Magnetoresistive sensor sensitive to out-of-plane magnetic fields

Abstract

The object of the invention is a magnetoresistive sensor (100) sensitive to a magnetic field applied out of plane, the magnetoresistive sensor (100) comprising:-a sensing layer (106); -a reference layer (104) having a fixed magnetization, the direction of said fixed magnetization being perpendicular to the plane of said reference layer; -a non-magnetic spacer layer (105), the non-magnetic spacer layer (105) separating the sensing layer (106) from the reference layer (104); the sensing layer (106) spontaneously has a magnetized vortex structure in the absence of the applied magnetic field, the vortex core diameter varying in the presence of the applied magnetic field perpendicular to the plane of the reference layer (104).

Description

Technical Field

[0001] The present invention belongs to the field of spin electronics and relates to a magnetoresistive sensor sensitive to the out-of-plane component of a magnetic field. For ease of description, the out-of-plane component of the magnetic field will be referred to as the out-of-plane magnetic field or the magnetic field perpendicular to the plane. Background Art

[0002] Spintronic magnetic field sensors are widely used in automotive applications, industrial applications or biomedical applications. (PP Freitas, R. Ferreira and S. Cardoso, 2016, "Spintronic Sensors," in 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 magnetoresistive sensors, superconducting quantum interference devices (SQUIDs), or fluxgates due to their combination of high sensitivity, compact size, low power consumption, compatibility with complementary metal oxide semiconductor (CMOS) integration, and low cost.

[0004] Spintronic sensors convert magnetic field changes into voltage changes or resistance changes under direct current (dc) or alternating current (ac) bias current. This resistance change is based on the giant magnetoresistance (GMR) effect in spin-valve (SV) or tunnel magnetoresistance (TMR) in magnetic tunnel junction (MTJ). Both magnetoresistive structures show that their resistance changes with the relative orientation between the fixed magnetization reference layer (RL) and the sensing layer (SL) magnetization.

[0005] It is well known that vortex-based magnetoresistive sensors exhibit high saturation fields, which constitute an attractive feature for applications with a 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] For some applications such as speed sensors or current sensors, a larger linear range is an important feature because it reduces the required accuracy of the amplitude of the field produced by the bias magnet on the sensor. However, this is also accompanied by a reduction in sensor sensitivity.

[0007] The stability of the vortex structure is due to the minimization of the total energy of the system, which includes the exchange energy due to the inhomogeneity of the vortex structure, the static magneto-crystalline anisotropy energy from the dipolar interaction, and the Zeeman energy (when an external field is applied). For certain geometries, the base energy state of the nanoscale cylinder tends to the vortex structure, 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) (Metlov, KL and Lee Y. Map, 2008, "Metastable map for thin circular magnetic nanocylinders", "Applied Physics Express" 92, 112506 (2008))). In such a geometry, the power of the static magnetoenergy exceeds the exchange energy, resulting in a vortex structure with an out-of-plane magnetized vortex core corresponding to the in-plane curling of the magnetic moment that achieves in-plane flux closure, to reduce the exchange energy cost that would otherwise be generated by the magnetic singularity at the center of the cylinder. In the usual case used in prior art sensors, the thickness of the cylinder is much smaller than its diameter. In this case, the diameter of the vortex core is defined by The exchange length is given. For Permalloy (Ni 80 Fe 20 ), cobalt, iron and other commonly used materials, this length is usually on the order of 5 nanometers (nm). Therefore, the area of ​​the vortex core is a very small fraction of the area of ​​the disk. For example, for a disk with a diameter of 1 micron, the ratio of the core area to the disk area is (5 / 1000) 2 =2.5 10 -5 of magnitude.

[0008] Prior art vortex-based sensors are sensitive to the in-plane component of the magnetic field. Throughout this specification, "in-plane" means parallel to the plane of the sensor's stack of layers, while "out-of-plane" or "perpendicular to the plane" means along the normal to that plane. Figure 1 A magnetic stack structure 1 of such a prior art sensor is shown.

[0009] The magnetic stack 1 comprises in sequence:

[0010] - Seed layer 2;

[0011] - Pinning synthetic antiferromagnetic structure 3;

[0012] - a spacer 4, which may be a tunnel barrier when the sensor is a magnetic tunnel junction, or a non-magnetic metal spacer when the sensor is a metal spin valve;

[0013] - a sensing layer 5 which spontaneously lies in a magnetic vortex structure in the absence of an applied magnetic field;

[0014] - Cover layer 6.

[0015] The pinned synthetic antiferromagnetic structure 3 comprises in sequence:

[0016] - an antiferromagnetic layer (AF) 7, which is usually based on an iridium-manganese (IrMn) alloy or a platinum-manganese (PtMn) alloy (e.g. Ir 20 Mn 80 or Pt 50 Mn 50 );

[0017] - ferromagnetic layer (ferromagnetic, FM) 8;

[0018] - a spacer layer 9 which provides an antiferromagnetic interlayer coupling (so-called RKKY coupling) between two adjacent magnetic layers, the spacer layer being made of ruthenium (Ru), for example;

[0019] - Reference layer 10.

[0020] The reference layer 10 is part of a synthetic antiferromagnet (SAF). The SAF includes two layers 8 and 10 separated by a metal spacer 9 and antiferromagnetically coupled. The two layers 8 and 10 are respectively considered as the reference (or polarizer) layer 10 and the pinned layer 8. The SAF is associated with an antiferromagnetic pinned 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 above the antiferromagnetic pinned layer 7. The form (AF / FM / Ru / reference) of the pinned synthetic antiferromagnetic structure (AF / SAF) is generally used as a pinned reference electrode, wherein the layer FM 8 is a ferromagnetic material such as cobalt or an alloy mainly based on iron (Fe), cobalt (Co), nickel (Ni). When the sensor is a metal spin valve with a non-magnetic metal spacer separating the reference layer from the sensing layer, the entire stack generally has a face centered cubic (fcc (111) texture). When the sensor is a magnetic tunnel junction, the spacing between the reference layer and the sensing layer is an oxide tunnel barrier, which is most commonly made of a body-centered cubic (bcc) (001) structure of magnesium oxide (MgO). In this case, in order to grow a four-fold symmetric bcc (001) oxide tunnel barrier on a three-fold symmetric fcc (111) AF / SAF structure, the reference layer 10 is usually composed of a composite three-layer, which includes in sequence:

[0021] - a Co layer or a cobalt iron (CoFe) layer, which is in contact with a spacer layer 9 of Ru or iridium (Ir), causing an antiparallel RKKY (Ruderman-Kittel-Kasuya-Yoshida) coupling;

[0022] - a nanocrystalline layer or an amorphous layer made of a material such as tantalum (Ta), tungsten (W), molybdenum (Mo) or hafnium (Hf), which is capable of ensuring a structural transition between an fcc structure and a bcc structure while absorbing an amorphous element (e.g., boron (B)) from an adjacent layer and providing interlayer ferromagnetic coupling,

[0023] - an amorphous cobalt iron boron (CoFeB) alloy in contact with the tunnel barrier 4. This amorphous CoFeB layer crystallizes during annealing at temperatures above 250 degrees Celsius (° C.) into the same crystal structure as the tunnel barrier, resulting in large tunnel magnetoresistance (TMR) values ​​known to those skilled in the art.

