Magnetoresistive element and magnetic sensor device with high sensitivity and low zero magnetic field offset shift

Abstract

A highly sensitive magnetoresistance element and a magnetic sensor device that can be used even in a strong magnetic field are provided. The magnetoresistive element (2) comprises a tunnel barrier layer (22) disposed between a ferromagnetic reference layer (21) having a fixed reference magnetization (210) and a ferromagnetic sensing layer (23) having a free sensing magnetization (230). The sense magnetization (230) comprises a ferromagnetic material composition and a vortex configuration that is stable in the absence of an applied magnetic field. The ferromagnetic material composition varies across the thickness of the sensing layer (23) from a composition having a high magnetization near the tunnel barrier layer (22) to a composition having a lower magnetization away from the tunnel barrier layer (22) such that the sense magnetization (230) and ferromagnetic exchange strength of the sensing layer (23) is higher near the tunnel barrier layer (22) than away from the tunnel barrier layer (22).

Description

Technical Field

[0001] The present invention relates to a magnetoresistive element adapted to detect an external magnetic field and having a wide linear response and a nominal performance that does not substantially change even after the magnetoresistive element is exposed to a high magnetic field. The present disclosure further relates to a magnetic sensor device including a plurality of the aforementioned magnetoresistive elements.

Background Art

[0002] Conventional magnetoresistive sensor elements typically include a ferromagnetic reference layer having a reference magnetization, a ferromagnetic detection layer having an averaged free magnetization, and a tunnel barrier layer between the reference ferromagnetic layer and the detection ferromagnetic layer. The reference magnetization remains substantially undisturbed, and the detection magnetization can be oriented in an external magnetic field. In this way, an external magnetic field can be detected by measuring the resistance of the magnetoresistive sensor element. The resistance value depends on the orientation and magnitude of the averaged detection magnetization with respect to the reference magnetization.

[0003] The detection magnetization can have a stable vortex configuration. In the vortex configuration, the magnetization winds in a circular path around a core that can move reversibly along the edge of the detection layer according to an external magnetic field. The vortex configuration provides linear and non-hysteretic behavior in a large magnitude range of an external magnetic field with respect to the practical size of the magnetoresistive sensor element and the thickness of the detection layer. Thus, the vortex structure is advantageous for magnetic sensor applications.

[0004] Vortex-based magnetoresistive sensors typically operate in an external magnetic field of low magnetic field, for example, 100 mT or less. The performance of vortex-based magnetoresistive sensors often changes after being exposed to a high magnetic field. This is because such a high magnetic field sufficiently saturates the magnetization of the sensor free layer, and the vortex configuration no longer exists. The disappearance or "expulsion" of this vortex may occur, for example, in a magnetic field exceeding 200 mT, which is often used in magnetic reliability tests. When a magnetoresistive sensor using a vortex is exposed to such a high magnetic field, the details of the magnetic arrangement of the sensor in a low magnetic field may be changed, and thus it tends to be troubled by a zero magnetic field offset shift that reduces the accuracy in low magnetic field measurement.

[0005] Patent Document 1 discloses a sensor comprising first and second magnetoresistive sensor elements configured to generate first and second output signals, respectively, in response to an external magnetic field. The first and second magnetoresistive sensor elements form a gradient unit, and each magnetoresistive sensor element comprises a sensing layer having an eddy magnetization pattern. A processing circuit is coupled to the sensor elements and is configured to generate a differential output signal as the difference between the first and second output signals of the first and second magnetoresistive sensor elements of the gradient unit. The system comprises an encoder that generates an external magnetic field and a sensor having one or more gradient units, the gradient units of which can be arranged in a secondary gradient detection configuration.

[0006] Patent document 2 discloses a magnetic tunnel junction apparatus comprising a substrate and a laminated structure on the substrate.

[0007] Patent Document 3 discloses that a stack of magnetic films comprises a synthetic antiferromagnetic material including a plurality of ferromagnetic layers (two adjacent ferromagnetic layers are antiferromagnetically coupled via a non-magnetic layer) and a reversal-induced layer that exhibits ferromagnetism.

