, the region of stability dramatically increases. This has been found to enable fabrication of a device capable of hosting skyrmions at room temperature, removing any requirements for cooling with anything more than cold water. Referring to Figures 13a and 13b, it has also been found that it is possible to fabricate a device 120 from beta-Mn type CoZnMn comprising regions with different thicknesses, a thin region 122 of thickness 300nm and a thick region of thickness 500nm. In such a device, the magnetic phase diagram shows that magnetic skyrmions 128 are only stable at room temperature within the thin region of the device under a specific applied magnetic field. By then utilising spin transfer torque and skyrmion motion under applied electrical currents, it is then possible to inject the skyrmion lattice across the thickness barrier where they will exist with an effectively infinite lifetime in a metastable state . The lost magnetic skyrmions 128 in the thin part region 122 will then spontaneously reformed due to the equilibrium conditions and the skyrmions within the thicker region 124 can be further manipulated with electrical current pulses for any specific application. The micromagnetic simulation of the skyrmion injector concept of Figures 13a and 13b shows in Figure 13a that magnetic skyrmions 128 are spontaneously formed in the thin region 122 which is in the equilibrium pocket of the phase diagram (Figure 2) whereas a conical phase is present in the thick region 124. After application of a current pulse, Figure 13b shows that in thick region 124 magnetic skyrmions 128 have been transported across the thickness barrier 123 and with such have become disordered due to edge effects and skyrmion Hall motion and exist in a disordered metastable state. This demonstrates successful injection of skyrmions across the thickness step 123. However, due to the limitations of the micromagnetic approximation, magnetic skyrmions 128 are not seen to spontaneously reform within the thinner section of the device. Similarly, due to this imitation, it would be expected that the magnetic skyrmions 128 would have enough thermal energy to reorganise themselves to reduce the amount of disorder post-injection. Instead these simulations show us what would be expected for a system quenched at zero temperature and also over a very small time step. Referring to Figures 14a and 14b, a sample of beta-Mn type CoZnMn hosting material 120 having a thin part 122 (300 nm) and a thick part 124 (500 nm) was held at 285 k and had a magnetic field of 80 mT applied thereto. This forms magnetic skyrmions 128 in the thin region 122. This is shown in a X-ray hologram in Figure 14a. The advantage of this technique is that it provides a direct visualisation of the skyrmions, rather than the indirect analysis of small angle x-ray scattering. Referring to Figure 14b, after application of 50mv current pulses (still at temperature of 285 k and applied field of 80 mT) magnetic skyrmions 128 are observed in the hologram on the opposite side of the thickness step 123 in thick region 128. These results are obtained using a holographic x-ray imaging technique. This is direct proof of the skyrmion injection process at room temperature under an applied magnetic field. It will be appreciated by persons skilled in the art that the above embodiment has been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departure from the 5 scope of protection as defined by the appended claims.

Claims

1. A method of moving magnetic skyrmions in a hosting material, the method comprising:providing a hosting material having a first part having a first thickness a second part having a second thickness which is greater than said first thickness;applying a magnetic field to and holding said hosting material in a predetermined temperature range to form magnetic skyrmions in said first part of said hosting material;passing an electric current through said hosting material to move said magnetic skyrmions to said second part of said hosting material, wherein the temperature of the hosting material and magnetic field applied to the hosting material are of values which cannot induce skyrmions into the thickness of said second part of the hosting material.

2. A method according to claim 1, wherein the step of passing an electric current through said hosting material to move said magnetic skyrmions to said second part of said hosting material comprises passing current pulses through the hosting material.

3. A method according to claim 2, wherein the current density of said current pulses is approximately 3 x 108 A / m2.

4. A method according to any one of the preceding claims, wherein the step of providing a hosting material having variable thickness comprises preparing a stepped lamella of a chiral magnetic material.

5. A method according to any one of the preceding claims, further comprising reversing the polarity of the electric current to move said magnetic skyrmions from said a second part of said hosting material to said a first part of said hosting material.

6. A method according to any one of the preceding claims, wherein providing a hosting material having a first part having a first thickness a second part having a second thickness which is greater than said first thickness comprises providing a stepped lamella of FeGe in which said first thickness is 300 nm and said second thickness is 500 nm; andwherein applying a magnetic field to and holding said hosting material in a predetermined temperature range to form magnetic skyrmions in said first part of said hosting material comprises applying a magnetic field of 75 mT and holding said hosting material at a temperature of 255 K.

7. A method according to any one of claims 1 to 5, wherein providing a hosting material having a first part having a first thickness a second part having a second thickness which is greater than said first thickness comprises providing a stepped lamella of beta-Mn type CoZnMn in which said first thickness is 300 nm and said second thickness is 500 nm; andwherein applying a magnetic field to and holding said hostingmaterial in a predetermined temperature range to formmagneticskyrmionsin said first part ofsaid hostingmaterial comprises applying a magnetic field of 80 mT and5 holding said hosting material at a temperature of 285 K.

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