[0024] The synthetic antiferromagnetic (SAF) structure allows to reduce the stray field (strayfield) applied by the hard reference stack to the sense layer and provides greater magnetic stability for the reference layer. The ferromagnetic reference layer has an in-plane magnetization and is exchange coupled with the 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 to the in-plane direction by annealing the stack under an externally applied magnetic field in that specific direction and cooling it down from above the blocking temperature of the antiferromagnetic layer to saturate the ferromagnetic layer magnetization in that desired direction.

[0025] In vortex-based sensors sensitive to in-plane fields, an almost linear variation of resistance with field is observed due to lateral vortex core motion in a direction perpendicular to the direction of the in-plane field to be sensed, within a certain range of fields limited by the vortex annihilation field. Current state-of-the-art magnetoresistive sensors with vortex structures in the sensing layer are attractive due to their linear R(H) characteristics and the stability of the vortex structure.

[0026] Figure 2 The operating mechanism of a prior art vortex sensor is depicted. Figure 2 The magnetic response of the vortex sensing layer along the x-axis (i.e., the direction of the magnetization of the pinned reference layer) is shown as a function of the 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 structure with a core positioned at the center of the disk (assuming no stray fields from the reference stack). Under a given non-zero in-plane field H (point B), the vortex magnetization is polarized along the field direction by the vortex core moving laterally in a direction perpendicular to the applied field direction. Figure 2 In the example, the field is applied in the x-direction, causing the vortex core to move in the y-direction by a distance Δd due to the balance between the Zeeman energy, the exchange energy, and the magnetostatic energy. The larger the field, the more the vortex core moves. This produces a linear M(H) characteristic and an R(H) characteristic until a certain characteristic field, called the annihilation field (Ha), where the vortex structure is completely annihilated and a single-domain micromagnetic structure (A) is formed. On the way back from saturation, the vortex moves in a field called the nucleation field (H n ) is formed again in the characteristic magnetic field.

[0027] A particular advantage of this known vortex-based sensor is the larger linear range (40 mT) compared to the linear range of single-domain elliptical sensors (5 millitesla (mT)), which represents an eight-fold increase in the sensing range, but a decrease in sensitivity by the same factor, as reported by Suess, D., Bachleitner-Hofmann, A., Satz, A., et al., "Topologically protected vortex structures for low-noise magnetic sensors with high linear range", Nature Electronics 1, 362-370, 2018.

[0028] A disadvantage of this type of sensor is its noise. This noise is caused by the vortex core being trapped in its lateral motion due to local fluctuations of the anisotropy. This may occur in particular in magnetic tunnel junctions due to the spatial variation of the interface anisotropy at the interface between the oxide tunnel barrier and the sensing layer. In fact, it is well known that this anisotropy is very sensitive to parameters such as the local oxygen concentration or the local boron concentration. Since in prior art vortex sensors the area of ​​the vortex core is very small (typically 5 nm in diameter), trapping may be very effective even for small defects of the order of a few nanometers (e.g. grain boundaries). This may cause an equivalent Barkhausen noise, which is known in domain wall propagation but is used here for vortex core propagation. This noise affects the signal-to-noise ratio of this type of sensor.

[0029] Additionally, these sensors are sensitive to the component of the field parallel to the pinning direction of the reference layer magnetization (ie, the in-plane direction).Therefore, there are limitations on the direction of measurement of the magnetic field.

[0030] In this context, it is an object of the present invention to provide a vortex-based magnetoresistive sensor capable of measuring out-of-plane components of a field and exhibiting an improved signal-to-noise ratio. Summary of the invention

[0031] To this end, the present invention proposes a magnetoresistive sensor that is sensitive to an externally applied magnetic field, the magnetoresistive sensor comprising:

[0032] - Sensing layer;

[0033] - a reference layer having a fixed magnetization, the direction of which is perpendicular to the plane of the reference layer;

[0034] - a non-magnetic spacer layer separating the sensing layer from the reference layer;

[0035] The sensing layer has a magnetized vortex structure in the absence of an applied magnetic field, and a vortex core diameter varies in the presence of an applied magnetic field perpendicular to the plane of the sensing layer.

[0036] Thanks to the invention, it is possible to sense magnetic fields oriented perpendicular to the plane of a sensing layer, which is in a vortex micromagnetic structure. The sensor principle is based on the expansion or contraction of an out-of-plane magnetized vortex core under the influence of the out-of-plane field to be sensed. By realizing such a sensing layer in a magnetic tunnel junction or spin valve comprising an out-of-plane magnetized reference layer, this expansion / contraction of the vortex core generates a magnetoresistive signal that can be used to measure the amplitude of the applied field to be sensed. The obtained magnetoresistive sensor shows a larger linear range and an improved signal-to-noise ratio compared to prior art vortex-based sensors.

[0037] The sensors according to the invention exhibit an almost linear variation of resistance with the magnitude of the out-of-plane field to be sensed. However, this linear variation is not associated with lateral motion of the vortex core as in prior art sensors, but rather with the expansion / contraction of the vortex core under the 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 greatly reduced, resulting in a higher signal-to-noise ratio than prior art vortex sensors.

[0038] In the present invention, as we will see further in the specification, the ratio of the thickness of the sensing layer divided by the transverse dimension of the sensor is advantageously much larger than in prior art vortex sensors. This makes the vortex core much wider than in prior art vortex sensors, where the diameter of the vortex core is given by the exchange length.

[0039] The sensor according to the invention may also exhibit one or more of the following properties, considered individually or in all technically possible combinations:

[0040] - the magnetoresistive sensor of the invention comprises means for applying a current through the sensor in a direction substantially perpendicular to the planes of the layers;

[0041] - The sensing layer is selected with an aspect ratio defined as the thickness of the sensing layer divided by the characteristic in-plane dimension of the sensing layer, the aspect ratio being 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 characteristic in-plane dimension of the sensing layer is its diameter. However, the invention is applicable to any type of shape that allows stabilizing the vortex structure of the sensing layer. This is also true for a square, a square with rounded corners, a slightly rectangular shape with possibly rounded corners (the long side does not exceed twice the short side), an ellipse with moderate ellipticity (the ratio between the major axis and the minor axis does not exceed 2), or any other shape that can stabilize the vortex state. For all these shapes, the sensing layer aspect ratio is defined by the ratio of its thickness to the shortest in-plane dimension. If the sensing layer has a disc shape, the characteristic dimension is its diameter. If the sensing layer is a square or close to a square, the characteristic dimension is the length of the side of the square. If the sensing layer is a rectangle, the characteristic dimension is the short side of the rectangle. For an ellipse, the characteristic dimension is the minor axis of the ellipse.

