MAGNETIC SENSOR ELEMENT AND DEVICE HAVING IMPROVED ACCURACY UNDER HIGH MAGNETIC FIELDS

Abstract

Magnetic angular sensor element destined to sense an external magnetic field, including a magnetic tunnel junction containing a ferromagnetic pinned layer having a pinned magnetization, a ferromagnetic sensing layer, and a tunnel magnetoresistance barrier layer; the ferromagnetic sensing layer including a first sensing layer being in direct contact with the barrier layer and having a first sensing magnetization, a second sensing layer having a second sense magnetization, and a metallic spacer between the first sensing layer and the second sensing layer; wherein the metallic spacer is configured to provide an antiferromagnetic coupling between the first sensing magnetization and the second sensing magnetization such that the first sensing magnetization is oriented substantially antiparallel to the second sensing magnetization; the second sensing magnetization being larger than the first sensing magnetization, such that the second sensing magnetization is oriented in accordance with the direction of the external magnetic field.

Claims

1. Magnetic angular sensor element destined to sense an external magnetic field, comprising: a magnetic tunnel junction containing a ferromagnetic pinned layer having a pinned magnetization; a ferromagnetic sensing layer; and a tunnel magnetoresistance barrier layer, between the ferromagnetic pinned layer and the ferromagnetic sensing layer, wherein the ferromagnetic sensing layer comprises a first sensing layer being in direct contact with the barrier layer and having a first sensing magnetization, a second sensing layer having a second sense magnetization, and a metallic spacer between the first sensing layer and the second sensing layer, wherein the metallic spacer is configured to provide an antiferromagnetic coupling between the first sensing magnetization and the second sensing magnetization such that the first sensing magnetization is oriented substantially antiparallel to the second sensing magnetization, wherein the second sensing magnetization being larger than the first sensing magnetization, such that the second sensing magnetization is oriented in accordance with the direction of the external magnetic field, wherein characterized in that the second sensing layercomprises a plurality of second sensing sublayers, each second sensing sublayer having a second sensing sub-magnetization amounting to said second sensing magnetization, and wherein two adjacent second sensing sublayers are separated from by a non-magnetic layer being configured to provide a magnetic coupling between the two adjacent second sensing sublayers.

2. The magnetic sensor element according to claim 1, wherein the non-magnetic layer is configured to provide a magnetic coupling such that the second sensing magnetization is oriented in a direction opposed to the one of the first sensing magnetization.

3. The magnetic sensor element according to claim 1, wherein the magnetic coupling is such that the second sensing sub-magnetization of one of the second sensing sublayers is oriented substantially parallel to the one of the adjacent second sensing sublayer.

4. The magnetic sensor element according to claim 1, wherein the non-magnetic layer is configured to have a strength of the magnetic coupling of 1 mJ/m.sup.2 or greater such that there is no reversal of the second sensing sub-magnetization within the second sensing sublayer for an amplitude of the external magnetic field up to 95493 A/m.

5. The magnetic sensor element according to claim 1, wherein the antiferromagnetic coupling of the metallic spacer is a RKKY coupling having an exchange coupling of 0.3 mJ/m.sup.2 or greater.

6. The magnetic sensor element according to claim 1, the pinned layer comprises ferromagnetic layers separated by and non-magnetic layers where the ferromagnetic layers farthest from the tunnel barrier layer are pinned by an antiferromagnet while the other ferromagnetic layers are coupled to the neighbouring ferromagnetic layers by an RKKY coupling mechanism through the non-magnetic layers separating them, wherein the RKKY exchange coupling is antiferromagnetic and has an absolute magnitude of 0.4 mJ/m.sup.2 or greater.

7. The magnetic sensor element according to claim 5, wherein the first sensing magnetization has a thickness of 1.5 nm or greater.

8. A magnetic angular sensor device comprising a plurality of the magnetic sensor element according to claim 1, wherein the magnetic sensor elements are arranged in a half-bridge or full bridge configuration.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The invention will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which:

[0028] FIG. 1 shows a single-branch sensing circuit comprising a plurality of electrically connected MTJ pillars fabricated from an MTJ film stack. All MTJ pillars share the same pinning direction of their reference layer.

