Magnetoresistive sensors sensitive to out-of-plane magnetic fields.

Abstract

The present invention relates to a magnetoresistive sensor (100) sensitive to out-of-plane applied magnetic fields, comprising: a sensing layer (106); a reference layer (104) with fixed magnetization, the direction of the fixed magnetization being perpendicular to the plane of the reference layer; a non-magnetic spacer layer (105) separating the sensing layer (106) and the reference layer (104); the magnetoresistive sensor comprising: a sensing layer (106) having a magnetization vortex configuration spontaneously without an applied magnetic field, the vortex core diameter changing in the presence of an applied magnetic field perpendicular to the plane of the reference layer (104).

Description

[Technical field]

[0001] The present invention is in the field of spin engineering and relates to magnetoresistive sensors that are sensitive to out-of-plane components of a magnetic field, which for ease of explanation will be called out-of-plane magnetic field, or magnetic field perpendicular to the plane. [Background technology]

[0002] Spintronic magnetic field sensors are used in a wide range of automotive, industrial, or biomedical applications (P.P.Freitas, R.Ferreira, and S.Cardoso, 2016, "Spintronic Sensors," Proceedings of the IEEE, vol.104, no.10, pp.1894-1918).

[0003] These types of magnetoresistive sensors offer several advantages over other alternatives such as Hall sensors, search coils, giant magnetoimpedance sensors, SQUIDs, or fluxgates due to a combination of high sensitivity, compact size, low power consumption, compatibility with CMOS integration, and low cost.

[0004] Spintronic sensors convert magnetic field changes into voltage or resistance changes under dc or ac bias currents. Such resistance changes are based on the giant magnetoresistance (GMR) effect in spin-valves (SVs) or tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJs). Both magnetoresistive structures exhibit changes in their electrical resistance depending on the relative orientation of the fixed magnetization of the reference layer (RL) and the sensing layer (SL) magnetizations.

[0005] Vortex-based magnetoresistive sensors are known to exhibit high saturation fields, which is an attractive feature for applications with large dynamic range (Suess, D., Bachleitner-Hofmann, A., Satz, A. et al., 2018, "Topologically protected vortex structures for low-noise Magnetic sensors with high linear range", Nat Electron 1, 362-370).

[0006] In certain applications, such as speed or current sensors, a large linear range is an important feature because it reduces the required precision in the amplitude of the field that the bias magnet generates on the sensor, however, it also comes at the expense of a reduced sensor sensitivity.

[0007] The stability of the vortex structure results from the minimization of the total energy of the system, including the exchange energy arising from the inhomogeneity of the vortex structure, the magnetostatic energy from dipole interactions, the magneto-crystalline anisotropy energy, and the Zeeman energy if an external field is applied. In certain geometries, the ground energy state of the nanoscale cylinder favors the vortex configuration, as predicted by theory (Metlov, KL and Lee Y., 2008, "Map of Metastable states for thin circular magnetic nanocylinders", Appl. Phys. Lett. 92, 112506 (2008)). In such geometries, the magnetostatic energy overwhelms the exchange energy, resulting in the formation of a vortex configuration corresponding to an in-plane curling of the magnetic moment, which allows in-plane flux closure with an out-of-plane magnetized vortex core to reduce the exchange energy cost generated by the magnetic singularity at the center of the cylinder. In the common case used in prior art sensors, the thickness of the cylinder is much smaller than its diameter. In this situation, the diameter of the vortex core is

number

[0008] Prior art vortex-based sensors are sensitive to in-plane components of the magnetic field. Throughout this description, "in-plane" means parallel to the plane of the sensor's stack layers, while "out-of-plane" or "perpendicular to the plane" means along the normal to this plane. Figure 1 illustrates a magnetic stack configuration 1 of such a prior art sensor.

[0009] The magnetic stack 1 is, in succession: - seed layer 2; - pinned synthetic antiferromagnetic structure3; a spacer 4 which can either be a tunnel barrier when the sensor is a magnetic tunnel junction, or a non-magnetic metallic spacer when the sensor is a metallic spin-valve; - a sensing layer 5 that spontaneously enters into a magnetic vortex configuration without an applied magnetic field; - Capping Layer 6 Includes.

[0010] The pin composite antiferromagnetic structure 3 is successively: - Typically IrMn or PtMn alloys (e.g. Ir 20 Mn 80 Or Pt 50 Mn 50 ) based antiferromagnetic layer (AF) 7; - Ferromagnetic layer (FM)8; a spacer layer 9 providing antiferromagnetic interlayer coupling (so-called RKKY coupling) between two adjacent magnetic layers, this spacer layer being made, for example, of Ru; - Standard layer 10 Equipped with.

[0011] The reference layer 10 is part of a synthetic antiferromagnet ("SAF"). The SAF comprises two antiferromagnetically coupled layers 8 and 10 separated by a metal spacer 9. The two layers 8 and 10 are considered as the reference (or polarizer) layer 10 and the pinned layer 8, respectively. The SAF is associated with an antiferromagnetic pinning layer 7, which provides an exchange bias to the magnetic pinned layer 8 of the SAF in contact with it. The SAF is then grown on this antiferromagnetic pinning layer 7. The pinned synthetic antiferromagnet structure (AF / SAF) of the form (AF / FM / Ru / Reference) is generally used as a pinned reference electrode, where the layer FM8 is a ferromagnetic material such as cobalt or an alloy mostly based on Fe, Co, Ni. When the sensor is a metallic spin-valve with a non-magnetic metallic spacer separating the reference layer from the sensing layer, the entire stack typically has a face-centered cubic (fcc(111) texture). When the sensor is a magnetic tunnel junction, the spacer between the reference and sensing layers is a tunnel barrier oxide, most often made with a MgO bcc(001) texture. In this case, to grow a four-fold symmetric bcc(001) tunnel barrier oxide on top of a three-fold symmetric fcc(111) AF / SAF structure, the reference layer 10 typically consists of, in succession: - a Co or CoFe layer in contact with the spacer layer 9 in Ru or Ir, inducing antiparallel RKKY (Ruderman-Kittel-Kasuya-Yoshida) coupling; - nanocrystalline or amorphous layers made of materials such as Ta, W, Mo or Hf, capable of absorbing amorphising elements (e.g. B) from adjacent layers and ensuring a structural transition between fcc and bcc structures while providing interlayer ferromagnetic coupling; - Amorphous CoFeB alloy in contact with tunnel barrier 4 Such an amorphous CoFeB layer crystallizes with the same crystal structure as the tunnel barrier during annealing at temperatures above 250° C., resulting in large tunneling magnetoresistance (TMR) as is well known to those skilled in the art.

[0012] The synthetic antiferromagnetic (SAF) configuration allows for the reduction of stray fields caused by this hard reference stack on the sensing layer, providing greater magnetic stability to the reference layer. The ferromagnetic reference layer has an in-plane magnetization and is exchange-coupled with a layer of antiferromagnetic material. As is well known to those skilled in the art, the ferromagnetic material layer 8 of the SAF adjacent to the antiferromagnetic layer 7 is pinned in the in-plane direction by annealing the stack and cooling it from a temperature above the blocking temperature of the antiferromagnetic layer under an externally applied magnetic field in that particular direction to saturate the ferromagnetic layer magnetization in that desired direction.