[0042] - the non-magnetic spacer layer is a tunnel barrier layer or a metal spacer;

[0043] - the reference layer is magnetically coupled to a synthetic antiferromagnetic layer with perpendicular anisotropy;

[0044] - the nonmagnetic 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 amorphous element, said first layer being in contact with the tunnel barrier layer;

[0045] - the sensing layer comprises at least one further layer of material suitable for: absorbing at least a portion of the amorphous elements present in the first layer and ensuring a structural transition between the layers comprised in the sensing layer;

[0046] - a material suitable for absorbing at least a part of the amorphous element is selected from the following materials: tantalum, molybdenum, tungsten, or hafnium or a mixture thereof;

[0047] - the sensing layer comprises one or more stacked oxide layers;

[0048] - the magnetoresistive sensor of the invention comprises an arrangement of one or more layers that avoids switching of the magnetization direction of the vortex cores;

[0049] - the reference layer is magnetically coupled to a layer made of a hard material with perpendicular anisotropy;

[0050] - the magnetoresistive sensor of the invention comprises an antiferromagnetic layer which induces a perpendicular exchange bias on the sensing layer, said antiferromagnetic layer being magnetically coupled to the sensing layer;

[0051] - the magnetoresistive sensor of the present invention comprises a non-magnetic spacer layer between the sensing layer and the antiferromagnetic layer, the non-magnetic spacer layer causing a perpendicular exchange bias on the magnetization of the sensing layer so as to adjust the exchange bias coupling strength;

[0052] The geometry and dimensions of the vortex sensing layer are chosen such that the vortex structure of the sensing layer is not annihilated within a range of at least +200 mT or -200 mT of the applied magnetic field to be sensed.

[0053] The invention also aims at a sensing device comprising a plurality of magnetoresistive sensors according to the invention, which are electrically coupled in series and / or in parallel.

[0054] The sensing device may include four magnetoresistive sensors according to the present invention, which are arranged according to a Wheatstone bridge structure and configured so that the reference layers of two sensors located at two opposite branches of the bridge are magnetized along a first out-of-plane direction, while the reference layers of two magnetoresistive sensors located at the other two branches of the bridge are magnetized along an opposite out-of-plane direction.

[0055] Other characteristics and advantages of the invention will clearly emerge from the following description given for illustrative and in no way limiting purposes, with reference to the accompanying drawings, in which:

[0056] - Figure 1 A vortex sensor according to the prior art is shown;

[0057] - Figure 2 Shows Figure 1 The operating mechanism of the prior art vortex sensor;

[0058] - Figure 3 A first embodiment of a sensor according to the invention is schematically shown;

[0059] - Figure 4 Shows Figure 3 The operating mechanism of the vortex sensor in

[0060] - Figure 5 Shows Figure 3 A detailed embodiment of a stack of sensors in;

[0061] - Figure 6 Shows Figure 5 Dependence of the resistance of the sensor in on the external magnetic field applied perpendicular to the plane of the magnetic stack;

[0062] - Figure 7a shows a cross-sectional view of magnetization reversal performed by micromagnetic simulation on a magnetic element with a diameter of 60 nm, a thickness of 60 nm, and a magnetic saturation of 0.8 mA / m;

[0063] - Figure 7 b shows the Figure 7 Two-dimensional (2D) view of the Mz component of the uppermost surface layer under the same external field applied in a;

[0064] - Figure 7 c shows a one-dimensional (1D) view of the Mz component of the uppermost surface layer at y = 0;

[0065] - Figure 8 shows the evolution of the magnetization Mz with the applied out-of-plane magnetic field Hz for sensing layers of different diameters D;

[0066] - Fig. 9 The electrical results of different nanopatterned sensors with different diameters are shown;

[0067] - Fig.10 shows the micromagnetic simulation of the inversion of a cylinder with a thickness of 60 nm and a diameter of 60 nm, where A ex =13×10 -12 J / m 2 (J / m 2 ) and α = 0.01;

[0068] - Fig.11 shows the evolution of the resistance of two sensors with similar diameters with the applied out-of-plane magnetic field, one sensor using nickel iron (NiFe) material (Ms=0.756 MA / m) as the sensing layer material and the other sensor using Co material (Ms=1.44 MA / m) as the sensing layer material;

[0069] - Fig.12 a and b show respectively:

[0070] o Schematic representation of the path followed by the magnetic vortex core of a prior art vortex sensor during displacement under an externally applied field in the plane of the layers and in the presence of an interface defect and its subsequent magnetization response; and

[0071] o Schematic representation of the expansion followed by the magnetic vortex core of the vortex sensor of the invention under an externally applied field perpendicular to the plane of the layers and in the presence of an interface defect and its subsequent magnetization response;

[0072] - Fig.13 a shows the experimental hysteresis behavior observed in the sensor due to the change in vortex core polarity.

[0073] - Fig.13 b shows the non-hysteretic behavior in the sensor of the present invention, which is attributed to the presence of stray fields from the incompletely compensated SAF structure, which stabilizes the vortex core polarity in a specific direction;

[0074] - Fig.14 a and b show a schematic representation and a detailed representation, respectively, of a magnetoresistive sensor according to the invention having an uncompensated magnetic reference layer magnetized in a direction perpendicular to the plane of the multilayer;

[0075] - Fig.15 Schematic and detailed representations, respectively, of a magnetoresistive sensor of the present invention having a compensated magnetic reference layer and a top exchange bias layer magnetized in a direction perpendicular to the planes of the layers;

[0076] - Fig.16 A sensing device of the invention is shown in a Wheatstone bridge configuration, the sensing device comprising four elementary magnetoresistive sensors of the invention. DETAILED DESCRIPTION

[0077] Figure 3 A first embodiment of a sensor 100 according to the invention is schematically shown.

[0078] The sensor 100 is a magnetic stack, which comprises in sequence:

[0079] - seed layer 101;

[0080] - Synthetic antiferromagnetic structure 102;

[0081] - structural damage layer 103;

[0082] - reference layer 104;

[0083] - a spacer 105, which may be a tunnel barrier when the sensor is a magnetic tunnel junction, or a non-magnetic metal spacer when the sensor is a metallic spin valve;

[0084] - a sensing layer 106 which spontaneously lies in a magnetic vortex structure in the absence of an applied magnetic field;

[0085] - Cover layer 107.

[0086] The above magnetic stack has the form of a cylinder whose cross section advantageously has a circular form.