[0008] Patent document 4 discloses a magnetoresistive sensor element in which the detection layer compensates for the temperature dependence of the tunnel magnetoresistance of the magnetoresistive sensor element. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] U.S. Patent Application Publication No. 2010 / 316890 [Patent Document 2] U.S. Patent Application Publication No. 2009 / 219754 [Patent Document 3] U.S. Patent Application Publication No. 2009 / 219754 [Patent Document 4] European Patent Application Publication No. 3862769 [Overview of the project]

[0010] This disclosure relates to a magnetoresistive element comprising a tunnel barrier layer between a ferromagnetic reference layer having a fixed reference magnetization and a ferromagnetic sensing layer having a free sensing magnetization. The sensing magnetization comprises a ferromagnetic material composition and has a stable vortex configuration in the absence of an applied magnetic field. The ferromagnetic material composition varies over the thickness of the sensing layer such that the sensing magnetization and ferromagnetic exchange intensity of the sensing layer are higher near the tunnel barrier layer than away from the tunnel barrier layer.

[0011] This disclosure further relates to a magnetic sensor device comprising multiple magnetoresistive elements.

[0012] With respect to those known in the art, the magnetoresistive elements disclosed herein have high sensitivity while reducing zero-field offset shift. This magnetic sensor device can be exposed to high magnetic fields without significantly altering its nominal performance.

[0013] Exemplary embodiments of the present invention are disclosed herein and shown by the drawings. [Brief explanation of the drawing]

[0014] [Figure 1] Figure 1 schematically shows a magnetoresistive element equipped with a sensing layer according to an embodiment. [Figure 2] Figure 2 shows a detection layer comprising a ferromagnetic material having a compositional gradient over the thickness of the detection layer, according to one embodiment. [Figure 3] Figure 3 shows a detection layer with a multilayer structure according to the embodiment. [Figure 4] Figure 4 shows a detection layer with a multilayer structure according to another embodiment. [Figure 5] Figure 5 shows a detection layer with a multilayer structure according to yet another embodiment. [Figure 6] Figure 6 shows experimental data measured for a magnetoresistive element. [Figure 7] Figure 7 shows a magnetic sensor device in which multiple magnetoresistive elements are arranged in a full-bridge configuration. [Modes for carrying out the invention]

[0015] Referring to Figure 1, a magnetoresistive element 2 according to an embodiment is shown. The magnetoresistive element 2 comprises a tunnel barrier layer 22 between a ferromagnetic reference layer 21 having a fixed reference magnetization 210 and a ferromagnetic sensing layer 23 having a free sensing magnetization 230. The sensing magnetization 230 is oriented in an external magnetic field 60 while the reference magnetization 210 remains substantially undisturbed. The external magnetic field 60 is detected by measuring the resistance value of the magnetoresistive sensor element 2. The resistance value depends on the average orientation of the sensing magnetization 230 relative to the reference magnetization 210.

[0016] In a preferred configuration, the sense magnetization 230 has a stable vortex configuration in the absence of an applied magnetic field. This vortex structure is composed of a magnetization that follows a circular path around the core 231 along the edge of the sense layer 23, and the position of the core is reversibly movable according 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 selected such that the sense layer 23 has a magnetization of a stable vortex configuration in the absence of an applied magnetic field.

[0017] In the example of FIG. 1, the reference magnetization 210 is substantially longitudinally oriented within the plane of the reference layer 21. The magnetoresistive element 2 may include a reference pinning layer 25 configured to exchange-couple the reference layer 21. When the reference pinning layer 25 is present, the orientation of the reference magnetization 210 is determined by the exchange coupling (generation of exchange bias) between the reference pinning layer 25 and the reference layer 21. The reference pinning layer 25 may include an antiferromagnet (AFM). The reference layer 21 may include a synthetic antiferromagnet (SAF).

[0018] In one embodiment, the sense layer 23 comprises or is formed of a ferromagnetic material. The chemical composition of the ferromagnetic material varies over the thickness of the sense layer 23 from a high magnetization composition close to the tunnel barrier layer 22 to a low magnetization composition away from the tunnel barrier layer 22.

[0019] A specific chemical composition of the ferromagnetic material provides a high-sensitivity magnetization 230. Here, the expression "magnetization" is used without distinction with respect to "saturation magnetization" or "spontaneous magnetization", and saturation magnetization has the ordinary meaning of the maximum induced magnetic moment. The ferromagnetic material composition that results in a high sense magnetization 230 provides a high ferromagnetic exchange strength. The ferromagnetic exchange strength can be adjusted by changing the sense magnetization 230 and thus by changing the chemical composition of the ferromagnetic material.

[0020] The magnetic properties of the detection layer change over the thickness of the detection layer as the ferromagnetic material composition changes over the thickness of the detection layer 23. Here, the detection layer 23 is configured such that the detection magnetization 230 and ferromagnetic exchange intensity of the detection layer 23 are higher near the tunnel barrier layer 22 than away from the tunnel barrier layer 22.

