MAGNETORESISTIVE SENSOR ELEMENT HAVING A WIDE LINEAR RESPONSE AND ROBUST NOMINAL PERFORMANCE AND MANUFACTURING METHOD THEREOF

Abstract

A magnetoresistive element for a magnetic sensor, the magnetoresistive element including a tunnel barrier layer between a reference layer having a fixed reference magnetization and a sense layer having a free sense magnetization, wherein the sense magnetization includes a stable vortex configuration. The magnetoresistive element further includes a reference pinning layer in contact with the reference layer and pining the reference magnetization by exchange-bias at a first blocking temperature. The magnetoresistive element further includes a sense pinning layer in contact with the sense layer and pining the sense magnetization by exchange-bias at a second blocking temperature lower that the first blocking temperature. Additionally, a method for manufacturing the magnetoresistive element.

Claims

1-9. (canceled)

10. A magnetoresistive element for a magnetic sensor, the magnetoresistive element comprising a tunnel barrier layer included between a reference layer having a fixed reference magnetization and a sense layer having a free sense magnetization; the sense magnetization comprising a stable vortex configuration in the absence of an applied magnetic field; the magnetoresistive element further comprising a reference pinning layer in contact with the reference layer and pining the reference magnetization by exchange-bias at a first blocking temperature; and a sense pinning layer in contact with the sense layer and pining the sense magnetization by exchange-bias at a second blocking temperature lower that the first blocking temperature, wherein the sense layer has a thickness between 15 nm and 80 nm, and wherein the strength of exchange-bias between the sense pinning layer and the sense layer is between 210.sup.8 J/cm.sup.2 and 410.sup.8 J/cm.sup.2.

11. The magnetoresistive element according to claim 10, wherein the sense pinning layer is configured such that the strength of exchange-bias between the sense pinning layer and the sense layer is lower than the strength of exchange-bias between the reference pinning layer and the reference layer.

12. The magnetoresistive element according to claim 10, wherein the sense layer comprises a CoFe, NiFe or CoFeB based alloy.

13. The magnetoresistive element according to claim 10, wherein the reference pinning layer or the sense pinning layer comprise, or are formed of, an antiferromagnetic material.

14. The magnetoresistive element according to claim 13, wherein the reference pinning layer or the sense pinning layer comprise an alloy based on Ir and Mn, Fe and Mn; Pt and Mn, Ni and Mn, Cr, NiO or FeO.

15. A method for manufacturing a magnetoresistive element comprising a tunnel barrier layer included between a reference layer having a fixed reference magnetization and a sense layer having a free sense magnetization; the sense magnetization comprising a stable vortex configuration in the absence of an applied magnetic field; the magnetoresistive element further comprising a reference pinning layer in contact with the reference layer and pining the reference magnetization by exchange-bias at a first blocking temperature; and a sense pinning layer in contact with the sense layer and pining the sense magnetization by exchange-bias at a second blocking temperature lower that the first blocking temperature; wherein the sense layer has a thickness between 15 nm and 80 nm, and the strength of exchange-bias between the sense pinning layer and the sense layer is between 210.sup.8 J/cm.sup.2 and 410.sup.8 J/cm.sup.2; the method comprising: depositing the reference pinning layer, reference layer, tunnel barrier layer, sense layer, and sense pinning layer, wherein the sense pinning layer is configured such that the magnitude of the exchange-bias generated in the sense layer by the sense pinning layer is between 210.sup.8 J/cm.sup.2 and 410.sup.8 J/cm.sup.2; annealing the magnetoresistive element with an applied external magnetic field at an annealing temperature higher than the first blocking temperature; and annealing the magnetoresistive element in the absence of an applied external magnetic field at an annealing temperature higher than the second blocking temperature and lower than the first blocking temperature.

16. The method according to claim 15, comprising forming the magnetoresistive element including depositing the reference pinning layer and depositing the sense pinning layer; wherein the reference layer is deposited on the reference pinning layer and the sense layer is deposited on the sense pinning layer.

