Technical domain

[0001] The present disclosure relates to a magnetoresistive element for a magnetic sensor destined to sense an external magnetic field. More particularly, the present disclosure relates to a magnetoresistive element comprising a sense layer having a stable vortex configuration and having a negative temperature coefficient of TMR of the magnetoresistive element that is compensated.

Related art

[0002] Fig. 1 shows a cross-section view of a magnetoresistive element 2 comprising a ferromagnetic reference layer 23 having a reference magnetization 230, a ferromagnetic sense layer 21 having a free sense magnetization 210 and a tunnel barrier layer 22 between the sense and reference ferromagnetic layers 21, 23. The sense magnetization 210 can be oriented in an external magnetic field 60 while the reference magnetization 230 remain substantially undisturbed. The external magnetic field 60 can thus be sensed by measuring a resistance of the magnetoresistive sensor element 2. The resistance depends on the relative orientation of the sense magnetization and the reference magnetization.

[0003] A low measured resistance (Rp) is measured in the magnetoresistive element 2 when the sense magnetization 210 is oriented parallel to the reference magnetization 230. A high resistance (RAP) is measured in the magnetoresistive element 2 when the sense magnetization 210 is oriented antiparallel to the reference magnetization 230. The difference between the value of the high and low resistance (RAP - Rp) is also known as the tunnel magnetoresistance (TMR). [0004] The sense magnetization 210 can comprise a a stable vortex configuration providing a linear and non-hysteretic behavior in a large magnitude range of an external magnetic field. Such magnetoresistive element is thus advantageous for 1D magnetic sensor applications and can be easily configured to adjust its sensitivity.

[0005] Fig. 2 shows a hysteresis response (or magnetization curve) to the external magnetic field 60 (Hext, in arbitrary unit) on the sense magnetization 210 (M, in arbitrary unit). The full hysteresis loop of a vortex sense magnetization 210 is characterized by a linear increase of magnetization M with the applied magnetic field Hext until the vortex expulsion field is reached at the H _expi point. At this point the sense magnetization 210 becomes magnetically saturated. To recover the vortex state in the sensing layer 21, one needs to reduce the magnetic field below the nucleation field Hnuci- As long as the applied magnetic field is within the magnitudes corresponding to the expulsion field (+/-H _exp i) of the vortex in the sense magnetization 210, the hysteresis response to the external magnetic field 60 (H _ex t) comprises a reversible linear portion corresponding to the movement of a core of the vortex with the external magnetic field H _ex t. The values and the slope of the linear part of hysteresis loop are strongly dependent on the size of the sense layer 21. The linear and non- hysteretic portion of the magnetization curve facilitates the measurement of small variations of the external magnetic field H _ex t.

[0006] The vortex is characterized by its susceptibility /, which corresponds to the slope of the linear region of the M(H) loop:

X = 5M / 5H _ex t (1).

[0007] The sensitivity S of the magnetoresistive element 2 then will be proportional to the product between the susceptibility x and the tunnel magnetoresistance (TMR) of the magnetoresistive sensor element 2:

S = x*TMR (2). [0008] A drawback of such magnetoresistive element is the temperature dependence of the TMR and the magnetic susceptibility of the sense layer 21 on the temperature T. When the temperature T increases, the sense magnetization 210 decreases which leads to an increase of the susceptibility X- On the other hand, the TMR diminishes when the temperature T is increased. This dependance results in a limited accuracy of the magnetoresistive element response over working temperatures and limits potential applications of the magnetoresistive element. Typically, the temperature coefficient of TMR of a conventional magnetoresistive element is large and negative leading to overall negative temperature coefficient of sensitivity (TCS) in the magnetoresistive element.

[0009] The TCS can be controlled by using an electronic circuit which compensates change of the sensitivity S of the magnetoresistive sensor element 2 by changing the magnetoresistive element bias voltage with respect to the temperature change. This solution however requires trimming to adjust the TCS. Moreover, using an additional electronic circuit requires larger die size, making the process and development of the magnetoresistive sensor element 2 more complicated.

[0010] In European patent application EP20200315015 by the present applicant, a magnetoresistive element comprises a sense layer having a portion comprising a transition metal element in a proportion such that a temperature dependence of a magnetic susceptibility of the sense layer substantially compensates a temperature dependence of the TMR of the magnetoresistive element.