[0021] In one view shown in Figure 2, the ferromagnetic material composition has a compositional gradient over the thickness of the detection layer 23. The compositional gradient may be linear or nonlinear over the thickness of the detection layer 23. Here, the detection layer 23 may be formed to consist of a single layer having the compositional gradient.

[0022] In another view shown in Figure 3, the detection layer 23 has a multilayer structure. The multilayer structure may include a first sublayer 232a that is close to the tunnel barrier layer 22 and has a chemical composition that results in high magnetization. The multilayer structure may further include a second sublayer 232b that is further away from the tunnel barrier layer 22 and has a chemical composition that results in low magnetization.

[0023] The multilayer structure may comprise two or more sublayers. In the example in Figure 4, the sensing layer 23 comprises a first sublayer 232a located near the tunnel barrier layer 22 and having a chemical composition that yields high magnetization. The sensing layer 23 further comprises a second sublayer 232b located away from the tunnel barrier layer 22 and having a chemical composition that yields low magnetization. An intermediate sublayer 232 is located between the first sublayer 232a and the second sublayer 232b. The intermediate sublayer 232 may comprise a ferromagnetic material composition that varies or is constant across the thickness of the sublayer. The ferromagnetic material of the intermediate sublayer 232 may comprise a chemical composition that yields a magnetization lower than that of the ferromagnetic material of the first sublayer 232a and higher than that of the second sublayer 232b.

[0024] Figure 5 shows a detection layer 23 in which the multilayer structure comprises more than three sublayers 232, 232a, and 232b. The ferromagnetic material composition may be constant across the thickness of each sublayer 232, 232a, and 232b. In such a configuration, the change in the chemical composition of the ferromagnetic material between the region of the detection layer 23 close to the tunnel barrier layer 22 and the region of the detection layer 23 far from the tunnel barrier layer 22 is achieved by varying the chemical composition of the ferromagnetic material between the different sublayers 232, 232a, and 232b. Alternatively, the chemical composition of the ferromagnetic material may be varied across the thickness of each sublayer 232, 232a, and 232b, or across the thickness of at least one of the sublayers 232, 232a, and 232b in the multilayer structure.

[0025] Each of the first sublayer 232a and the second sublayer 232b may have a constant composition over the thickness of sublayers 232a and 232b. Here, the variation in magnetization of the ferromagnetic material is obtained by the first sublayer 232a having a chemical composition that yields a higher magnetization and the second sublayer 232b having a chemical composition that yields a lower magnetization.

[0026] Alternatively, at least one of the first sublayer 232a or the second sublayer 232b, or both of the first sublayer 232a and the second sublayer 232b, may have a ferromagnetic material composition that is modulated over the thickness of the sublayer.

[0027] In Figures 1 to 5, the first portion of the detection layer 23 near the tunnel barrier layer 22 is indicated by the number 23a, and the second portion 23b of the detection layer 23, which is further away from the tunnel barrier layer 22, is indicated by the number 23b. In all configurations, the composition of the ferromagnetic material is varied across the thickness of the detection layer 23, from a composition that yields high magnetization near the tunnel barrier layer 22 (first portion 23a) to a composition that yields low magnetization further away from the tunnel barrier layer 22 (second portion 23b).

[0028] In some embodiments, the detection magnetization 230 in the first portion 23a of the detection layer 23 is at least 30% higher than the detection magnetization 230 in the second portion 23b of the detection layer 23. The first and second portions 23a and 23b may correspond to about one-third of the thickness of the detection layer 23. Depending on the configuration of the detection layer 23 (as in the exemplary configurations shown in Figures 1 to 5), the first and second portions 23a and 23b may consist of a single layer or may encompass one of more than one sublayer of the multilayer structure of the detection layer 23.

[0029] In some embodiments, the reference layer and the sensing layer 21, 23 may comprise or be formed from a ferromagnetic material such as a cobalt ("Co"), iron ("Fe"), or nickel ("Ni") based alloy, preferably a CoFe, NiFe, or CoFeB based alloy. The reference layer 21 may have a thickness between 2 nm and 10 nm. The reference and sensing magnetizations 210, 230 may have magnetic anisotropy such that they are substantially parallel to the plane of layers 21, 23 (in-plane, as shown in Figure 1) and substantially perpendicular to the plane of layers 21, 23 (out-of-plane).

[0030] The tunnel barrier 22 may comprise an insulating material. Suitable insulating materials include oxides such as aluminum oxide (e.g., Al2O3) and magnesium oxide (e.g., MgO). The thickness of the tunnel barrier layer 22 can be in the range of nm, such as from about 1 nm to about 3 nm.