17. A magnetic sensor comprising a plurality of the magnetoresistive elements according to claim 10.

Description

SHORT DESCRIPTION OF THE DRAWINGS

[0016] Exemplar embodiments of the invention are disclosed in the description and illustrated by the drawings in which:

[0017] FIG. 1 shows a cross-section view of a conventional magnetoresistive sensor element comprising a sense layer;

[0018] FIGS. 2a and 2b illustrate a top view of the sense layer having a sense magnetization comprising a vortex configuration in absence (FIG. 2a) and in the presence (FIG. 2b) of an external magnetic field;

[0019] FIG. 3 shows a hysteresis response of the conventional magnetoresistive sensor element;

[0020] FIG. 4 shows a cross-section view of a magnetoresistive sensor element comprising a sense pinning layer and a sense layer, wherein the sense layer has a sense magnetization comprising a vortex configuration, according to an embodiment;

[0021] FIG. 5 shows a vortex expulsion field and a vortex nucleation field of the vortex configuration, as a function of the thickness of the sense layer;

[0022] FIG. 6 reports the sensitivity of the magnetoresistive element as a function of the thickness of the sense layer, for a range of the external magnetic field of 1.610.sup.4 A/m and for a strength of the exchange-bias generated by the sense pinning layer of 210.sup.8 J/cm.sup.2 and 410.sup.8 J/cm.sup.2;

[0023] FIG. 7 reports the linearity of the magnetoresistive element response in term of linear error as a function of the thickness of the sense layer, for a range of the external magnetic field of 1.610.sup.4 A/m and for a strength of the exchange-bias generated by the sense pinning layer of 210.sup.8 J/cm.sup.2 and 410.sup.8 J/cm.sup.2;

[0024] FIG. 8 report the linearity of the magnetoresistive element response in term of linear error as a function of the sensitivity of the magnetoresistive element for a strength of the exchange-bias generated by the sense pinning layer of 210.sup.8 J/cm.sup.2 and 410.sup.8 J/cm.sup.2 and for a range of the external magnetic field of 1.610.sup.4 A/m; and

[0025] FIG. 9 report the linearity of the magnetoresistive element response in term of linear error as a function of the sensitivity of the magnetoresistive element for a strength of the exchange-bias generated by the sense pinning layer of 210.sup.8 J/cm.sup.2 and 410.sup.8 J/cm.sup.2 and for a range of the external magnetic field of 3.210.sup.4 A/m.

EXAMPLES OF EMBODIMENTS

[0026] With reference to FIG. 4, a cross-section view of a magnetoresistive sensor element 2 is shown according to an embodiment. The magnetoresistive sensor element 2 comprises a ferromagnetic reference layer 21 having a reference magnetization 210, a ferromagnetic sense layer 23 having a free sense magnetization 230 and a tunnel barrier layer 22 between the reference and sense ferromagnetic layers 21, 23. The sense magnetization 230 can be oriented in an external magnetic field 60 while the reference magnetization 210 remains 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 orientation of the sense magnetization 230 relative to the reference magnetization 210.

[0027] The sense magnetization 230 comprises a stable vortex configuration rotating in a circular path along the edge of the sense layer 23 and around a core 231, reversibly movable in accordance to the external magnetic field 60. For a given lateral dimension of the magnetoresistive sensor element 2, the thickness of the sense layer 23 is chosen such that the sense layer 23 has a stable vortex configuration magnetisation in the absence of an applied magnetic field.

[0028] The reference magnetization 210 is substantially longitudinally oriented in the plane of the reference layer 21. The orientation of the reference magnetization 210 is determined by the exchange coupling (generating an exchange-bias) between the reference pinning layer 24 and the reference layer 21. The reference layer 21 can comprise a synthetic antiferromagnetic (SAF).