[0029] FIG. 2 reports the variation of the total angular error as a function of the magnitude of an external magnetic field;

[0030] FIG. 3 shows a sensing circuit comprising two MTJ sensing circuits arranged in a half-bridge configuration;

[0031] FIG. 4 compares the variation of the total angular error as a function of the magnitude of an external magnetic field, calculated for the single-branch (SB) sensing circuit of FIG. 1 and for the half-bridge (HB) sensing circuit of FIG. 3;

[0032] FIGS. 5a and 5b represent a longitudinal component and a transversal component of the external magnetic field;

[0033] FIG. 6 reports the deviation of the reference layer magnetization relative to its pinned orientation as a function of the orientation of the external magnetic field for the half-bridge sensing circuit of FIG. 3;

[0034] FIG. 7 reports the variation of the derivative of the resistance with the angle as a function of the orientation of the reference layer magnetization relative to the sensing layer magnetization for different TMR;

[0035] FIG. 8 shows a magnetic angular sensor element, according to an embodiment;

[0036] FIG. 9 shows total angular error as a function of the magnitude of the external magnetic field for several values of the pinning field strength;

[0037] FIG. 10 shows total angular error as a function of the magnitude of the external magnetic field for several values of TMR in MTJ stack.

DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS

[0038] FIG. 8 shows a magnetic angular sensor element 20 destined to sense an external magnetic field (H.sub.ext) 60, according to an embodiment. The magnetic sensor element 20 comprises a magnetic tunnel junction containing a ferromagnetic pinned layer 21 having a pinned magnetization 210, a ferromagnetic sensing layer 23, and a tunnel magnetoresistance barrier layer 22, between the ferromagnetic pinned layer 21 and the ferromagnetic sensing layer 23. The ferromagnetic sensing layer 23 comprises a first sensing layer 23a being in direct contact with the barrier layer 22 and having a first sensing magnetization 230a, a second sensing layer 23b having a second sense magnetization 230b, and a metallic spacer 24 between the first sensing layer 23a and the second sensing layer 23b.

[0039] The metallic spacer 24 is configured to provide an antiferromagnetic coupling between the first sensing magnetization 230a and the second sensing magnetization 230b such that the first sensing magnetization 230a is oriented substantially antiparallel to the second sensing magnetization 230b.

[0040] The second sensing magnetization 230b is larger than the first sensing magnetization 230a, such that the second sensing magnetization 230b is oriented in accordance with the direction of the external magnetic field 60.

[0041] The present invention further pertains to a magnetic angular sensor device comprising a plurality of the magnetic sensor element 20.

[0042] In a preferred embodiment, the magnetic sensor elements 20 are arranged in a half-bridge circuit 30, as represented in FIG. 3. The magnetic sensor elements 20 can also be arranged in a full bridge configuration.

[0043] When the external magnetic field H.sub.ext is applied in a direction that is close to an antiparallel orientation with respect to the pinned direction of the reference layer magnetization 210, a higher deviation of the reference layer magnetization 210 occurs (first factor). However, the first sensing layer magnetization 230a is oriented close to a parallel orientation with respect to the pinned direction of the reference layer magnetization 210, reducing the resistance variation per one degree of the reference layer deviation (second factor). The deviation of the reference layer magnetization 210 is thus at least partially compensated by a decrease of the angular variation of the resistance R.

[0044] Using the magnetic angular sensor element 20 in the half-bridge circuit 30 (or a full-bridge circuit) improves the compensation of the angular error AE.sub.T compared to known arrangement of the magnetic angular sensor element, not only at low magnitudes of the external magnetic field 60 but also at high magnitudes of the external magnetic field 60.

[0045] In an embodiment, the second sensing layer 23b comprises a plurality of second sensing sublayers 231, each second sensing sublayer 231 having a second sensing sub-magnetization 2310 amounting to said second sensing magnetization 230b. Two adjacent second sensing sublayers 231 are separated from by a non-magnetic layer 232 being configured to provide a magnetic coupling between the two adjacent second sensing sublayers 231.

[0046] In another embodiment, the non-magnetic layer 232 is configured to provide a magnetic coupling such that the second sensing magnetization 230b is oriented in a direction opposed to the one of the first sensing magnetization 230a.

[0047] In yet another embodiment, the magnetic coupling is such that the second sensing sub-magnetization 2310 of one of the second sensing sublayers 231 is oriented substantially parallel to the one of the adjacent second sensing sublayer 231.

[0048] In yet another embodiment, the non-magnetic layer 232 is configured to have a strength (a minimal required strength) of the magnetic coupling such that there is no reversal of the second sensing sub-magnetization 2310 within the second sensing sublayer 231 for an amplitude of the external magnetic field 60 up to 1200 Oe (95493 A/m).

[0049] In yet another embodiment, the antiferromagnetic coupling of the metallic spacer 24 is a RKKY coupling having an exchange coupling of 0.3 mJ/m.sup.2 or greater.

[0050] In yet another embodiment, the first sensing magnetization 230a has a thickness of 1.5 nm or greater.

[0051] A RKKY coupling having an exchange coupling of 0.3 mJ/m.sup.2 or greater is enough to stabilize the first sensing layer 23a and second sensing layer 23b in the antiferromagnetic configuration for an external magnetic field H.sub.ext that is below 1200 Oe (95493 A/m).