[0031] In one preferred embodiment, the detection layer 23 comprises or is formed from a ferromagnetic material including one or a combination of CoFe, NiFe, or CoFeB-based alloys.

[0032] In one embodiment, the ferromagnetic material comprises a mixture containing CoFe and a NiFe-based alloy, where the concentration of CoFe relative to NiFe is high near the tunnel barrier layer 22 and decreases away from the tunnel barrier layer 22. In one view, the sensing layer 23 can be configured such that the ferromagnetic material composition varies over the thickness of the sensing layer 23, from a concentration of the CoFe-based alloy higher than the concentration of the NiFe-based alloy near the tunnel barrier layer 22 to a concentration of the CoFe-based alloy lower than the concentration of the NiFe-based alloy away from the tunnel barrier layer 22. In another view, the concentration of CoFe relative to NiFe can be higher in the first portion 23a and lower in the second portion 23b. In one view, the ferromagnetic material composition of the first portion 23a may comprise at least 95 volume% of the CoFe-based alloy, and the second portion 23b may comprise at least 95 volume% of the NiFe-based alloy. In a further view, the first portion 23a may have a thickness of approximately 75% of the thickness of the detection layer 23, and the second portion 23b may have a thickness of approximately 25% of the thickness of the detection layer 23. Different thickness ratios of the first portion 23a and the second portion 23b may also be considered.

[0033] Referring again to Figure 2, the magnetoresistive element 2 may include an interface layer 24 between the sensing layer 23 and the tunnel barrier layer 22. The interface layer 24 is configured to increase the tunnel magnetoresistance (TMR) of the magnetoresistive element 2. More specifically, the interface layer 24 may comprise CoFe or a CoFeB alloy and have a thickness between 1 nm and 3 nm. Preferably, the tunnel barrier layer 22 comprises MgO.

[0034] Figure 6 reports experimental data measured for a magnetoresistive element with a lateral dimension (diameter) of 450 nm, comprising an MgO tunnel barrier layer 22, a reference layer 21 with an SAF structure, and an in-plane reference magnetization 210. The reference magnetization 210 is fixed by an antiferromagnetic layer 25. Measurements were performed on a detection layer 23 having a constant ferromagnetic material composition over its thickness, comprising either a 50 volume% CoFe and NiFe-based alloy (A) or a 75 volume% CoFe and NiFe-based alloy (B). Measurements were also performed on a detection layer 23 with varying ferromagnetic material composition over its thickness, and with 75 volume% CoFe(C) near the tunnel barrier layer 22.

[0035] Figure 6 shows that as the volume percentage of the CoFe-based alloy increases relative to the volume percentage of the NiFe-based alloy, the sensitivity decreases along with the total detected magnetization 230. For the detection layer 23, which has a constant ferromagnetic material composition across its thickness and contains 75 volume% of the CoFe-based alloy, a sensitivity of approximately 1.75 mV / V / mT and an offset shift centered around 5 mV / V were measured. When the detection layer 23 has a ferromagnetic material composition that varies across its thickness, and the first portion 23a has a ferromagnetic material composition of approximately 75 volume% of CoFe, a sensitivity similar to that measured when it has a constant ferromagnetic material composition across its thickness and contains 75 volume% of the CoFe-based alloy is measured. However, a much lower offset shift (centered around 0 mV / V) is measured.

[0036] A detection layer 23 having a ferromagnetic material composition that varies over its thickness can be obtained using various manufacturing methods. For example, the detection layer 23 can be formed using a combination of lamination and alloying, such as continuous alloying, multilayer formation, alloying or lamination with a nonmagnetic material, or alloying or lamination with a nonmagnetic material. One method for reducing the detection magnetization 230 and ferromagnetic exchange intensity is dilution of the ferromagnetic material with a nonmagnetic transition metal. The compositional gradient of the ferromagnetic material over the thickness of the detection layer 23 is obtained within a single-phase ferromagnetic material with a dilution gradient over its thickness. Preferred manufacturing methods for the detection layer 23 include lamination and co-deposition.

[0037] Deposition methods include chemical vapor deposition (CVD, MOCVD, etc.), molecular beam epitaxy (MBE), pulsed laser deposition (PLD), and sputtering (RF sputtering, DC sputtering, etc.).

[0038] In the embodiment shown in Figure 7, the magnetic sensor device 50 comprises a plurality of magnetoresistive elements 2. In the figure, the magnetic sensor device 50 is arranged in a full-bridge configuration, but other configurations of the magnetic sensor device 50, such as a half-bridge, are also possible.