[0029] In one aspect, the reference and sense layers 21, 23 comprise, or are formed of, a ferromagnetic material such as a cobalt (Co), iron (Fe) or nickel (Ni) based alloy and preferentially a CoFe, NiFe or CoFeB based alloy. The reference layer 21 can have a thickness between 2 nm and 7 nm. The reference and sense layers 21, 23 can comprise a multilayer structure where each layer can include a ferromagnetic material such as a Co, Fe or Ni based alloy and preferentially a CoFe, NiFe or CoFeB based alloy, and non-magnetic layers such as Ta, Ti, W, Ru, Ir.

[0030] The reference and sense magnetizations 210, 230 can have magnetic anisotropy substantially parallel to the plane of the layers 21, 23 (in-plane, as shown in FIG. 4) and/or substantially perpendicular to the plane of the layers 21, 23 (out-of-plane).

[0031] The magnetoresistive sensor element 2 further comprises a reference pinning layer 24 pining the reference magnetization 210 by exchange-bias at a first threshold temperature Tb1. The expression threshold temperature can correspond to a blocking temperature, such as a Neel temperature, or another threshold temperature of the reference pinning layer 24. The reference pinning layer 24 unpins, or decouples, the reference magnetization 210 when the temperature above the first threshold temperature Tb1.

[0032] The magnetoresistive sensor element 2 further comprises a sense pinning layer 25 pining the sense magnetization 230 by exchange-bias at a second threshold temperature Tb2 lower that the first threshold temperature Tb1.

[0033] In one aspect, the sense pinning layer 25 can be configured such that the strength of exchange-bias generated in the sense layer 23 by the sense pinning layer 25 is less than that generated in the reference layer 21 by the reference pinning layer 24. For example, the magnitude of the exchange-bias generated in the sense layer 23 by the sense pinning layer 25 is substantially between 210.sup.8 J/cm.sup.2 and 410.sup.8 J/cm.sup.2 (0.2 erg/cm.sup.2 to 0.4 erg/cm.sup.2).

[0034] In some aspects, the thickness of the sense layer 23 can be selected such that the strength of exchange-bias generated in the sense layer 23 by the sense pinning layer 25 allows the sense magnetization 230 can be aligned with respect to the external magnetic field 60 to be measured in a magnetization changeable state.

[0035] In some aspects, the reference pinning layer 24 and the sense pinning layer 25 comprises, or is formed of, an antiferromagnetic material pinning the reference magnetization 210 and respectively the sense magnetization 230, through exchange coupling. In particular, the reference pinning layer 24 and the sense pinning layer 25 comprise, or are formed of, a magnetic material of the antiferromagnetic type, including alloys 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) or alloys based on chromium (Cr), NiO or FeO.

[0036] In some aspects, the thickness of the reference pinning layer 24 and of the sense pinning layer 25 can be between 4 nm and 15 nm.

[0037] The tunnel barrier layer 22 comprises, or is formed of, an insulating material. Suitable insulating materials include oxides, such as aluminum oxide (e.g., Al.sub.2O.sub.3) 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 10 nm. Large TMR for example of up to 200% can be obtained for the magnetic tunnel junction 2 comprising a crystalline MgO-based tunnel barrier layer 22.

[0038] FIG. 5 shows the vortex expulsion field H.sub.expl and the vortex nucleation field H.sub.nucl as a function of the thickness of the sense layer 23. The values of the vortex expulsion field H.sub.expl and of the vortex nucleation field H.sub.nucl are calculated for a sense layer 23 comprising a NiFe alloy, a TMR of the magnetoresistive sensor element 2 of 140%, a reference magnetization 210 pinned in-plane, i.e., in the plane of the reference layer 21. The calculations were performed for a strength of the exchange-bias generated in the sense layer 23 by the sense pinning layer 25 of 210.sup.8 J/cm.sup.2 and 410.sup.8 J/cm.sup.2. The calculated vortex expulsion field H.sub.expl and the vortex nucleation field H.sub.nucl as a function of the thickness of the sense layer 23 in the absence of an exchange-bias generated by the sense pinning layer 25 are also shown.