[0087] As described above, the sensing layer 106 has a magnetized vortex structure. In addition, the sensing layer 106 exhibits an aspect ratio (i.e., thickness divided by the in-plane characteristic dimension (here, diameter)) of between 0.2 and 2, and advantageously, between 0.2 and 1. The aspect ratio of the sensing layer in this sensor is closer to 1 than in the prior art vortex sensors. Therefore, the vortices here are more confined to a narrow cylinder than in the prior art in-plane sensitive sensors. Unexpectedly, due to this confinement, the vortex core becomes much larger than that in the prior art vortex sensor. In addition, as we refer to Figure 4 It will be seen that when an out-of-plane field is applied to the device, the vortex core diameter varies dramatically, which is not the case in prior art vortex sensors.

[0088] The reference layer 104 has a fixed single domain magnetization perpendicular to the plane of the reference layer 104. Figure 3 As shown, the reference layer RL is most often used with a synthetic antiferromagnet ("SAF") 102. However, the present invention does not necessarily require a SAF.

[0089] The sensing layer 106 in the vortex structure is part of a magnetoresistive element. The magnetoresistive element can be a tunnel junction or a giant magnetoresistive metal stack. In both cases, the element includes a top electrode and a bottom electrode so that a current can flow through the device in a direction roughly perpendicular to the planes of the layers. As is well known to those skilled in the art, the requirements for the properties of the layers are different in a magnetic tunnel junction and in an all-metal giant magnetoresistive stack. In fact, in a tunnel junction, there are different crystal structures in the stack (body-centered (bcc) near the tunnel barrier and face-centered cubic (fcc) at the bottom and top of the stack); while in a giant magnetoresistive stack, the structure can remain fcc throughout the stack.

[0090] Compared with the prior art, Figure 4 Describes Figure 3 The operating mechanism of the sensor 100 in FIG.

[0091] 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, the sensor 100 of the present invention does not rely on the lateral in-plane movement of the vortex core when the in-plane field is applied, but relies on the expansion / contraction of the vortex core of the sensing layer 106 under the out-of-plane field Hz (i.e., along the z-axis, the Oxy plane is the plane of the different layers of the sensor).

[0092] exist Figure 4The transfer curve of FIG. 1 depicts the variation of the magnetization response Mz of the sensing layer 106 along the z-axis with the out-of-plane component Hz of the applied magnetic field, which shows the variation of the magnetization intensity Mz of the sensing layer 106 with the z-axis component of the applied magnetic field Hz. As for the prior art magnetoresistive sensor, the transfer curve shows: a linear response region and two characteristic fields along both the rising branch and the falling branch thereof; an annihilation field Ha, in which the vortex core expands to the entire sensing layer area, so that the micromagnetic structure becomes a single domain that is almost saturated in the out-of-plane direction; and a nucleation field Hn, in which the vortex is formed again when the field drops from saturation.

[0093] As mentioned above, in the prior art vortex sensors, the sensing layer diameter is usually large, in the order of one or several microns, at least one order of magnitude larger than the thickness of the sensing layer. Therefore, the vortex core is very narrow, having a diameter of a few nanometers given by the exchange length. On the other hand, advantageously, according to the present invention, in a cylinder with a smaller diameter, the vortex core diameter becomes wider as the diameter becomes closer to the film thickness. If a field is applied in an out-of-plane direction parallel to the vortex core magnetization, the core diameter tends to increase: this is shown by state B compared to state C where no field is applied. In state B, assuming that the magnetization direction of the vortex core is downward along the z-axis (i.e., the z component of the magnetization of the vortex core is negative), the component Hz is parallel to the vortex core magnetization 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 where no field is applied. On the contrary, if a field is applied antiparallel to the nuclear magnetization, the diameter of the core decreases. It should also 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 annihilation field Ha, where, for example, Figure 4 As shown in state A, the vortex core extends to the entire sensing layer area. We will return to this issue later in the description to show how to avoid this annihilation and keep the sensor in its linear response.

[0094] If the sensing layer 106 is implemented in a spin valve or magnetic tunnel junction including an out-of-plane magnetized reference layer 104, these changes in the vortex core diameter of the sensing layer 106 produce resistance changes within the field range defined by the positive vortex annihilation field and the negative vortex annihilation field. Based on the resistance changes, the value of the corresponding out-of-plane component of the applied magnetic field can be obtained.

[0095] Figure 5 A first possible embodiment of the vortex sensor of the invention is shown, wherein the sensing layer is inserted into a magnetic tunnel junction, the reference layer of which is magnetized in the out-of-plane direction.

[0096] If the device is a magnetic tunnel junction, the tunnel barrier 105 is an oxide layer or a nitride layer, such as magnesium oxide (MgO), aluminum oxide (AlOx), titanium oxide (TiOx) or aluminum nitride (AlN), titanium nitride (TiN), but preferably MgO which is known to provide a large TMR signal. In order to obtain a large TMR with MgO (large TMR generally means TMR above 80%, preferably above 140%, preferably above 200%), the MgO layer and the two magnetic electrodes sandwiched between the MgO layers must have a crystalline bcc structure with a (100) structure. As is well known to those skilled in the art, this can be achieved by using an iron cobalt boron (FeCoB) layer as the magnetic electrode, which is amorphous when deposited, but can be recrystallized with a bcc structure when annealed at a temperature generally in the range of 250°C to 400°C.

[0097] In order for this recrystallization to occur, the B element or more generally the amorphous element contained in the alloy based on iron-cobalt (FeCo) must be expelled from the alloy. For this purpose, a thin layer of a material capable of absorbing this amorphous element is deposited next to the FeCo-based layer at its interface opposite to the interface with the MgO layer. This amorphous element absorbing layer can be made of Ta, W, Mo or Hf, for example.

[0098] Taking the above factors into account, the vortex sensing layer 106 may first include a thin layer based on FeCoB in contact with the MgO barrier 105. In this layer, B is an amorphous element. B can be replaced by other amorphous elements such as zirconium (Zr) or niobium (Nb). The first layer typically has a thickness in the range of 1.5nm to 5nm. Then, the FeCoB is in contact with a thin layer a few angstroms thick that can absorb amorphous elements when the structure is annealed. The layer can be made of W, Mo, Ta, for example. The sensing layer 106 also includes a magnetic layer preferably made of a soft magnetic material such as permalloy NiFe (with a composition close to Ni80Fe20), or a soft FeCo layer such as Co90Fe10 or a FeCoB alloy, so as to minimize the hysteresis of the layer when the micromagnetic structure of the layer changes under the influence of the field to be sensed. The sensing layer 106 can also be a multilayer including several such layers. The sensing layer 106 may also include thin stacks (thickness in the order of 0.1 nm to 0.5 nm) of non-magnetic metals such as W, Mo, Ta, etc., in order to absorb any amorphous elements or disrupt 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, tantalum oxide (TaOx), AlOx, TiOx, etc., in order to induce some interface perpendicular anisotropy in the sensing layer, thereby allowing a slight reduction in the thickness of the sensing layer. The product of the resistance and the area of ​​these oxide stacks (RA) should then be adjusted to a lower value than the RA of the main barrier providing the tunnel magnetoresistance signal. This will avoid excessive attenuation of the magnetoresistance signal due to the increase in resistance caused by the introduction of these oxide stacks.