[0013] In vortex-based sensors that are sensitive to in-plane fields, in a certain range of fields limited by the vortex annihilation field, a nearly linear change in resistance versus field is observed due to the lateral vortex core motion in a direction perpendicular to the in-plane field direction to be sensed. State-of-the-art magnetoresistive sensors with vortex configurations in the sensing layer are attractive due to their linear R(H) characteristics and the stability of the vortex configurations.

[0014] The mechanism of operation of the prior art vortex sensor is depicted in FIG. 2. FIG. 2 shows the magnetic response along the x-axis of the vortex sensing layer (i.e., the direction of magnetization of the pinned reference layer) in response to a field to be sensed, which is applied in the plane of the layer along the x-direction of the magnetization of the pinned reference layer. At zero applied field (point C), the equilibrium state is a vortex configuration with the core located at the center of the disk (assuming there is no stray field from the reference stack). Under a given non-zero in-plane field H (point B), the vortex magnetization is polarized along the field direction with a lateral shift of the vortex core in a direction perpendicular to the applied field direction. In FIG. 2, the field is applied along the x-direction, which shifts the vortex core in the y-direction by a distance Δd, which results from the balance of the Zeeman energy, the exchange energy, and the magnetostatic energy. The larger the field, the more the vortex core shifts. This results in linear M(H) and R(H) characteristics up to a certain characteristic field called the annihilation field (Ha), where the vortex structures completely annihilate and single-domain micromagnetic structures form (A). On the way back from saturation, the nucleation field (H n vortices form again in the characteristic magnetic field known as the vortex.

[0015] One particular advantage of such known vortex-based sensors is their large linear range (40 mT) compared to that of single-domain elliptical sensors (5 mT), which represents an 8-fold enhancement in detection range, but a reduction in sensitivity by the same factor as reported by Suess, D., Bachleitner-Hofmann, A., Satz, A. et al., 2018, "Topologically protected vortex structures for low-noise magnetic sensors with high linear range", Nat Electron 1, 362-370.

[0016] A drawback of this type of sensor is their noise. This noise arises from the trapping of the vortex core in its lateral motion due to local variations in anisotropy. This can occur especially in magnetic tunnel junctions due to spatial variations in the interfacial anisotropy at the interface between the tunnel barrier oxide and the sense layer. In fact, this anisotropy is known to be very sensitive to parameters such as the local oxygen or boron concentration. Since the vortex core has a small area in prior art vortex sensors (typically 5 nm in diameter), the trapping can be very effective even for small defects of the order of a few nm, such as grain boundaries. This can induce an equivalent of the Barkhausen noise known in domain wall propagation, but now for vortex core propagation. This noise affects the signal-to-noise ratio of this type of sensor.

[0017] In addition, these sensors are sensitive to the field components parallel to the pinning direction, i.e., in-plane, of the reference layer magnetization, and therefore have limitations on the direction of magnetic field measurement. [Prior art documents] [Non-patent literature]

[0018] [Non-Patent Document 1] PPFreitas, R. Ferreira, and S. Cardoso, 2016, "Spintronic Sensors", Proceedings of the IEEE, vol. 104, no. 10, pp. 1894-1918. [Non-Patent Document 2] Suess, D., Bachleitner-Hofmann, A., Satz, A. et al., 2018, "Topologically protected vortex structures for low-noise Magnetic sensors with high linear range", Nat Electron 1, 362-370. [Non-Patent Document 3] Metlov, KL and Lee Y., 2008, "Map of Metastable states for thin circular magnetic nanocylinders", Appl.Phys.Lett.92, 112506 (2008) Summary of the Invention [Problem to be solved by the invention]

[0019] In this context, it is an object of the present invention to provide an eddy-based magnetoresistive sensor capable of measuring the out-of-plane component of a field and offering an improved signal-to-noise ratio. [Means for solving the problem]

[0020] To this end, the present invention provides a magnetoresistive sensor that is sensitive to an out-of-plane applied magnetic field, comprising: - Sensing layer; - a reference layer with fixed magnetization, the direction of the fixed magnetization being perpendicular to the plane of the reference layer; a non-magnetic spacer layer separating the sensing layer and the reference layer; Equipped with A magnetoresistive sensor is proposed in which the sensing layer has a magnetized vortex configuration without an applied magnetic field, and the vortex core diameter changes in the presence of an applied magnetic field perpendicular to the plane of the sensing layer.

[0021] Thanks to the present invention, it is possible to sense magnetic fields oriented perpendicular to the plane of the sensing layer, where the sensing layer is in a vortex micromagnetic configuration. The sensor principle is based on the expansion or contraction of an out-of-plane magnetized vortex core under the effect of an out-of-plane field to be sensed. By implementing such a sensing layer in a magnetic tunnel junction or spin-valve that includes an out-of-plane magnetized reference layer, this expansion / contraction of the vortex core creates a magnetoresistance signal that can be used to measure the amplitude of the applied field to be sensed. The resulting magnetoresistance sensor exhibits a large linear range and an improved signal-to-noise ratio compared to prior art vortex-based sensors.

[0022] The sensors according to the invention exhibit an approximately linear change in resistance versus the amplitude of the out-of-plane field to be sensed. However, this linear change is not related to the lateral motion of the vortex core as in prior art sensors, but is instead based on the expansion / contraction of the vortex core under an out-of-plane applied magnetic field. Due to the much larger size of the vortex core and the fact that the center of the core does not move laterally during field sensing, the noise of these sensors is much reduced, resulting in a higher signal-to-noise ratio compared to prior art vortex sensors.

[0023] In the present invention, as will be seen further in the description, the ratio of the thickness of the sensing layer divided by the lateral dimension of the sensor is advantageously much larger than in prior art vortex sensors, which results in the vortex core being much wider than in prior art vortex sensors, where the diameter of the vortex core is given by the exchange length.