Summary

[0011] The present disclosure concerns a magnetoresistive element comprising a reference layer having a pinned reference magnetization, a ferromagnetic sense layer having a free sense magnetization comprising a stable vortex configuration, and a tunnel barrier layer between the reference layer and the sense layer. The sense layer comprises a first sense layer portion in contact with the tunnel barrier layer and a second sense layer portion in contact with the first sense layer portion. The first sense layer portion is configured such that a magnetic coupling between the first and second sense layer portions is between ±10 ⁴ J/m ² and ±10’ ³ J/m ² ; and a perpendicular magnetic anisotropy (PMA) originating from the interface between the first sense layer portion and the tunnel barrier layer is between 8x10 ⁴ A/m and 8x10 ⁵ A/m, such as to shift positively the TCS of the magnetoresistive element and compensate the negative temperature coefficient of TMR of the magnetoresistive element.

[0012] The magnetoresistive element disclosed herein can achieve a high degree of TCS compensation by a relatively modest modification of the of layers in the magnetoresistive element. By contrast, compensation of TCS only by dilution of the sense layer magnetization requires a high concentration of nonmagnetic impurities. This results in nonlinear temperature dependence of magnetic susceptibility which is transferred to nonlinearity of the TSC of the magnetoresistive element.

Short description of the drawings

[0013] Exemplar embodiments of the invention are disclosed in the description and illustrated by the drawings in which:

Fig. 1 shows a cross-section view of a magnetoresistive element comprising a sense layer;

Fig. 2 shows the full hysteresis response to the external magnetic field on the sense magnetization;

Fig. 3 illustrates a magnetoresistive element comprising a first sense layer portion a second sense layer portion, according to an embodiment; and

Fig. 4 represent the first sense layer portion comprising a plurality of ferromagnetic sense sublayers, according to an embodiment. Examples of embodiments

[0014] Fig. 3 illustrates a magnetoresistive element 2 according to an embodiment. The magnetoresistive element 2 comprises a ferromagnetic reference layer 23 having a pinned reference magnetization 230, a ferromagnetic sense layer 21 having a sense magnetization 210, and a tunnel barrier layer 22 between the sense and reference ferromagnetic layers 21, 23. The sense magnetization 210 is configured to be orientable in an external magnetic field 60 while the reference magnetization 230 remains substantially undisturbed. The external magnetic field 60 can thus be sensed by measuring a resistance of the magnetoresistive element 2. The resistance depends on the orientation of the sense magnetization 210 relative to the reference magnetization 230.

[0015] The ferromagnetic layers can be made of a Fe based alloy, such as CoFe, NiFe or CoFeB. The reference layer 23 can be pinned by an antiferromagnetic layer 24 by magnetic exchange bias coupling. The antiferromagnetic layer 24 can comprise an alloy based on manganese Mn, such as alloys based on iridium Ir and Mn (e.g., IrMn); alloys based on Fe and Mn (e.g., FeMn); alloys based on platinum Pt and Mn (e.g., PtMn); and alloys based on Ni and Mn (e.g., NiMn). The reference layer 23 can comprise one or a plurality of ferromagnetic layers. In the example illustrated in Fig. 3, the reference layer 23 comprises a synthetic antiferromagnet (SAF) structure comprising at least a first ferromagnetic layer 231 and a second ferromagnetic layer 232 that are separated and coupled antiferromagnetically through an intervening non-magnetic layer 233. The non-magnetic layer is typically a Ru layer but can also comprise any of: Ru, Ir or Cu or a combination of these elements. In a SAF structure, a first reference magnetization 230 of the first ferromagnetic layer 231 is coupled antiparallel to a second reference magnetization 235 by the nonmagnetic layer 233. In the case the magnetoresistive element 2 comprise an antiferromagnetic layer 24, the second reference magnetization 235 of the ferromagnetic layer 232 adjacent to the antiferromagnetic layer 24 is pinned by the antiferromagnetic layer 24. The thickness of the reference layer 23 will depend on the material selected. In one example, the reference layer 23 may have a thickness from 0.3 nm to 3 nm.

[0016] The tunnel barrier 22 can comprise an insulating material. Suitable insulating materials include oxides, such as aluminum oxide (e.g., AI2O3) and magnesium oxide (e.g., MgO). A thickness of the tunnel barrier layer 22 can be in the nm range, such as from about 1 nm to about 3 nm.

[0017] According to some embodiments, the sense layer 21 comprises a first sense layer portion 211 in contact with the tunnel barrier layer 22 and a second sense layer portion 212 in contact with the first sense layer portion 211.

[0018] The second sense layer portion 212 can be configured to allow for a vortex state. For instance, the sense magnetization 210 can comprise a stable vortex configuration. The second sense layer portion 212 can comprise a soft ferromagnetic alloy. For example, the second sense layer portion 212 can comprise an Ni alloy or NiFe alloy possibly comprising a few wt% of Cr, Si, B or V. In one aspect, the second sense layer portion 212 has a thickness that is greater than 15 nm. The second sense layer portion 212 can be up to 120 nm in thickness.