[0099] As already explained, the total thickness of the sensing layer is advantageously chosen with an aspect ratio (thickness divided by the lateral dimension (here the diameter of the circular sensing layer)) between 0.2 and 2, so that its micromagnetic equilibrium structure is 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). An example of the dimensions of the sensing layer is a thickness of the order of 60 nm to stabilize the vortex formation at zero field with a diameter range between 50 nm and 200 nm (e.g. 60 nm).

[0100] Micromagnetic simulations can be used to calculate the thickness range that satisfies the condition for a vortex core structure where the vortex core diameter is at least twice the exchange length.

[0101] With respect to the reference layer 104, its magnetization is fixed in a direction perpendicular to the stack of magnetic layers. Since the measured magnetoresistance variation depends on the variation of the perpendicular magnetization Mz of the vortex sensing layer 106, this perpendicular structure of the reference layer 104 is required. If the stack is a MgO-based magnetic tunnel junction, the reference layer 104 is preferably made of a FeCoB alloy in contact with the MgO barrier 105. The magnetization of the reference layer 104 remains fixed out of the plane by the interfacial anisotropy present at the FeCoB / MgO interface and by exchange coupling with out-of-plane magnetization multilayers such as (Co / platinum (Pt)) multilayers. Other multilayers such as Co / X (where X is palladium (Pd), Ni or Ir) are well known by those skilled in the art for providing larger perpendicular magnetic anisotropy (PMA) values. Alloys such as CoPt ordered alloys, CoPd ordered alloys or FePt ordered alloys or FePd ordered alloys or some rare earth transition alloys such as TbCo, TbFe, etc. can also provide strong PMA. This PMA multilayer or alloy itself can be part of a synthetic antiferromagnetic SAF layer 102, which is composed of two antiferromagnetically coupled multilayers, such as (Co / Pt) multilayers separated by a non-magnetic spacer, usually Ru or Ir, and the thickness of the non-magnetic spacer is adjusted to provide a strong antiferromagnetic coupling between two adjacent multilayers (for Ru spacers, typically 0.8nm). This type of arrangement is well known to those skilled in the art. It is also well known that this SAF structure can include more than two antiferromagnetically coupled multilayers to further reduce the net stray field applied by the SAF and reference layers on the sensing layer. Since these PMA multilayers have an fcc structure, and FeCoB is bcc after annealing, a thin structural transition layer 103, such as made of Ta, Mo or W, must be introduced again between the FeCoB layer and the PMA multilayer. This layer 103 is also intended to absorb amorphous elements from the FeCoB alloy upon annealing. In other words, a structurally disruptive layer 103 is introduced between the reference layer 104 formed of FeCoB in contact with the barrier 105 and the rest of the SAF 102 to ensure a structural transition between the FeCoB layer, which must be body-centered cubic (bcc with 4-fold symmetry) in the final device, and the rest of the SAF, which generally has an fcc structure with (111 structure) and 3-fold symmetry.

[0102] In addition, the SAF layer 102 is grown on the seed layer 101 to promote a (111) structure. The seed layer 101 may be a multilayer in the form of Ta / Pt. The material used in the seed layer 101 may also be used for the cap layer 107.

[0103] Figure 6The experimental results show that the resistance of the sensor changes with the direction perpendicular to the Figure 5 Shown is the correlation of the plane of the sensor's magnetic stack as the applied external magnetic field varies. Figure 6 It is shown that a linear range of up to greater than 200 mT is achievable, which is much larger than the reported linear range (40 mT or 80 mT) of prior art vortex sensors developed for in-plane field sensing.

[0104] Figure 7 a shows Figure 5 Cross-sectional view of the evolution of the sense layer magnetization under an out-of-plane field as qualitatively described in . Figure 7 The micromagnetic structure shown in a was obtained by micromagnetic simulation using the micromagnetic simulation software MuMax3. The simulation was performed on a cylinder with a thickness of 60 nm and a diameter of 60 nm, which had the following sensing layer parameters:

[0105] -Saturation magnetization: M sat =800×10 3 Ampere / meter (A / m),

[0106] -Exchange stiffness: A ex =13×10 -12 J / m 2 .

[0107] Figure 7 b represents a 2D xy view of the top magnetization during the reversal process. The grid in the simulation has a cell size of 1 nm. At H = 0 Tesla (T), the vortex core is stable, with a core magnetized in the downward direction. By increasing the field (see snapshots at 0.125 T and 0.25 T), the negatively magnetized core gradually shrinks and concomitantly, the in-plane curled magnetization cells become increasingly polarized toward the out-of-plane applied field direction. Finally, the vortex is annihilated and a single domain is observed above H = 0.5 T or below H = -0.5 T.

[0108] Figure 7 c shows the change of the magnetization saturation value Mz in the vertical direction of the uppermost surface layer (1 nm unit) along the diameter of the cylindrical sensing layer.

[0109] like Figure 8 The characteristic nucleation and annihilation fields can be controlled by varying the sensing layer diameter, as shown by micromagnetic simulations of Figure 5 Evolution of the magnetization Mz with the applied out-of-plane magnetic field Hz for sensing layers with different diameters D (D is equal to 40nm, 60nm, 80nm and 100nm respectively) with the same material parameters. Figure 8It is shown that for an element with a diameter of 40 nm, the reversal between the two single domain states is abrupt, since the shape anisotropy (the thickness of the sensing layer is greater than the diameter of the sensing layer) favors the single domain state along the longest dimension of the magnetic element (here its thickness). However, for diameters of 60 nm or more, vortex formation is observed and the linear region in which the vortices are stable under an external field expands with increasing diameter of the sensing element. In view of the foregoing, the diameter of the sensing layer of the sensor according to the invention (i.e. the characteristic length) 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 allows increasing the range of the linear part of the response while reducing the slope of the linear part of the response. This diameter dependence allows creating sensor devices with a larger linear range simply by increasing the diameter of the sensor. Furthermore, it is possible to Figure 8 It is further seen in Figure 1 that there is hysteresis in the response of Mz to Hz, which manifests itself as two parallel linear responses separated by a constant magnetization difference. Advantageously, the sensor according to the invention will be used for one or the other of these two parallel linear responses. We will see later in the description how to avoid switching from one linear section to the other for a single sensor according to the invention.

[0110] Figure 8 The simulations reported in Fig. 9 This is a confirmation of the experimental results plotted in , which show the electrical results for different nanopatterned sensors with different diameters. Fig. 9 The resistance of the sensor is shown as a function of the component Hz of the applied magnetic field. The different curves confirm the larger linear range predicted by the simulation for devices with smaller resistance (corresponding to larger diameters). The multilayer stack components of the sensor deposited by direct current (DC) magnetron sputtering at room temperature are shown in Figure 2. Figure 5 As shown:

[0111] Ta3 / Pt30 / 4×[Co0.5 / Pt0.25] / Co0.5 / Ru0.9 / 3×[Co0.5 / Pt0.25] / Co0.5 / Ta0.2 / FeCoB1.1 / MgO1.25 / FeCoB1.4 / Ta0.2 / NiFe58 / Ta1 / Pt3.