[0052] I yet another embodiment, the layers order as shown in FIG. 8 is inverted. Such configuration corresponds to a so-called top-pinned MTJ stack.

[0053] For any given TMR magnitude, increasing the pinning strength of the reference layer 21 reduces the asymmetry in the reference magnetic susceptibility χ.sub.R of the reference layer 21 in the magnetic sensor element 20 in the top branch and bottom branch of the half-bridge circuit 30. Increasing the pinning strength of the reference layer 21 thus diminishes the impact of the effect of the second factor as defined above.

[0054] FIG. 9 shows the angular error in the orientation of the pinned reference layer magnetization 210 as a function of the magnitude of the external magnetic field H.sub.ext for several values of the pinning field H.sub.pin (pinning strength) and for the magnetic sensor elements 20 having a TMR of 50%.

[0055] For any given reference layer pinning field H.sub.pin, there is an optimal TMR value which provides optimal compensation at a desired magnitude of the external magnetic field H.sub.ext. FIG. 10 shows total angular error as a function of the magnitude of the external magnetic field H.sub.ext for several values of TMR in the magnetic sensor elements 20 and for a pinning field H.sub.pin of 4 kOe (318 A/m). The optimum (minimum angular error) can be found numerically and/or optimized by adequate designing of the magnetic sensor element 20.

[0056] The strength of the antiferromagnetic coupling (the magnitude of the pinning field H.sub.pin) should be high enough to keep the reference layer magnetization 210 rigid when submitted to the external magnetic field H.sub.ext.

[0057] FIG. 11 is a partial view of the magnetic sensor element 20 according to yet another embodiment, showing the ferromagnetic pinned layer 21, the tunnel magnetoresistance barrier layer 22 and the sensing layer 23. The pinned layer 21 comprises a synthetic antiferromagnet (SAF) structure, i.e., a magnetic multilayer comprising comprises ferromagnetic pinned sublayers separated by non-magnetic sublayers. In FIG. 11 the magnetic sensor element 20 is represented comprising three ferromagnetic pinned sublayers 211 separated by two non-magnetic sublayer 212, however the magnetic sensor element 20 can have at least two ferromagnetic pinned sublayers 211 separated by and non-magnetic sublayer 212. The ferromagnetic layer farthest from the tunnel barrier layer 22 is pinned by an antiferromagnet 213 while the other ferromagnetic layers are coupled to the neighbouring ferromagnetic layers by an RKKY coupling mechanism through the non-magnetic layers separating them. Here, the RKKY coupling is essentially antiferromagnetic.

[0058] FIG. 12 shows the dependence of angular error ΔφH on the external magnetic field H.sub.ext for different TMR magnitudes for the magnetic sensor element 20 in the configuration of FIG. 11. Here, calculations were performed assuming an exchange pinning of 0.66 mJ/m.sup.2 and the RKKY coupling of -0.66 mJ/m.sup.2 for the ferromagnetic layers farthest from the tunnel barrier layer 22 (in the bottom part of the stack). As can be seen from FIG. 12, the SAF configuration of the pinned layer 21 can provide the same improved compensation effect of the angular error as for the configuration shown in FIG. 8. In fact, the improved compensation effect can be obtained for the RKKY exchange coupling being antiferromagnetic and having an absolute magnitude of 0.4 mJ/m.sup.2 or greater.

TABLE-US-00001 Reference Numbers and Symbols 10 magnetic angular sensor device, sensing circuit 20 magnetic angular sensor element, MTJ 21 ferromagnetic pinned layer 210 reference layer magnetization, pinned magnetization 211 ferromagnetic pinned sublayer 212 non-magnetic sublayer 213 antiferromagnet 22 barrier layer 23 ferromagnetic sensing layer 23a first sensing layer 230 sensing layer magnetization 230a first sensing magnetization 23b second sensing layer 230b second sensing magnetization 24 metallic spacer 231 second sensing sublayer 2310 second sensing sub-magnetization 232 non-magnetic layer 30 sensing circuit 60 external magnetic field AE.sub.T total angular error AE.sub.R angular error in reference layer orientation AE.sub.S angular error in sensing layer orientation H.sub.ext magnitude of an external magnetic field H.sub.long longitudinal component H.sub.pin pinning field H.sub.trans transversal component H.sub.trans transversal component R resistance R.sub.ap resistance when the sensing and reference layer magnetization are antiparallel V.sub.in inutput voltage V.sub.out output voltage θ angle χ.sub.R magnetic susceptibility of the reference layer χ.sub.trans transversal magnetic susceptibility