[0019] In an embodiment, the first sense layer portion 211 is configured such that a magnetic coupling between the first and second sense layer portions 211, 212 is between 10' ⁴ J/m ² and 10' ³ J/m ² , where the sign of the exchange bias coupling can be positive or negative. The first sense layer portion 211 can be further configured such that a PMA field originating from the interface between the first sense layer portion 211 and the tunnel barrier layer 22 is between 8x10 ⁴ A/m and 8x10 ⁵ A/m.

[0020] Adjusting the exchange bias coupling between the first and second sense layer portions 211, 212 to the desired exchange bias coupling values between ±10' ⁴ J/m ² and ±10' ³ J/m ² and adjusting the PMA field to the desired PMA field values between 8x10 ⁴ A/m and 8x10 ⁵ A/m modifies the magnetic susceptibility of the first sense layer portion 211 and makes its temperature coefficient more positive. The positive temperature coefficient of the first sense layer portion 211 shifts positively the TCS of the magnetoresistive element 2. The positive shift of the magnetoresistive element 2 TCS can be such as to compensate the negative temperature coefficient of TMR of the magnetoresistive element 2. For instance, the TCS of the magnetoresistive element 2 can be shifted more positively by about 200 to 700 ppm/°C, depending on the diameter of the magnetoresistive element 2 and on material type of the second sense layer portion 212.

[0021] The first sense layer portion 211 is configured such that a magnetic coupling between the first and second sense layer portions 211, 212 is between 10' ⁴ J/m ² and 2x1 O' ³ J/m ² , or 2x1 O' ⁴ J/m ² and 10' ³ J/m ² , or 2x10' ⁴ J/m ² and 2x1 O' ³ J/m ² . The magnetic coupling between the first and second sense layer portions 211, 212 is typically four to five time less than the one in a sense layer of a conventional magnetoresistive element 2.

[0022] In an embodiment, the magnetoresistive element 2 further comprises a mediation layer 213 between the first and second sense layer portions 211, 212. The mediation layer 213 is configured to reduce the magnetic coupling between the first sense layer portion 211 and the second sense layer portion 212. More particularly, the mediation layer 213 can be configured to adjust the exchange bias coupling between the first and second sense layer portions 211, 212 to the desired exchange bias coupling values between ±10 ⁴ J/m ² and ±10 ³ J/m ² , and adjust the PMA field to the desired PMA field values between 8x10 ⁴ A/m and 8x10 ⁵ A/m. The mediation layer 213 can comprise a nonmagnetic layer, preferably a nonmagnetic layer.

[0023] In one aspect, the thickness and composition of the mediation layer 213 can be adjusted to arrive at the desired exchange bias coupling values between ±10' ⁴ J/m ² and ±10' ³ J/m ² and to the desired PMA field values between 8x10 ⁴ A/m and 8x10 ⁵ A/m. In an example, the mediation layer 213 can comprise or can be made of any one or a combination of: Nb, Ti, Ru, W, Ta, Ir, Mo or Cu. In another example, a thickness of the mediation layer 213 can range between 0.5 nm and 2.5 nm.

[0024] It was also shown that the exchange bias coupling between the first and second sense layer portions 211, 212 between ±10 ⁴ J/m ² and ±10 ³ J/m ² is strong enough to allow the transfer of the vortex state from the second sense layer portion 212 to the first sense layer portion 211. Therefore, the hysteresis response (or magnetization curve) of the magnetoresistive element 2 with the mediation layer 213 can be similar to the one of the magnetoresistive element 2 without the mediation layer 213.

[0025] The effective TMR and magnetic susceptibility in the first sense layer portion 211 decreases with a decrease in the magnitude of the PMA field. Since the PMA field decreases quickly with the temperature, it provides a compensation effect to the TCS of the first sense layer portion 211. This effect is more important when the first sense layer portion 211 is thin. For example, the first sense layer portion 211 can be at least about four times thinner than the second sense layer 212.

[0026] In another embodiment, the first sense layer portion 211 comprises a Co _x Fe _y B _z alloy with x and y varying independently from 0 to 100 %vol and z varying from 0 to 20 %vol. The thickness the first sense layer portion 211 can be further adjusted to arrive at the desired exchange bias coupling values between ±10 ^-4 J/m ² and ±10 ³ J/m ² and to the desired PMA field values between 8x10 ⁴ A/m and 8x10 ⁵ A/m. For example, but without limitation, the first sense layer portion 211 has a thickness which can be from 1.5 nm to 4 nm. It should be noted that both PMA field and exchange bias coupling will be decreased by substantially the same amount when the thickness of the first sense layer portion 211 increases.