[0039] FIG. 5 shows that the strength of the expulsion and nucleation fields H.sub.nucl, H.sub.expl are generally higher for thicknesses of the sense layer 23 smaller than about 40 nm, when the sense layer 23 is subjected to the exchange-bias generated by the sense pinning layer 25 and compared to the expulsion and nucleation fields H.sub.nucl, H.sub.expl in the absence of the exchange-bias generated by the sense pinning layer 25. High values of the expulsion and nucleation fields H.sub.nucl, H.sub.expl can be obtained for a thickness of the sense layer 23 between 15 nm and 80 nm. The higher strength of the expulsion and nucleation fields H.sub.nucl, H.sub.expl allows for increasing the stability of the vortex configuration. It further allows for wider linear response region of the magnetoresistive sensor element 2 and for a reduced change in the response of the magnetoresistive sensor element 2 when the latter is subjected to a high magnetic field used in magnetic permeability testing.

[0040] FIG. 6 reports the sensitivity S of the magnetoresistive element 10 as a function of the thickness of the sense layer 23, for a range of the external magnetic field 60 of 1.610.sup.4 A/m (200 Oe) and for a strength of the exchange-bias generated by the sense pinning layer 25 of 210.sup.8 J/cm.sup.2 (open circles) and 410.sup.8 J/cm.sup.2 (open squares). The sensitivity S of a conventional magnetoresistive element not comprising a sense pinning layer 25 is also reported for an external magnetic field 60 of 1.610.sup.4 A/m (200 Oe) (plain squares). The magnetization vortex state results from an equilibrium between the vortex magnetostatic energy and exchange energy from the sense pinning layer 25. Due to the competition between the magnetostatic energy at larger thickness of the sense layer 23 and increased exchange energy (or pinning field) for small thicknesses of the sense layer 23, a maximum sensitivity S of the magnetoresistive element 10 can be obtained by adjusting the strength of the exchange-bias generated by the sense pinning layer 25. Here, an exchange-bias of 410.sup.8 J/cm.sup.2 yields a sensitivity S between 3 and 5 mV/V/mT for the sense layer 23 having a thickness between 15 nm and 80 nm. Such values of the sensitivity S are suitable for magnetic sensor applications.

[0041] FIG. 7 reports the linearity of the magnetoresistive element 10 response in term of linear error in % as a function of the thickness of the sense layer 23, for a range of the external magnetic field 60 of 1.610.sup.4 A/m (200 Oe) and for a strength of the exchange-bias generated by the sense pinning layer 25 of 210.sup.8 J/cm.sup.2 (open circles) and 410.sup.8 J/cm.sup.2 (open squares). The linearity of the response obtained for a conventional magnetoresistive element not comprising a sense pinning layer 25 is also reported as a function of the thickness of the sense layer, for an external magnetic field 60 of 1.610.sup.4 A/m (200 Oe) (plain squares). FIG. 7 shows that the linear error for a range of the external magnetic field 60 of 1.610.sup.4 A/m (200 Oe) can be reduced for the sense layer 23 having a thickness less than 20 nm, compared to that of a magnetoresistive element without the sense pinning layer 25. FIG. 7 shows that the linear error of the magnetoresistive element 10 response, for a range of the external magnetic field 60 of 1.610.sup.4 A/m (200Oe) is below 4% for the sense layer 23 having a thickness between 15 nm and 80 nm.

[0042] FIGS. 8 and 9 report the linearity of the magnetoresistive element 10 response in term of linear error in % as a function of the sensitivity S of the magnetoresistive element 10 for a strength of the exchange-bias generated by the sense pinning layer 25 of 210.sup.8 J/cm.sup.2 (plain circles) and 410.sup.8 J/cm.sup.2 (plain squares). In FIG. 8, the linearity was calculated for a range of the external magnetic field 60 of 1.610.sup.4 A/m (200 Oe) and in FIG. 9, the linearity was calculated for a range of the external magnetic field 60 of 3.210.sup.4 A/m (400 Oe). The linearity of the response obtained for a conventional magnetoresistive element not comprising a sense pinning layer 25 is also reported as a function of the sensitivity S of the conventional magnetoresistive element, for an external magnetic field 60 of 1.610.sup.4 A/m (open circles) in FIG. 8 and for an external magnetic field 60 of 3.210.sup.4 A/m (open circles) in FIG. 9.