[0024] The sensor according to the invention may also present one or more of the following characteristics, considered individually or in all technically possible combinations: - the magnetoresistive sensor of the invention comprises means for applying a current through the sensor in a direction approximately perpendicular to the plane of the layers; - the sensing layer is selected with an aspect ratio defined as the thickness of the sensing layer divided by its in-plane characteristic dimension, comprised between 0.2 and 2, and advantageously between 0.2 and 1; when the sensor of the invention has a cylindrical shape with a circular cross section, the in-plane characteristic dimension of the sensing layer is its diameter. However, the invention applies to any type of shape that makes it possible to stabilize a vortex configuration in the sense layer. This applies, among others, to a square, a square with rounded corners, a slightly rectangular shape (the long side does not exceed twice the short side), possibly with rounded corners, an ellipse with a gentle ellipticity (the ratio between the long and short axes does not exceed 2), or any other shape in which a vortex state can be stabilized. For all these shapes, the aspect ratio of the sense layer is defined by the ratio of its thickness to its shortest in-plane dimension. If the sense layer has a disk shape, this characteristic dimension is its diameter. If it is a square or close to a square, this is the length of the side of the square. If it is a rectangle, it is the short side of the rectangle. In the case of an ellipse, it is the minor axis of the ellipse; - the non-magnetic spacer layer is a tunnel barrier layer or a metal spacer; - the reference layer is magnetically coupled to a synthetic antiferromagnetic layer with perpendicular anisotropy; - the non-magnetic spacer layer is a tunnel barrier layer and the sensing layer comprises a first layer made of an alloy based on iron, cobalt and an amorphizing element, said first layer being in contact with the tunnel barrier layer; - the sensing layer comprises at least another layer of a material adapted to absorb at least a portion of the amorphizing element present in the first layer and to ensure a structural transition between the layers comprised in the sensing layer; the material adapted to absorb at least a portion of the amorphizing element is chosen from the following materials: Ta, Mo, W or Hf, or mixtures thereof; - the sensing layer comprises one or several stacked oxide layers; - the magnetoresistive sensor of the present invention comprises an arrangement of one or more layers that avoids switching of the vortex core magnetization direction; - the reference layer is magnetically coupled to a layer made of a hard material with perpendicular anisotropy; - the magnetoresistive sensor of the present invention comprises an antiferromagnetic layer for inducing a perpendicular exchange bias in the sensing layer, said antiferromagnetic layer being magnetically coupled to the sensing layer; - the magnetoresistive sensor of the present invention comprises a non-magnetic spacer layer between the sensing layer and an antiferromagnetic layer that induces a perpendicular exchange bias in the sensing layer magnetization to adjust the exchange bias coupling strength; The geometry and dimensions of the vortex sensing layer are selected such that the vortex configurations in the sensing layer are not annihilated over a range of applied magnetic fields to be sensed of at least + or - 200 mT.

[0025] The present invention also comprises for the purpose a sensing device comprising a plurality of magnetoresistive sensors according to the present invention electrically coupled in series and / or in parallel.

[0026] The sensing device may comprise four magnetoresistive sensors according to the invention arranged according to a Wheatstone bridge configuration such that the reference layers of two sensors located on two opposing branches of the bridge are magnetized in a first out-of-plane direction, while the two magnetoresistive reference layers located on two other branches of the bridge are magnetized in an opposite out-of-plane direction.

[0027] Other characteristics and advantages of the invention will appear clearly from the description given below, which is illustrative and in no way limiting, with reference to the attached drawings, in which: [Brief description of the drawings]

[0028] [Figure 1] FIG. 1 illustrates a vortex sensor according to the prior art. [Diagram 2] 2 illustrates the mechanism of operation of the prior art vortex sensor of FIG. 1. [Diagram 3]FIG. 1 illustrates generally a first embodiment of a sensor according to the invention. [Figure 4] 4A-4C are diagrams illustrating the mechanism of operation of the vortex sensor of FIG. 3. [Diagram 5] FIG. 4 shows a detailed embodiment of the stack of sensors of FIG. 3. [Figure 6] 6 illustrates the dependence of the electrical resistance of the sensor of FIG. 5 as a function of an external magnetic field applied perpendicular to the plane of the magnetic stack. [Figure 7] a) shows a longitudinal cross-section of magnetization reversal of a magnetic element of 60 nm diameter, 60 nm thickness and 0.8 MA / m magnetic saturation performed by micromagnetic simulation; b) shows a 2D view of the Mz component of the topmost surface layer in the same external field applied in a; and c) shows a 1D view of the Mz component of the topmost surface layer at y=0. [Figure 8] FIG. 13 shows the evolution of the magnetization Mz as a function of the applied out-of-plane magnetic field Hz for different diameters D of the sensing layer. [Figure 9] FIG. 13 shows electrical results for different nanopatterned sensors with different diameters. [Figure 10] FIG. 13 shows micromagnetic simulations of the reversal of a 60 nm thick and 60 nm diameter cylinder with Aex=13×10−12 J / m2 and α=0.01 for different saturation magnetization values. [Figure 11] FIG. 14 shows the evolution of electrical resistance of two sensors with similar diameters, one having NiFe material (Ms=0.756 MA / m) as the sensing layer material and the other having Co material (Ms=1.44 MA / m) as the sensing layer material, as a function of the applied out-of-plane magnetic field. [Figure 12] FIG. 1A is a schematic diagram of the path taken by a magnetic vortex core during displacement in the presence of an interfacial defect under an external applied field in the plane of the layers and the resulting magnetization response for a prior art vortex sensor; FIG. 1B is a schematic diagram of the expansion taken by a magnetic vortex core during displacement in the presence of an interfacial defect under an external applied field perpendicular to the plane of the layers and the resulting magnetization response for a vortex sensor of the present invention. [Figure 13] (a) shows experimental hysteretic behavior observed in the sensor due to vortex core polarity changes, and (b) shows non-hysteretic behavior in the sensor of the present invention due to the presence of a stray magnetic field from a non-perfectly compensated SAF structure that stabilizes the vortex core polarity in a particular direction. [Figure 14] FIG. 1A is a schematic diagram of a magnetoresistive sensor of the present invention having a non-compensated magnetic reference layer magnetized in a direction perpendicular to the plane of the multilayer, and FIG. 1B is a detailed diagram of a magnetoresistive sensor of the present invention having a non-compensated magnetic reference layer magnetized in a direction perpendicular to the plane of the multilayer. [Figure 15] FIG. 1A is a schematic diagram of a magnetoresistive sensor of the present invention having a compensation magnetic reference layer and an upper exchange biasing layer magnetized in a direction perpendicular to the plane of the layers, and FIG. 1B is a detailed diagram of a magnetoresistive sensor of the present invention having a compensation magnetic reference layer and an upper exchange biasing layer magnetized in a direction perpendicular to the plane of the layers. [Figure 16] FIG. 2 shows a sensing device of the present invention in a Wheatstone bridge configuration, comprising four elementary magnetoresistive sensors of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] FIG. 3 illustrates diagrammatically a first embodiment of a sensor 100 according to the invention.

[0030] The sensor 100 continuously: - Seed layer 101; - Synthetic antiferromagnetic structures 102; - texture breaking layer 103; - Reference layer 104; a spacer 105, which can be either a tunnel barrier when the sensor is a magnetic tunnel junction, or a non-magnetic metallic spacer when the sensor is a metallic spin-valve; - a sensing layer 106 that spontaneously resides in a magnetic vortex configuration without an applied magnetic field; - Capping Layer 107 A magnetic stack comprising:

[0031] The magnetic stack has the form of a pillar, the cross section of which is advantageously of circular form.

[0032] As mentioned above, the sensing layer 106 has a magnetized vortex configuration. Moreover, the sensing layer 106 exhibits an aspect ratio (i.e., thickness divided by in-plane feature dimension, here diameter) of 0.2 to 2 and advantageously 0.2 to 1. In contrast to prior art vortex sensors, the aspect ratio of the sense layer in this sensor is much closer to 1. As a result, the vortex is now confined to a narrow cylinder more than in prior art in-plane sensitive sensors. Unexpectedly, due to this confinement, the vortex core results to be significantly larger than in prior art vortex sensors. Moreover, as can be seen with respect to FIG. 4, the vortex core diameter changes significantly when an out-of-plane field is applied to the device, which is not the case in prior art vortex sensors.