[0112] The samples were then annealed at 300°C for 10 minutes. From the highest resistance junction to the lowest resistance junction (meaning from smaller diameter to larger diameter), Fig. 9 The annihilation field represented by gray dots gradually increases from 53mT to 178mT.

[0113] The characteristic nucleation and annihilation fields can also be controlled by varying the saturation magnetization Ms of the sensing layer. Fig.10Micromagnetic simulations of the inversion of a cylinder with a thickness of 60 nm and a diameter of 60 nm are shown, where A ex =13×10 -12 J / m 2 and α = 0.01. It is observed that the linear range corresponding to the vortex stabilization region can also be adjusted by modifying the saturation magnetization of the sensing material. Increasing the saturation magnetization Ms of the sensing layer allows increasing the range of the linear part of the response (by decreasing the slope of the linear part).

[0114] For those with Figure 5 When comparing two sensor devices (one sensor device uses NiFe material (Ms=0.756 MA / m) as the sensing layer material, and the other sensor device uses Co material (Ms=1.44 MA / m) as the sensing layer material), the electrical diameter of the sensor device is similar to that of the embodiment proposed in FIG. 1 (52 nm and 58 nm, respectively). As shown in FIG. Fig.11 As observed in , the experiments confirm that the linear range increases with Ms. For the case where the sensing layer is made of cobalt, very large fields up to more than 200 mT can be achieved without annihilating the vortex state.

[0115] Another advantage of the vortex sensor according to the present invention is its reduced hysteresis compared to the prior art vortex sensors. In fact, in the prior art vortex sensors sensitive to the in-plane field, the presence of randomly distributed defects at the interface between the vortex sensing layer and the tunnel barrier changes the vortex core displacement path, because the vortex core will be randomly trapped in the energy well associated with such defects. This effect is particularly evident in the prior art vortex sensors, because the vortex core is very small (exchange length of about 5nm), making it easy for the vortex core to be trapped by such small defects. In the magnetic tunnel junction, these defects can be attributed to the spatial variation of the oxygen concentration or the boron concentration at the MgO / FeCoB interface, or to the diffusion of a third element (such as Ta, manganese (Mn), W or Mo) present in the stack to the MgO interface. These defects can cause local variations in the anisotropic energy, which then act as trapping centers for the vortex core. Fig.12a shows a plan view of a prior art sensing layer including a defect 30. When a vortex core 31 moves in a first displacement direction 32, the vortex core is trapped in a defect 30 encountered in the displacement path (and then released from the defect). Similarly, a vortex core 31 moving in a second displacement direction 33 is trapped in a defect 30 encountered in the displacement path (and released from the defect). Depending on the position of the vortex core when it is nucleated, the vortex core 31 may be trapped in a different defect 30 when moving along the first displacement direction 32 than when moving along the second displacement direction 33 and released from a different defect 30 when moving along the second displacement direction 33. Due to this entrapment into / release from different defects, the vortex core 31 may follow different paths in the first displacement direction and the second displacement direction. In this prior art sensor, the vortex core follows different paths in the first displacement direction and the second displacement direction, resulting in different magnetization distributions of the magnetization of the vortex core, and therefore, the resistance of the magnetic sensor at each value of the externally applied in-plane magnetic field H is different.

[0116] Due to this trapping, the transfer curve (of Mx versus Hx) may differ depending on the path followed, resulting in a different resistance of the sensor at each value of the external field, as shown in European Patent No. 19315026.5 of 2020 by S. Dounia, C. Baraduc and B. Dieny.

[0117] In contrast, the sensor according to the invention is much less sensitive to the presence of such defects and their trapping. In fact, the vortex core does not move laterally in the sensor of the invention: the vortex core only expands or contracts when the curled magnetization in the outer part of the device is reversibly pulled out of the plane or back into the in-plane direction under the influence of the out-of-plane field to be sensed. Fig.12 As schematically depicted in b, the vortex deformation of the sensor of the present invention under a magnetic field is more reversible than in prior art vortex sensors. This results in a significant reduction in noise in the sensor of the present invention.

[0118] In an embodiment of the sensor of the invention, the reference layer and the associated SAF layer may be compensated, which means that they apply no field or only a very small field (i.e. a field that is negligible relative to the field required to reverse the polarity of the vortex core). In this case, a possible disadvantage of the sensor of the invention may be that the polarity of the vortex core changes when a magnetic field greater than the annihilation field is applied in a direction opposite to the polarity of the vortex core. This change in the polarity of the vortex will produce effects such as Figure 8 As has been observed in Fig.13Unwanted hysteresis shown in a. If the sensor is always operated with the same field polarity (small circle shown in black), this hysteresis will never appear. However, if the vortex core polarity changes, a certain difference (ΔR) may appear between the resistance of the two states (A, B) at zero field. This resistance difference (ΔR) can be clearly observed if the entire cycle is followed (0.04T to -0.04T). If one starts with a positive field greater than the annihilation field (H>+Ha) and reduces the field to zero, the vortex core exhibits a positive polarity (A, vortex core up). In contrast, if one starts with a negative field greater in absolute value than the annihilation field and reduces the field to zero (see Fig.13 If the vortex core is turned down (grey line in a), the vortex polarity is reversed (B, vortex core pointing downward). This results in the resistance difference ΔR observed at zero applied field.

[0119] This problem can be avoided by limiting the range of fields to which the sensor is exposed. The field value should be kept below the vortex annihilation field. However, in this case, there is still a risk of unexpected switching of the vortex core polarity if the sensor is accidentally exposed to a field that is too large.

[0120] In order to better circumvent this problem of unexpected switching of vortex core polarity, a second embodiment is proposed, in which the reference layer is coupled to an associated PMA multilayer or SAF layer, the reference layer and the PMA multilayer (or the reference layer and the SAF layer, respectively) being uncompensated. As a result, these layers generate an out-of-plane stray field on the sensing layer magnetization. Due to this out-of-plane stray field, vortices always re-nucleate with the same polarity, regardless of the saturation field direction (positive or negative). This produces a phenomenon such as Fig.13 b The best response is shown in Figure 2. It is noteworthy that this stray field translates into only a weak asymmetry in the annihilation field with respect to the opposite field, which can be explained by the larger volume of the sensing layer.

[0121] exist Fig.14 Such a sensor 200 with an uncompensated reference layer and a PMA multilayer is schematically shown in a and Fig.14 More details are shown in b.

[0122] The sensor 200 is a magnetic stack, which comprises:

[0123] - seed layer 201;

[0124] - A multilayer 202 of perpendicular magnetic anisotropy PMA material;

[0125] - structural damage layer 203;

[0126] - reference layer 204;

[0127] - a spacer 205, which may be a tunnel barrier when the sensor is a magnetic tunnel junction, or a non-magnetic metal spacer when the sensor is a metallic spin valve;

[0128] - a sensing layer 206 which spontaneously lies in a magnetic vortex structure in the absence of an applied magnetic field;

[0129] - Cover layer 207.