[0027] In an embodiment shown in Fig. 4, the first sense layer portion 211 can comprises a plurality of ferromagnetic sense sublayers 2111, each sense sublayer 2111 containing Co _x Fe _y B _z alloy wherein x, y, z can vary from one sense sublayers 2111 to another. The sense sublayers 2111 can have a thickness between 0.3 nm and 3 nm. The first sense layer portion 211 can further comprise nonmagnetic spacer sublayers 2112 between the sense sublayers 2111. The spacer sublayers 2112 can comprise a metallic nanolayer and have a thickness smaller than 0.4 nm.

[0028] In a further embodiment, the second sense layer portion 212 can comprise a dilution element. The dilution element can be alloyed in the second sense layer portion 212 or the second sense layer portion 212 can comprise one or a plurality of dilution nanolayers. The dilution element or dilution nanolayer is configured to dilute the sense magnetization 210 and decreases the Curie temperature Tc of the sense layer 21. Decreasing the Curie temperature Tc of the sense layer 21 results in a faster drop in magnetization with increasing temperature T in the working temperature range of the magnetoresistive element 2. The decrease in magnetization with increasing temperature T results in an increase of the susceptibility / with increasing temperature. By adjusting the dilution of the sense magnetization 210 it is then possible to compensate the decrease of the TMR with the increase of the susceptibility / with increasing temperature. Adjusting the dilution of the sense magnetization 210 thus allows for further controlling the TCS, for instance shifting positively the TCS of the magnetoresistive element 2 and compensate the negative temperature coefficient of TMR of the magnetoresistive element 2. The dilution element or dilution nanolayer can comprise a transition metal element, for instance Ta, W or Ru.

[0029] In an embodiment, the second sense layer portion 212 comprises the dilution element in a proportion less than 15%vol (% by volume).

[0030] Compensation of the TCS by dilution of the second sense layer portion 212 can occurs independently from the compensation due to the first sense layer portion 211 and of the compensation due to the mediation layer 213. Compensation of the TCS by dilution of the second sense layer portion 212 can thus be used in addition the compensation due to the first sense layer portion 211 and of the mediation layer 213 to fine tune the TCS of the magnetoresistive element 2.

[0031 ] The inventors have shown that the TCS of the magnetoresistive element 2 disclosed herein can have a TCS about +600 ppm of positive shift when the first sense layer portion 211 is separated from the second sense layer portion 212 by the mediation layer 213. The mediation layer 213 provides the first sense layer portion 211 with a strong (for example between ±10 ^-4 J/m ² and ±10 ^-3 J/m ² ) positive temperature coefficient of magnetic susceptibility. Thus, the positive temperature coefficient of the first sense layer portion 211 in the presence of the mediation layer 213 shifts positively the TCS of the magnetoresistive element 2 and can compensate the negative temperature coefficient originating from the TMR effect. This applies for both positive and negative magnitude of the exchange bias coupling between first and second sense layer portions 211, 212.

[0032] The inventors have also shown that the temperature coefficient of TMR in the magnetoresistive element 2 and the temperature dependence of the magnetic susceptibility of the second sense layer portion 212 are not substantially modified between the magnetoresistive element 2 comprising the mediation layer 213 and without the mediation layer 213.

[0033] In fact, the magnetoresistive element 2 was characterized by using a physical property measurement system (PPMS). PPMS characterization has shown that there is no difference in temperature dependence of magnetic susceptibility of the second sense layer portion 212 and no significant change in temperature dependence of TMR compared to a conventional sense layer 21. At the same time, one observes noticeable modification of temperature dependence of magnetic susceptibility in the first sense layer portion 211. In fact, the first sense layer portion 211 acquires significantly higher positive temperature coefficient that is responsible for improved TCS of the vortex-based magnetoresistive element.

[0034] The PPMS characterization has further shown an increase in the temperature coefficient of the first sense layer portion 211. It is believed that the increase in the temperature coefficient is linked with the PMA field (and the temperature dependence of the PMA field) in the first sense layer portion 211. The interfacial energy of the first sense layer portion 211 is believed to be higher than that of the interfacial energy of an antiferromagnetic coupling through a Ru spacer between the first and second sense layer portions 211, 212.

Reference numbers and symbols

2 magnetoresistive sensor element 20 magnetoresistive sensor

21 sense layer 210 sense magnetization 211 first sense layer portion 2111 sense sublayer 2112 spacer sublayer 212 second sense layer portion 213 mediation layer 22 tunnel barrier layer

23 reference layer 230 reference magnetization, first reference magnetization 231 first ferromagnetic layer 232 second ferromagnetic layer 233 non-magnetic layer 235 second reference magnetization 24 antiferromagnetic layer 60 external magnetic field