[0043] For an external magnetic field 60 range of 1.610.sup.4 A/m and an exchange bias of 210.sup.8 J/cm.sup.2, a thickness of the sense layer 23 between 15 nm and 80 nm yields a linear error in the response of the magnetoresistive element 10 that is below 3.5%. For larger external magnetic field 60 range of 3.210.sup.4 A/m and an exchange bias of 210.sup.8 J/cm.sup.2, a thickness of the sense layer 23 between 15 nm and 80 nm yields a linear error in the response of the magnetoresistive element 10 that is below 2%.

[0044] The thickness of the sense layer between 15 nm and 80 nm allows for obtaining strong exchange coupling of the sense magnetization in a portion of the layer near the sense pinning layer and the lesser exchange coupling of the sense magnetization in a portion of the layer far from the sense pinning layer such that vortex behave in a linear fashion in that farther portion, in the presence of the external magnetic field. The thickness of the sense layer between 15 nm and 80 nm allows for combining the effect if the exchange coupling of the sense magnetization (nominal performance being unchanged after the magnetoresistive element has been subjected to high magnetic fields) and obtaining a wide linear response.

[0045] A low sensitivity S of the magnetoresistive element 10 response (for example a sensitivity S smaller than 5%) can then be obtained by adjusting the strength of the exchange-bias generated by the sense pinning layer 25 and by adjusting the thickness of the sense layer 23.

[0046] According to an embodiment, a method for manufacturing the magnetoresistive element 10 comprises the steps of: [0047] annealing the magnetoresistive element 10 with an applied external magnetic field sufficient to saturate the magnetization of the reference layer 21, and at an annealing temperature higher than the first blocking temperature Tb1, which pins de reference layer 21 in a direction along the direction of the applied magnetic field; and [0048] annealing the magnetoresistive element 10 in the absence of an applied external magnetic field at an annealing temperature higher than the second blocking temperature Tb2 and lower than the first blocking temperature Tb1, which pins the sense layer 21 in a magnetic vortex configuration.

[0049] Prior to the annealing steps, the method can comprise forming the magnetoresistive element 10, including the steps of depositing the reference pinning layer 24 and the sense pinning layer 25. The reference layer 21 can be deposited directly on the reference pinning layer 24 and the sense layer 23 can be deposited directly on the sense pinning layer 25.

[0050] Forming the magnetoresistive element 10 can further comprise the step of depositing the tunnel barrier layer 22, wherein the sense layer 23 is deposited on the tunnel barrier layer 22. Depositing the tunnel barrier layer 22 can be performed by using an RF magnetron sputtering technique or any other suitable techniques.

[0051] In some aspects forming the magnetoresistive element 10 comprises depositing, in this order, the reference pinning layer 24, reference layer 23, tunnel barrier layer 22, sense layer 23 and sense pinning layer 25. The magnetoresistive element 10 can further comprise depositing, in this order, the sense pinning layer 25, sense layer 23, tunnel barrier layer 22, reference layer 23 and reference pinning layer 24.

[0052] In an embodiment, a magnetic sensor comprises a plurality of the magnetoresistive element 2 disclosed herein.

REFERENCE NUMBERS AND SYMBOLS

[0053] 2 magnetoresistive element [0054] 21 reference layer [0055] 210 reference magnetization [0056] 22 tunnel barrier layer [0057] 23 sense layer [0058] 230 sense magnetization [0059] 231 core [0060] 24 reference pinning layer [0061] sense pinning layer [0062] 60 external magnetic field [0063] H.sub.ext external magnetic field [0064] H.sub.expl expulsion field [0065] H.sub.nucl nucleation field [0066] S sensitivity [0067] susceptibility