[0033] The reference layer 104 has a fixed mono-domain magnetization perpendicular to the plane of the reference layer 104. The reference layer RL is most often used in conjunction with a synthetic antiferromagnet ("SAF") 102, as shown in Figure 3. However, the present invention does not necessarily require a SAF.

[0034] The sensing layer 106 in the vortex configuration is part of a magnetoresistance element. The magnetoresistance element can be either a tunnel junction or a giant magnetoresistance metal stack. In both cases, the element comprises top and bottom electrodes such that the current flows through the entire device in a direction approximately perpendicular to the plane of the layers. As is well known by those skilled in the art, the requirements for the nature of the layers are different in a magnetic tunnel junction and a fully metallic giant magnetoresistance stack. Indeed, in a tunnel junction, different crystallographic structures exist in the stack (body-centered (bcc) near the tunnel barrier and face-centered cubic (fcc) in the lower and upper parts of the stack), while in a giant magnetoresistance stack, the structure can remain fcc throughout the stack.

[0035] In contrast to this prior art, the mechanism of operation of the sensor 100 of FIG. 3 is explained with respect to FIG.

[0036] Unlike prior art vortex-based sensors that are sensitive to in-plane fields, the sensor 100 is intended to sense out-of-plane fields. From a physical point of view, instead of relying on the lateral in-plane shift of the vortex core upon application of an in-plane field, the sensor 100 of the present invention relies on the expansion / contraction of the vortex core of the sensing layer 106 under an out-of-plane field Hz (i.e., the Oxy plane, which is the plane of the different layers of the sensor along the z-axis).

[0037] The magnetization response Mz along the z-axis of the sensing layer 106 as a function of the out-of-plane component Hz of the applied magnetic field is depicted in the transfer curve of Figure 4, which shows the magnetization Mz of the sensing layer 106 as a function of the z-axis component of the applied magnetic field Hz. For a prior art magnetoresistive sensor, the transfer curve shows a linear response region and two characteristic fields along both the ascending and descending branches: the annihilation field Ha, where the vortex core expands to the entire sensing layer area and the micro-magnetic configuration becomes a nearly saturated single domain in the out-of-plane direction, and the nucleation field Hn, where the field is reduced from saturation and the vortex forms again.

[0038] As mentioned above, in prior art vortex sensors, the sensing layer diameter is usually large, on the order of one or a few microns, at least an order of magnitude larger than the thickness of the sensing layer. As a result, the vortex core is rather narrow, with a diameter of a few nanometers given by the exchange length. On the other hand, advantageously according to the present invention, the vortex core diameter is much wider in a cylinder of smaller diameter when the diameter approaches the film thickness more closely. If a field is applied in an out-of-plane direction parallel to the vortex core magnetization, the core diameter tends to increase: this situation is illustrated by state B compared to state C where no field is applied. Assuming that in state B the magnetization of the vortex core direction along the z-axis is downwards (i.e. the z-component of the vortex core magnetization is negative), the component Hz is parallel and in the same direction as the vortex core magnetization (Mz is negative), and the diameter of the vortex core diameter increases compared to its value in state A without an applied field. Conversely, if a field is applied anti-parallel to the core magnetization, its diameter decreases. It should be noted that advantageously, the sensor of the present invention should be used within its linear response range; in other words, preferably the applied field should not be used above the vanishing field Ha where the vortex core would expand to the entire sensing layer area as shown in state A of Figure 4. This issue will be addressed later in the description to show how to avoid such vanishing and keep the sensor within its linear response.

[0039] When this sensing layer 106 is implemented in a spin-valve or magnetic tunnel junction that includes an out-of-plane magnetized reference layer 104, these changes in the vortex core diameter of the sensing layer 106 result in resistance changes in the field range bounded by the positive and negative vortex annihilation fields. Based on the resistance changes, the value of the corresponding out-of-plane component of the applied magnetic field can be accessed.

[0040] FIG. 5 illustrates a first possible embodiment of the vortex sensor of the present invention, in which the sensing layer is inserted into a magnetic tunnel junction in which the reference layer is magnetized in the out-of-plane direction.

[0041] If the device is a magnetic tunnel junction, the tunnel barrier 105 is an oxide or nitride layer such as MgO, AlOx, TiOx, or AlN, TiN, but preferably MgO, which is known to provide a large TMR signal. To obtain a large TMR with MgO (large TMR typically means a TMR of more than 80%, preferably more than 140%, preferably more than 200%), the MgO layer and the two magnetic electrodes sandwiching the MgO layer must have a crystalline bcc structure with a (100) texture. As is well known by those skilled in the art, this can be achieved by using a FeCoB layer as the magnetic electrode, which is amorphous when deposited, but can recrystallize in a bcc structure upon annealing, typically at temperatures in the range of 250°C to 400°C.

[0042] For such recrystallization to occur, the B, or more generally the amorphizing element, contained in the FeCo-based alloy must be expelled from the alloy. A thin layer of a material capable of absorbing this amorphizing element is then deposited next to the FeCo-based layer on its interface opposite the interface with the MgO layer. Such an amorphizing element absorbing layer can be made of, for example, Ta, W, Mo, or Hf.

[0043] Taking into account the above considerations, the vortex sensing layer 106 initially comprises a thin FeCoB-based layer in contact with the MgO barrier 105. In this layer, B is an amorphizing element. B can be replaced with other amorphizing elements such as Zr or Nb. This first layer typically has a thickness in the range of 1.5 to 5 nm. This FeCoB is then in contact with a thin layer of a few angstroms thick that can absorb the amorphizing element upon annealing of the structure. This layer can be made of W, Mo, Ta, for example. The sensing layer 106 further comprises a magnetic layer preferably made of a soft magnetic material such as permalloy NiFe (composition close to Ni80Fe20) or a soft FeCo layer such as Co90Fe10 or FeCoB alloy, in order to minimize hysteresis in this layer when its micromagnetic configuration changes under the effect of the field to be sensed. This sensing layer 106 can also be a multilayer including several such layers. The sensing layer 106 may also comprise thin stacks (on the order of 0.1 to 0.5 nm thick) of non-magnetic metals such as W, Mo, Ta, etc., intended to absorb any amorphizing elements or interrupt grain growth to reduce the top roughness of the sensing layer. If the device is a magnetic tunnel junction, these stacks may also be made of oxide materials such as MgO, TaOx, AlOx, TiOx, etc., intended to induce some interfacial perpendicular anisotropy in the sense layer, thus allowing the thickness of the sense layer to be slightly reduced. The resistance-area product (RA) of these oxide stacks must then be adjusted to a value lower than the RA of the main barrier providing the tunnel magnetoresistance signal. This avoids excessive weakening of the magnetoresistance signal due to the increased resistance caused by the introduction of these oxide stacks.