[0130] In addition to the PMA multilayer 202, the other layers 201, 203, 204, 205, 206 and 207 may be Figure 3 and Figure 5 The sensors in the diagram are the same as shown in the layers.

[0131] As mentioned above, in this case, the reference layer does not have to be Figure 3 and Figure 5 ) is not coupled to the SAF layer as in . Instead, the reference layer 204 is coupled to a single high PMA material 202. Such a high PMA material can be made of Co / Pt or Co / X repetitions, where X represents Pd, Ni or Ir, which are known to provide large PMA values. Alloys such as CoPt ordered alloys, CoPd ordered alloys, or FePt ordered alloys or FePd ordered alloys, or some rare earth transition alloys such as TbCo, TbFe, etc. can also provide strong PMA. The number of repetitions in the (Co / Pt) multilayer or the thickness of the alloy can be adjusted to provide sufficient stray fields so that vortices always nucleate with the same polarity, but not too high so that the response remains symmetrical when a positive or negative field is applied to the sensor. For the (Pt / Co) multilayer, a number of repetitions n between 1 and 6 can be advantageously used in the PMA multilayer.

[0132] Another possible embodiment aimed at stabilizing the polarity of the vortex core includes coupling the sensing layer to an antiferromagnetic layer to provide an out-of-plane exchange bias to the vortex core. Fig.15 Such a sensor 300 is schematically shown in a and Fig.15 More details are shown in b.

[0133] The sensor 300 is a magnetic stack, which comprises in sequence:

[0134] - seed layer 301;

[0135] -SAF layer 302;

[0136] - structural damage layer 303;

[0137] - reference layer 304;

[0138] - a spacer 305, which may be a tunnel barrier when the sensor is a magnetic tunnel junction, or a non-magnetic metal spacer when the sensor is a metallic spin valve;

[0139] - a sensing layer 306 which spontaneously lies in a magnetic vortex structure in the absence of an applied magnetic field;

[0140] - exchange bias layer 308;

[0141] - Cover layer 307.

[0142] In addition to the added exchange bias layer 308, the other layers 301, 302, 303, 304, 305, 306 and 307 may be connected with Figure 3 and Figure 5 The corresponding layers 101, 102, 103, 104, 105, 106 and 107 shown in the sensor are the same.

[0143] The exchange bias layer 308 is located above the vortex sensing layer 306 and in contact with the vortex sensing layer. The exchange bias layer 308 made of an antiferromagnetic material (AF) such as an IrMn alloy or a platinum manganese (PtMn) alloy, located at the interface of the sensing layer opposite to the tunnel barrier, can be used for this purpose. A unidirectional magnetic anisotropy appears at the interface between the ferromagnetic sensing layer 306 and the AF layer 308, which is caused by the exchange coupling between the two layers. In order to provide an exchange bias, the stack must be annealed above the antiferromagnetic layer blocking temperature and cooled down in a magnetic field with the field direction perpendicular to the plane of the layers. As is well known to 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 copper (Cu), W, Mo or Ta at the interface between the sensing layer 306 and the antiferromagnetic layer 308. Fig.15 The main advantage of the sensor in is that the individual vortex polarity stabilization is not caused by stray fields from the reference layer (which can be difficult to control in practice due to its variability with junction size), but only by the exchange between the antiferromagnetic layers grown in contact with the sensing layer. The use of such AF layers is quite common in spintronic sensors, since the reference layer of conventional sensors is usually pinned in the plane of the layers by exchange biasing with such AF structures.

[0144] Fig.16 An embodiment of a sensing device D is shown. D comprises a Wheatstone bridge arrangement with four magnetoresistive sensors R1 , R2, R3 and R4, each sensor being a sensor according to the invention.

[0145] The sensing device D in the Wheatstone bridge structure includes four basic sensors R1, R2, R3 and R4 of the present invention, which are configured so that the vortex core magnetization in the reference layer and the sensing layer of R1 and R4 is oriented in a first out-of-plane direction, while the vortex core magnetization in the reference layer and the sensing layer of R2 and R3 is oriented in the opposite out-of-plane direction.

[0146] 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 the vortex core magnetizations in their reference and sensing layers are oriented in a first out-of-plane direction, while R2 and R3 also have the same response, and the vortex core magnetizations in their reference and sensing layers are oriented in opposite out-of-plane directions.

[0147] A sense current, I 入 represents the current flowing into the bridge, and I 出 represents the outflow current. In this Wheatstone bridge structure D, the elementary sensors are configured so that under the applied magnetic field to be sensed, the resistance of two of these elementary sensors (here R1 and R4) located on opposite branches increases (decreases respectively), while the resistance of the other two sensors (in this example R2 and R3) decreases (increases respectively). Advantageously, this enables the measurement of the potential difference Vout between the two contacts C3 and C4, which is directly related to the external out-of-plane magnetic field to be sensed.

[0148] according to Fig.16 In order to configure these basic sensors so as to obtain an opposite response of the group of sensors R1 and R4 to 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, a relatively high (typically in the region of 100 Ω.μm) magnetic field is chosen. 2A magnetic tunnel junction with a resistance-area product (RA) of 100°C (or more) is formed. This allows the power consumption of the sensor to be reduced and the tunnel magnetoresistance amplitude to be increased. In addition, the reference layer is selected to have an uncompensated reference layer so that the stray field from the reference layer is large enough to keep the vortex core magnetization always parallel to the reference layer magnetization. Then, in order to initialize the Wheatstone bridge structure, an out-of-plane field is first applied that is large enough to fully saturate the magnetization of all layers in a first out-of-plane direction (e.g., upward). Current is then sent between contacts C1 and C3 and between contacts C4 and C2 to generate Joule heating in sensors R1 and R4. This Joule heating, which typically raises the temperature of the junction by 80°C to 200°C, reduces the coercive field of the reference layer and the coercive field of the vortex core magnetization in the two sensors R1 and R4. When the junction is heated, a field is applied with a direction opposite to the first direction of the initial saturation field, where the amplitude 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, which are kept at standby temperature. Finding such an intermediate field amplitude is possible because the coercive field in this type of sensor varies with temperature. As a result of this initialization process, the four magnetic sensors included in the bridge will eventually form a bridge as shown in FIG. Fig.16 The magnetic structure shown. 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 sensing layer in all junctions is parallel to its corresponding reference layer magnetization. Then, under the application of the field to be sensed, the vortex core of the sensor whose magnetization is parallel to the applied field will widen, while the vortex core of the sensor whose magnetization is anti-parallel to the applied field will shrink. For the former junction, this will cause the resistance of the junction to decrease, while for the latter, this will cause the resistance to increase. Therefore, the imbalance of the Wheatstone bridge will change significantly with the applied field, thereby producing the sensor output signal Vout.