[0044] As already explained, the total thickness of this sensing layer is advantageously selected with an aspect ratio (thickness divided by the lateral dimension, here the diameter of the circular sensing layer) of 0.2 to 2 so that the micro-magnetic balance configuration is in a vortex state with a core diameter significantly larger than the exchange length, preferably at least twice the exchange length (i.e. at least 10 nm for materials such as Permalloy or Co90Fe10). Examples of dimensions of the sensing layer range from 50 to 200 nm, e.g. a thickness of approximately 60 nm to stabilize the vortex formation at zero field for a diameter of 60 nm.

[0045] Micromagnetic simulations can be used to calculate the range of thicknesses that satisfy the condition for a vortex configuration with a vortex core diameter at least twice the exchange length.

[0046] As for the reference layer 104, its magnetization is pinned in a direction perpendicular to the stack of magnetic layers. This perpendicular configuration of the reference layer 104 is required because the measured magnetoresistance change depends on the change in the perpendicular magnetization Mz of the vortex sensing layer 106. If the stack is a MgO-based magnetic tunnel junction, this reference layer 104 is preferably made of FeCoB alloy in contact with the MgO barrier 105. The magnetization of this reference layer 104 is kept pinned out-of-plane by both the interface anisotropy present at the FeCoB / MgO interface and exchange coupling with an out-of-plane magnetized multilayer such as a (Co / Pt) multilayer. Other multilayers such as Co / X (X is Pd, Ni, or Ir) are well known to those skilled in the art to provide large PMA (perpendicular magnetic anisotropy) values. CoPt, CoPd, or alloys such as FePt or FePd ordered alloys, or some rare earth transition alloys such as TbCo, TbFe, may also provide strong PMA. This PMA multilayer or alloy may itself be part of a synthetic antiferromagnetic SAF layer 102 consisting of two antiferromagnetically coupled multilayers, such as (Co / Pt) multilayers separated by a nonmagnetic spacer, typically Ru or Ir, whose thickness is adjusted to provide a strong antiferromagnetic coupling between two adjacent multilayers (typically 0.8 nm for Ru spacers). This type of arrangement is well known by those skilled in the art. It is also well known that such SAF structures may include three or more antiferromagnetically coupled multilayers to further reduce the net stray field that the SAF and reference layers exert on the sensing layer. Since these PMA multilayers have an fcc structure, while FeCoB is bcc after annealing, a thin structural transition layer 103, made of, for example, Ta, Mo, or W, must still be introduced between the FeCoB layer and the PMA multilayer. This layer 103 is also intended to absorb amorphizing elements away from the FeCoB alloy upon annealing.In other words, the texture-disrupting layer 103 is introduced between the reference layer 104 in FeCoB in contact with the barrier 105 and the remainder of the SAF 102 to ensure a structural transition between the FeCoB layer, which must be body-centered cubic (bcc with four-fold symmetry) in the final device, and the remainder of the SAF, which generally has an fcc structure with (111 texture) and three-fold symmetry.

[0047] Additionally, the SAF layer 102 is grown on the seed layer 101 to promote a (111) texture. The seed layer 101 can be a multilayer of the form Ta / Pt. The materials used in the seed layer 101 can also be used for the capping layer 107.

[0048] Figure 6 illustrates experimental results showing the dependence of the electrical resistance of the sensor as a function of an external magnetic field applied perpendicular to the plane of the magnetic stack of the sensor depicted in Figure 5. Figure 6 shows that a linear range up to more than 200 mT can be obtained, much larger than what has already been reported for prior art vortex sensors developed for in-plane field sensing (40 mT or 80 mT).

[0049] Figure 7a shows a cross section of the evolution of the sensing layer magnetization under an out-of-plane field as qualitatively described in Figure 5. The micromagnetic configuration depicted in Figure 7a was obtained by micromagnetic simulation using the micromagnetic simulation MuMax3 software. The simulation was performed for a 60 nm thick and 60 nm diameter cylinder with the following sensing layer parameters: - Saturation magnetization: M sat =800×10 3 A / m, - Exchange Stiffness: A ex =13×10 -12 J / m 2 .

[0050] Figure 7b represents a 2D xy view of the top layer magnetization during the reversal process. The mesh in the simulation has a cell size of 1 nm. At H=0T, the vortex core is stabilized with the core now magnetized in the downward direction. By increasing the field (see snapshots at 0.125T and 0.25T), the negatively magnetized core gradually and simultaneously shrinks and the in-plane curl magnetized cells become increasingly polarized towards the out-of-plane applied field direction. Eventually, the vortex disappears and single domains are observed above H=0.5T or below H=-0.5T.

[0051] The evolution of the magnetization Mz value along the vertical direction for the topmost surface layer (1 nm cells) along the diameter of the cylindrical sensing layer is represented in Figure 7c.

[0052] The characteristic nucleation and annihilation fields can be controlled by varying the sensing layer diameter, as shown in the micromagnetic simulations of FIG. 8, which show the evolution of the magnetization Mz as a function of the applied out-of-plane magnetic field Hz for different diameters D of the sensing layer (D equals 40 nm, 60 nm, 80 nm, and 100 nm, respectively), using the same material parameters as in FIG. 5. FIG. 8 shows that for an element with a diameter of 40 nm, the reversal is abrupt between the two single domain states, because the shape anisotropy (thickness of the sensing layer larger than the diameter of the sensing layer) favors the single domain state along the longest dimension of the magnetic element, which is here its thickness. However, for diameters of 60 nm and above, vortex formation is observed, and the linear region in which the vortex is stable under the external field expands with the sensing element diameter. In view of the preceding, the diameter (i.e., the characteristic length) of the sensing layer of the sensor according to the invention is therefore greater than or equal to the thickness of the sensing layer. It can be seen that increasing the diameter of the sensing layer makes it possible to increase the range of the linear part of the response while decreasing the slope of the linear part of the response. Such a diameter dependence makes it possible to make sensor devices with a large linear range simply by increasing the diameter of the sensor. Furthermore, in FIG. 8 one can further see the presence of hysteresis in the response Mz vs. Hz, said hysteresis appearing as two parallel linear responses separated by a constant magnetization difference. Advantageously, a sensor according to the invention shall be used either for one or the other of these two parallel linear responses. Later in this document it will be mentioned how to avoid switching from one linear part to the other for a single sensor according to the invention.

[0053] The simulations reported in Figure 8 are confirmed by the experimental results plotted in Figure 9, which show the electrical results of different nanopatterned sensors with different diameters. Figure 9 shows the electrical resistance of the sensor as a function of the component Hz of the applied magnetic field. The different curves confirm the larger linear range predicted by the simulations of devices with smaller resistances (corresponding to larger diameters). The multilayer stack composition of the sensor, deposited at room temperature by DC magnetron sputtering, is the one shown in Figure 5: Ta3 / Pt30 / 4x[Co0.5 / Pt0.25] / Co0.5 / Ru0.9 / 3x[Co0.5 / Pt0.25] / Co0.5 / Ta0.2 / FeCoB1.1 / MgO1.25 / FeCoB1.4 / Ta0.2 / NiFe58 / Ta1 / Pt3.

[0054] The sample was then annealed for 10 min at 300° C. The annihilation field, represented by the grey dots in FIG. 9, gradually increases from 53 mT to 178 mT from the most to the least resistive junction, meaning from smaller to larger diameters.

[0055] The characteristic nucleation and annihilation fields can also be controlled by varying the saturation magnetization Ms of the sensing layer. Figure 10 shows the A for different saturation magnetization values ​​(Ms). ex =13×10 -12 J / m 2 Figure 1 shows a micromagnetic simulation of the reversal of a 60 nm thick and 60 nm diameter cylinder with and α = 0.01. It is observed that the linear range corresponding to the vortex stability region can be adjusted by modifying the saturation magnetization of the sensing material. Increasing the saturation magnetization Ms of the sensing layer makes it possible to increase the range of the linear part of the response (by decreasing the slope of the linear part).

[0056] The increase in the linear range with increasing Ms is experimentally confirmed as observed in Fig. 11 when comparing two sensor devices, one with NiFe material (Ms = 0.756 MA / m) and the other with Co material (Ms = 1.44 MA / m) as the sensing layer material, for sensors with electrical diameters similar to the proposed embodiment of Fig. 5 (52 and 58 nm, respectively). In the case of the sensing layer made of cobalt, very large fields up to more than 200 mT can be achieved without the collapse of the vortex state.

[0057] Another advantage of the vortex sensor according to the invention compared to prior art vortex sensors is its reduced hysteresis. Indeed, in prior art vortex sensors sensitive to in-plane fields, the presence of randomly distributed defects at the interface between the vortex sensing layer and the tunnel barrier modifies the vortex core displacement path when the vortex core is randomly trapped into the energy wells associated with such defects. This effect is particularly pronounced in prior art vortex sensors since the vortex core is very narrow (exchange length .about.5 nm) and, as a consequence, the vortex core can be easily trapped by such small defects. In magnetic tunnel junctions, these defects can be due to spatial variations in oxygen or boron concentration above the MgO / FeCoB interface or the diffusion of third elements present in the stack, such as Ta, Mn, W, or Mo, towards the MgO interface. These defects can induce local changes in the anisotropic energy, which then act as trapping centers for the vortex core. FIG. 12 shows a top view of a prior art sensing layer with defects 30. As the vortex core 31 moves in the first displacement direction 32, it is trapped into (and subsequently detrapped from) defects 30 encountered in the path of the displacement. Similarly, the vortex core 31 moving in the second displacement direction 33 is trapped into (and detrapped from) defects 30 encountered in the path of the displacement. Depending on its position in the nucleation, the vortex core 31 may be trapped and detrapped into different defects 30 when moving in the first displacement direction 32 than when moving in the second displacement direction 33. Due to such trapping / detrapping into different defects, the vortex core 31 may follow different paths in the first and second displacement directions. In such prior art sensors, a vortex core following different paths in the first and second displacement directions results in a different magnetization distribution of its magnetization, and hence a different resistance of the magnetic sensor at each value of the externally applied in-plane magnetic field H.

[0058] As a result of this trapping, the transfer curve (Mx vs. Hx) may differ depending on the path taken, resulting in a different resistance of the sensor at each value of the external field as shown by S. Dounia, C. Baraduc, and B. Dieny, 2020, European Patent No. 19315026.5.

[0059] In contrast, the sensor according to the invention is much less sensitive to the presence of such defects and their trapping. Indeed, the vortex core does not move laterally in the sensor of the invention: it only expands or contracts, while the curled magnetization in the outer parts of the device is reversibly pulled out-of-plane or back to the in-plane direction under the influence of the out-of-plane field to be sensed. Thus, the vortex deformation under a magnetic field is much more reversible in the sensor of the invention than in the prior art vortex sensor, as depicted diagrammatically in FIG. 12b. This results in significantly reduced noise in the sensor of the invention.

[0060] In an embodiment of the sensor of the present invention, the reference layer and the associated SAF layer may be compensated, which means that they do not apply a field or only apply a very small field (i.e., a field that is negligible with respect to the field required to reverse the vortex core polarity). In such a case, a possible drawback of the sensor of the present invention may be a change in the vortex core polarity when a magnetic field larger than the annihilation field is applied in a direction opposite to the vortex core polarity. Such a change in the vortex polarity creates an undesirable hysteresis, as already observed in FIG. 8 and illustrated in FIG. 13a. Such a hysteresis never appears if the sensor is always operated under the same field polarity (minor loop represented in black). However, if the vortex core polarity is changed, a certain difference (ΔR) in resistance between the two states (A, B) at zero field may appear. Such a difference in resistance (ΔR) is well observed when going through a full cycle (0.04T to -0.04T). Starting with a positive field (H>+Ha) larger than the annihilation field, if the field is reduced to zero, the vortex core assumes a positive polarity (A, vortex core upwards). In contrast, starting with a negative field larger in absolute value than the annihilation field, if the field is reduced to zero (see grey line in FIG. 13a), the vortex polarity is reversed (B, vortex core downwards). This results in a difference in the observed resistance ΔR at zero applied field.

[0061] This problem can be avoided by limiting the range of fields to which the sensor is exposed. The field value must remain below the vortex annihilation field. However, there is still a risk of unintentional switching of the vortex core polarity in this case if the sensor is unintentionally exposed to an excessively large field.

[0062] To better avoid this problem of unintentional switching of the vortex core polarity, a second embodiment is proposed in which the reference layer is coupled to the associated PMA multilayer or SAF layer, and the reference layer and the PMA multilayer (or the reference layer and the SAF layer, respectively) are not compensated. As a result, they create an out-of-plane stray field on the sense layer magnetization. Thanks to this out-of-plane stray field, the vortices always nucleate back with the same polarity, independent of the saturation field direction (positive or negative). This results in the optimal response shown in Figure 13b. Notably, this stray field only leads to a weak asymmetry of the annihilation field for the opposite field, which can be explained by the large volume of the sensing layer.

[0063] Such a sensor 200 having an uncompensated reference layer and a PMA multilayer is illustrated generally in FIG. 14a and with details in FIG. 14b.

[0064] The sensor 200 continuously: - seed layer 201; - a multilayer having a perpendicular magnetic anisotropy PMA material 202; - Texture destruction layer 203; - Reference layer 204; a spacer 205 which can be either a tunnel barrier when the sensor is a magnetic tunnel junction, or a non-magnetic metallic spacer when the sensor is a metallic spin-valve; - a sensing layer 206 that spontaneously resides in a magnetic vortex configuration without an applied magnetic field; - Capping Layer 207 A magnetic stack comprising:

[0065] Except for PMA multilayer 202, the other layers 201, 203, 204, 205, 206, and 207 can be identical to the layers illustrated in the sensors of FIGS.