[0149] In an alternative embodiment, the sensors R1, R2, R3, R4 may be formed by several individual sensors connected in series or in parallel. In this case, all individual sensors constituting one of the resistors Ri (i from 1 to 4) will be configured identically by applying a saturation field only in the upward direction or in the downward direction.

[0150] Of course, the sensor according to the invention is not limited to the embodiments that have just been described with reference to the accompanying drawings, for illustrative and in no way limiting purposes.

[0151] In all the above disclosed embodiments, the sensor according to the invention is a "bottom" structure, which means 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 a "bottom" structure; the sensor of the invention may instead be in a "top" structure. This means that the stack may be grown in a bottom pinned structure or in a top pinned structure, i.e. the sensing layer may be above or below the tunnel junction, while the reference layer is correspondingly below or above the tunnel barrier. Since the sensing layer in the sensor is preferably rather thick, it may be advantageous to grow it above the tunnel barrier so as not to create large roughness for the growth of the tunnel barrier.

[0152] Furthermore, although the detailed embodiments of the sensor according to the present invention are carried out for sensors based on magnetic tunnel junctions, it is known to those skilled in the art that the above embodiments can also be applied to the case of all-metal magnetoresistive sensors based on giant magnetoresistance. In this case, the tunnel barrier will be replaced by a non-magnetic metal spacer such as Cu. From a material point of view, these metal stacks are easier because all the materials involved can have an fcc structure, so there are fewer material issues to consider than in magnetic tunnel junctions, where some parts of the stack are fcc and other parts are bcc.

[0153] In addition, detailed embodiments have been disclosed assuming that the sensor of the present invention has a cylindrical shape with a circular cross-section, thereby characterizing the size of the sensing layer by the diameter and thickness of the sensing layer. However, the present invention will be applicable to any type of shape that allows the vortex structure of the sensing layer to be stabilized. This is also true for a square, a square with rounded corners, a slightly rectangular shape that may have rounded corners (the long side does not exceed twice the short side), an ellipse with moderate ellipticity (the ratio between the major axis and the minor axis does not exceed 2), or any other shape that can stabilize the vortex state. For all these shapes, the aspect ratio of the sensing layer is defined by the ratio of its thickness to the shortest in-plane dimension. If the sensing layer has a disc shape, the characteristic dimension is its diameter. If the sensing layer is a square or nearly a square, the characteristic dimension is the length of the square side. If the sensing layer is a rectangle, the characteristic dimension is the short side of the rectangle. For an ellipse, the characteristic dimension is the minor axis.

[0154] These devices can be manufactured using a very similar manufacturing process to prior art TMR sensors. The process involves the following steps: 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. During this process, it is well known that ion beam etching of magnetic materials often produces tapered pillars (especially in the present case), where high aspect ratio pillars are etched. The presence of this tapered shape is acceptable and does not hinder the working principle of the sensor.

[0155] In addition, as is well known to those skilled in the art, a plurality of sensors of the present invention may be connected in series and / or in parallel to improve the signal-to-noise ratio of the entire sensor.

Claims

1. A magnetoresistive sensor (100, 200, 300) sensitive to an externally applied magnetic field, comprising: - a sensing layer (106, 206, 306); - a reference layer (104, 204, 304) having a fixed magnetization, the direction of which is perpendicular to the plane of the reference layer; - a non-magnetic spacer layer (105, 205, 305) separating the sensing layer (106, 206, 306) from the reference layer (104, 204, 304); The sensing layer (106, 206, 306) has a magnetized vortex structure 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 layer (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 an in-plane characteristic dimension of the sensing layer, being between 0.2 and 2 inclusive.

3. The magnetoresistive sensor according to claim 2, characterized in that: The aspect ratio is between 0.2 and 1 included.

4. A magnetoresistive sensor according to any one of the preceding claims, characterized in that The nonmagnetic spacer layer is a tunnel barrier layer or a metal spacer.

5. Magnetoresistive sensor according to any one of the preceding claims, characterized in that The reference layer is magnetically coupled to a synthetic antiferromagnetic layer having perpendicular anisotropy.

6. Magnetoresistive sensor according to any one of the preceding claims, characterized in that The nonmagnetic spacer layer is a tunnel barrier layer, and the sensing layer includes a first layer made of an alloy based on iron, cobalt and an amorphous element, the first layer being in contact with the tunnel barrier layer.

7. The magnetoresistive sensor according to claim 6, characterized in that: The sensing layer comprises at least one further layer of material suitable for: absorbing at least a portion of the amorphous elements present in the first layer and ensuring a structural transition between the layers comprised in the sensing layer.

8. The magnetoresistive sensor according to claim 7, characterized in that: The material suitable for absorbing at least a portion of the amorphous element is selected from the following materials: tantalum, molybdenum, tungsten, or hafnium or mixtures thereof.

9. A magnetoresistive sensor according to any one of the preceding claims, characterized in that The sensing layer comprises a stack of one or more oxide layers.

10. A magnetoresistive sensor according to any one of the preceding claims, characterized in that The magnetoresistive sensor comprises an arrangement of one or more layers that prevents switching of the magnetization direction of the vortex cores.

11. Magnetoresistive sensor according to any one of the preceding claims, characterized in that The reference layer is magnetically coupled to a layer made of a hard material having perpendicular anisotropy.

12. Magnetoresistive sensor according to any one of the preceding claims, characterized in that The magnetoresistive sensor includes an antiferromagnetic layer that induces a perpendicular exchange bias on the sensing layer, the antiferromagnetic layer being magnetically coupled to the sensing layer.

13. The magnetoresistive sensor according to claim 12, characterized in that: The magnetoresistive sensor includes a nonmagnetic spacer layer between the sensing layer and the antiferromagnetic layer, the nonmagnetic spacer layer between the sensing layer and the antiferromagnetic layer causing a perpendicular exchange bias on the magnetization of the sensing layer so as to adjust the exchange bias coupling strength.

14. A magnetoresistive sensor according to any one of the preceding claims, characterized in that The geometry and dimensions of the vortex sensing layer are selected such that the vortex structure of the sensing layer is not annihilated within a range of at least +200 mT or -200 mT of the applied magnetic field to be sensed.

15. A sensing device, characterized in that: The sensing device comprises a plurality of magnetoresistive sensors according to any one of the preceding claims, the plurality of magnetoresistive sensors being electrically coupled in series and / or in parallel.

16. A sensing device according to claim 15, comprising four magnetoresistive sensors according to any one of claims 1 to 14, wherein the four magnetoresistive sensors are arranged according to a Wheatstone bridge structure and are configured so that the reference layers of two sensors located at two opposite branches of the bridge are magnetized along a first out-of-plane direction, while the reference layers of two magnetoresistive sensors located at the other two branches of the bridge are magnetized along an opposite out-of-plane direction.

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