[0066] As mentioned above, in this case the reference layer does not need to be bonded to the SAF layer as in Figures 3 and 5. Rather, the reference layer 204 is bonded to a single high PMA material 202. Such high PMA materials can be made of Co / Pt or Co / X repeats, where X represents Pd, Ni, or Ir, which are known to provide large PMA values. CoPt, CoPd, or alloys such as FePt or FePd ordered alloys, or some rare earth transition alloys such as TbCo, TbFe, can also provide strong PMA. The number of repeats of the (Co / Pt) multilayer or the thickness of the alloy can be adjusted to provide a stray field that is sufficient so that vortices always nucleate with the same polarity, but not too high so that the response remains symmetric when positive or negative fields are applied to the sensor. In the case of a (Pt / Co) multilayer, a number of repeats n between 1 and 6 in the PMA multilayer can be advantageously used.

[0067] Another possible embodiment aimed at stabilizing the vortex core polarity consists in coupling the sensing layer to an antiferromagnetic layer to provide an out-of-plane exchange bias to the vortex core. Such a sensor 300 is illustrated diagrammatically in Figure 15a and in more detail in Figure 15b.

[0068] The sensor 300 continuously: - seed layer 301; - SAF layer 302; - Texture destruction layer 303; - Reference layer 304; - a spacer 305 which can be either a tunnel barrier when the sensor is a magnetic tunnel junction, or a non-magnetic metallic spacer when the sensor is a metallic spin-valve; - a sensing layer 306 that spontaneously resides in a magnetic vortex configuration without an applied magnetic field; - exchange biasing layer 308; - Capping Layer 307 A magnetic stack comprising:

[0069] With the exception of the added exchange biasing layer 308, the other layers 301, 302, 303, 304, 305, 306, and 307 may be identical to the respective layers 101, 102, 103, 104, 105, 106, and 107 illustrated in the sensors of FIGS.

[0070] The exchange biasing layer 308 is on top of and in contact with the vortex sensing layer 306. Said exchange biasing layer 308 made of an antiferromagnetic material (AF) such as an iridium manganese (IrMn) or platinum manganese (PtMn) alloy at the interface of the sensing layer facing the tunnel barrier can be used for this purpose. A unidirectional magnetic anisotropy arises at the interface between the ferromagnetic sensing layer 306 and the AF layer 308 and is caused by exchange coupling between the two layers. To provide the exchange bias, the stack must be annealed above the antiferromagnetic layer blocking temperature and cooled in a magnetic field with the field direction perpendicular to the plane of the layers. As is well known by those skilled in the art, the strength of the exchange bias can be adjusted by inserting a thin layer (not shown) of a non-magnetic element such as Cu, W, Mo, or Ta at the interface between the sensing layer 306 and the antiferromagnetic layer 308. The main advantage of the sensor of Figure 15 is that the single vortex polarity stabilization does not arise from a stray field from the reference layer, which may be difficult to control in practice due to its variations in junction dimensions, but rather arises solely from exchange with an antiferromagnetic layer that is grown in contact with the sensing layer. The use of such an AF layer is fairly common in spintronic sensors, since reference layers in conventional sensors are typically pinned by exchange biasing with such an AF structure in the plane of the layer.

[0071] 16 shows an embodiment of a sensing device D. D includes a Wheatstone bridge arrangement with four magnetoresistive sensors R1, R2, R3 and R4, each sensor being a sensor according to the invention.

[0072] The sensing device D in a Wheatstone bridge configuration comprises four elementary sensors R1, R2, R3, and R4 of the present invention, where R1 and R4 have their reference layers and vortex core magnetizations in their sensing layers oriented in a first out-of-plane direction, while R2 and R3 have their reference layers and vortex core magnetizations in their sensing layers oriented in the opposite out-of-plane direction.

[0073] Sensors R1 and R4 each have an opposite response compared to sensors R2 and R3, in other words, R1 and R4 (sensors located on opposite branches) have the same response and have their reference layers and vortex core magnetizations in their sense layers oriented in a first out-of-plane direction, while R2 and R3 also have the same response and have their reference layers and vortex core magnetizations in their sense layers oriented in the opposite out-of-plane direction.

[0074] The sense current is supplied between contacts C1 and C2, and I in represents the current entering the bridge, and I out represents the outgoing current. In such a Wheatstone bridge configuration D, the elementary sensors are configured such that, under an applied magnetic field to be sensed, the resistance of two of the elementary sensors located in the opposite branches (here R1 and R4) increases (respectively decreases), while the resistance of the two other sensors (in this example R2 and R3) decreases (respectively increases). Advantageously, this makes it possible to measure the potential difference Voutput between the two contacts C3 and C4, which is directly related to the external out-of-plane magnetic field to be sensed.

[0075] According to the embodiment of Fig. 16, it is possible to start with four sensors R1, R2, R3, R4 with the same stack of layers (i.e. realized on the same wafer using the same technology and the same geometry and materials). To configure these elementary sensors to obtain an opposite response of the group of sensors R1 and R4 versus the group of sensors R2 and R3, one possibility is to use the combined effect of Joule heating and the application of an external magnetic field. Preferably, the magnetic tunnel junctions have a relatively high, typically 100 Ωm 2The reference layer is selected to have a resistance area product (RA) greater than 1000 Å. This allows for reducing the power consumption of the sensor and increasing the tunnel magnetoresistance amplitude. The reference layer is also selected to have an uncompensated reference layer such that the stray field from the reference layer is large enough to always keep the vortex core magnetization parallel to that of the reference layer magnetization. Then, to start the Wheatstone bridge configuration, an out-of-plane field large enough to fully saturate the magnetization of all layers in a first out-of-plane direction (e.g., upwards) is first applied. Then, a current is sent between contacts C1 and C3 and between contacts C4 and C2 to create Joule heating in sensors R1 and R4. This Joule heating, which increases the temperature of the junctions by typically 80° C. to 200° C., reduces the coercive field of the reference layer and vortex core magnetization in those two sensors R1 and R4. While the junctions are heated, a field is applied that is large enough to switch the reference layer magnetization and the vortex core magnetization in sensors R1 and R4, but low enough not to switch the magnetization of sensors R2 and R3 that remain at standby temperature, and that is opposite in direction to the first direction of the initial saturation field. Finding such intermediate field amplitudes is possible thanks to the change in the coercive field with temperature in this type of sensor. As a result of this initialization procedure, the four magnetic sensors included in the bridge end up in a magnetic configuration as represented in FIG. 16. The reference layer magnetization in R2 and R3 is still in the upward direction, while the reference layer magnetization in R1 and R4 is in the downward direction, and the vortex core magnetization of the sense layer in all junctions is parallel to the respective reference layer magnetization. Then, under the application of the field to be sensed, the vortex cores of the sensors whose magnetization is parallel to the applied field become wider, while the vortex cores of the sensors whose magnetization is antiparallel to the applied field shrink. In the case of the former junctions, this leads to a decrease in their resistance, while in the latter case, this leads to an increase in resistance. Therefore, the imbalance of the Wheatstone bridge changes significantly with the applied field, thus producing the sensor output signal Voutput.

[0076] In an alternative embodiment, the sensors R1, R2, R3, R4 may be formed by several individual sensors connected in series or in parallel, in which case all the individual sensors constituting one of the resistors Ri (i from 1 to 4) are configured identically, simply by applying a saturating field in the upward or downward direction.

[0077] Naturally, the sensor according to the invention is not limited to the embodiments that have just been described with reference to the figures, for guidance and in no way for limiting purposes.

[0078] In all the embodiments disclosed above, the sensor according to the invention is of the "bottom" configuration, meaning that the sensing layer is above the tunnel barrier and the reference layer is below the tunnel barrier. However, the invention is of course not limited to the "bottom" configuration, and the sensor of the invention can alternatively be in the "top" configuration. This means that the stack can be grown in a bottom pin or top pin configuration, i.e. the sensing layer can be above or below the tunnel junction, while the reference layer is below or above the tunnel barrier accordingly. Since the sensing layer is preferably somewhat thick in the sensor, it can be advantageous to grow the sensing layer above the tunnel barrier in order not to create a large roughness due to the growth of the tunnel barrier.

[0079] Furthermore, although the detailed embodiments of the sensor according to the invention are made for sensors based on magnetic tunnel junctions, those skilled in the art will understand that the above described embodiments can also be adapted to the case of fully metallic magnetoresistive sensors based on giant magnetoresistance. In this case, the tunnel barrier is replaced by a non-magnetic metal spacer such as Cu. From a material point of view, these metal stacks are easier since all the materials involved have an fcc structure, so fewer material issues need to be considered than in the case of magnetic tunnel junctions where some parts of the stack are fcc while others are bcc.

[0080] Moreover, detailed embodiments are disclosed under the assumption that the sensor of the present invention has a cylindrical shape with a circular cross section such that the sensing layer dimensions are characterized by its diameter and thickness. However, the present invention applies to any type of shape that allows the sense layer to stabilize a vortex configuration. This is true, among others, of a square, a square with rounded corners, a slightly rectangular shape (where the long side is not more than twice the short side) possibly with rounded corners, an ellipse with mild ellipticity (where the ratio between the long and short axes is not more than 2), or any other shape in which a vortex state can be stabilized. For all these shapes, the sense layer aspect ratio is defined by the ratio of its thickness to its shortest in-plane dimension. If the sense layer has a disk shape, this characteristic dimension is its diameter. If it is a square or close to a square, this is the length of the side of the square. If it is a rectangle, it is the short side of the rectangle. If it is an ellipse, it is the short axis.

[0081] These devices can be fabricated using a manufacturing process quite similar to that of prior art TMR sensors. The process involves depositing all layers by physical vapor deposition, etching the sensing layer by reactive or ion beam etching, and etching the other layers by ion beam etching. In this process, it is known that ion beam etching of magnetic materials often results in tapered pillars, especially in the present case where high aspect ratio pillars are etched. The presence of such tapered shapes is acceptable and does not interfere with the working principle of the sensor.

[0082] Additionally, as will be familiar to those skilled in the art, several sensors of the present invention may be connected in series and / or parallel to increase the overall sensor signal-to-noise ratio.

Claims

1. A magnetoresistive sensor (100, 200, 300) that is sensitive to out-of-plane magnetic fields, - Sensing layer (106, 206, 306), - A reference layer (104, 204, 304) with fixed magnetization, wherein the direction of the fixed magnetization is perpendicular to the plane of the reference layer, - Non-magnetic spacer layers (105, 205, 305) separating the sensing layers (106, 206, 306) and the reference layers (104, 204, 304) Equipped with, A magnetoresistive sensor in which the sensing layers (106, 206, 306) have a magnetized vortex configuration in the absence of an applied magnetic field, and the vortex core diameter changes in the presence of an applied magnetic field perpendicular to the plane of the reference layers (104, 204, 304).

2. The magnetoresistive sensor according to claim 1, characterized in that the sensing layer is selected with an aspect ratio defined as the thickness of the sensing layer divided by the in-plane feature dimensions of the sensing layer, which ranges from 0.2 to 2.

3. The magnetoresistive sensor according to claim 2, characterized in that the aspect ratio is in the range of 0.2 to 1.

4. A magnetoresistive sensor according to any one of claims 1 to 3, characterized in that the non-magnetic spacer layer is a tunnel barrier layer or a metal spacer.

5. A magnetoresistive sensor according to any one of claims 1 to 3, characterized in that the reference layer is magnetically coupled to a synthetic antiferromagnetic layer having perpendicular anisotropy.

6. A magnetoresistive sensor according to any one of claims 1 to 3, characterized in that the non-magnetic spacer layer is a tunnel barrier layer, and the sensing layer comprises a first layer made of an alloy based on iron, cobalt, and amorphous elements, wherein the first layer is in contact with the tunnel barrier layer.

7. The magnetoresistive sensor according to claim 6, characterized in that the sensing layer comprises at least another layer of material adapted to absorb at least a portion of amorphous elements present in the first layer and to ensure interlayer structural transitions within the sensing layer.

8. The magnetoresistive sensor according to claim 7, characterized in that the material adapted to absorb at least a portion of the amorphous elements is selected from the following materials, namely Ta, Mo, W, or Hf, or a mixture thereof.

9. A magnetoresistive sensor according to any one of claims 1 to 3, characterized in that the sensing layer comprises one or more stacked oxide layers.

10. The magnetoresistive sensor according to any one of claims 1 to 3, characterized in that the magnetoresistive sensor comprises an arrangement of one or more layers to avoid switching of the vortex core magnetization direction.

11. A magnetoresistive sensor according to any one of claims 1 to 3, characterized in that the reference layer is magnetically coupled to a layer made of a rigid material having perpendicular anisotropy.

12. The magnetoresistive sensor according to any one of claims 1 to 3, characterized in that the magnetoresistive sensor comprises an antiferromagnetic layer that induces a vertical exchange bias in the sensing layer, and the antiferromagnetic layer is magnetically coupled to the sensing layer.

13. The magnetoresistive sensor according to claim 12, characterized in that the magnetoresistive sensor comprises a non-magnetic spacer layer between the sensing layer and an antiferromagnetic layer that induces a vertical exchange bias in the magnetization of the sensing layer, in order to adjust the exchange bias coupling strength.

14. The magnetoresistive sensor according to any one of claims 1 to 3, characterized in that the geometry and dimensions of the vortex sensing layer are selected such that the vortex configuration of the sensing layer does not disappear over an applicable magnetic field range to be detected of at least + or -200 mT.

15. A detection device comprising a plurality of magnetoresistive sensors according to any one of claims 1 to 3, electrically coupled in series and / or parallel.

16. A sensing device comprising four magnetoresistive sensors according to any one of claims 1 to 3, wherein the four magnetoresistive sensors are arranged according to a Wheatstone bridge configuration, and the reference layers of two sensors located on two opposing branches of the bridge are magnetized in a first out-of-plane direction, while the reference layers of two magnetoresistive sensors located on the other two branches of the bridge are magnetized in the opposite out-of-plane direction.

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