[0039] The magnetoresistive element 2 disclosed herein exhibits good sensitivity while reducing the zero-field offset shift. The magnetoresistive element 2 is advantageous for use in magnetic sensor devices with reduced zero-field offset shift even after the magnetic sensor device has been exposed to a high magnetic field. For example, the magnetic sensor device can be exposed to high magnetic fields, such as those exceeding 200 mT used during magnetic reliability testing, without significantly altering its nominal performance. [Explanation of symbols]

[0040] 2 magnetoresistance element 21 Reference layer 210 Reference magnetization 22 Tunnel barrier layer 23 detection layer 23a First part of the detection layer 23b Second part of the detection layer 230 Magnetization detection 231 cores 232 sublayer 232a, 232b sublayer 24 Interface layer 25 Antiferromagnetic layer 50 Magnetic sensor device 60 External magnetic field

Claims

1. A magnetoresistive element comprising a tunnel barrier layer between a ferromagnetic reference layer having a fixed reference magnetization and a ferromagnetic sensing layer having a free sensing magnetization, The detected magnetization comprises a ferromagnetic material composition and a stable vortex configuration in the absence of an applied magnetic field. In the magnetoresistive element, the ferromagnetic material composition is varied over the thickness of the detection layer, from a composition having high magnetization near the tunnel barrier layer to a composition having lower magnetization as it moves away from the tunnel barrier layer, such that the detection magnetization and ferromagnetic exchange intensity of the detection layer are higher near the tunnel barrier layer than farther away from the tunnel barrier layer. The detection magnetization of the first portion of the detection layer near the tunnel barrier layer is at least 30% higher than the detection magnetization of the second portion of the detection layer away from the tunnel barrier layer, and the concentration of CoFe relative to NiFe is higher in the first portion and lower in the second portion. A magnetoresistive element in which the first portion has a thickness of approximately 75% of the thickness of the detection layer.

2. The magnetoresistive element according to claim 1, wherein the ferromagnetic material composition has a compositional gradient over the thickness of the detection layer.

3. The magnetoresistive element according to claim 1, wherein the sensing layer has a multilayer structure comprising a first sublayer having a composition with higher magnetization and being close to the tunnel barrier layer, and a second sublayer having a composition with lower magnetization and being further away from the tunnel barrier layer.

4. The magnetoresistive element according to claim 3, wherein the multilayer structure comprises more than two sublayers.

5. The magnetoresistive element according to claim 3, wherein each sublayer has a constant composition over the thickness of the sublayer, and the constant composition varies between different sublayers.

6. The magnetoresistive element according to claim 3, wherein at least one of the sublayers has a composition that varies over the thickness of the sublayer.

7. A magnetoresistive element comprising a tunnel barrier layer provided between a ferromagnetic reference layer having a fixed reference magnetization and a ferromagnetic detection layer having a free detection magnetization, The detected magnetization comprises a ferromagnetic material composition and a stable vortex configuration in the absence of an applied magnetic field. In the magnetoresistive element, the ferromagnetic material composition is varied over the thickness of the detection layer, from a composition having high magnetization near the tunnel barrier layer to a composition having lower magnetization as it moves away from the tunnel barrier layer, such that the detection magnetization and ferromagnetic exchange intensity of the detection layer are higher near the tunnel barrier layer than farther away from the tunnel barrier layer. The detection magnetization of the first portion of the detection layer near the tunnel barrier layer is at least 30% higher than the detection magnetization of the second portion of the detection layer away from the tunnel barrier layer, and the concentration of CoFe relative to NiFe is higher in the first portion and lower in the second portion. The first and second portions are magnetoresistive elements, each corresponding to approximately one-third of the thickness of the detection layer.

8. The magnetoresistive element according to claim 1, wherein the ferromagnetic material composition contains an alloy based on CoFe, CoFeB, or NiFe.

9. The ferromagnetic material composition contains an alloy based on CoFe and NiFe, wherein the concentration of CoFe relative to NiFe is high near the tunnel barrier layer and decreases as it moves away from the tunnel barrier layer. The magnetoresistive element according to claim 8.

10. The magnetoresistive element according to claim 1, wherein the magnetoresistive element comprises an interface layer between the sensing layer and the tunnel barrier layer, and the interface layer contains CoFe or a CoFeB alloy and has a thickness of 1 nm to 3 nm.

11. A magnetic sensor device comprising multiple magnetoresistive elements, wherein each magnetoresistive element is It comprises a ferromagnetic reference layer having a fixed reference magnetization and a ferromagnetic detection layer having a free detection magnetization, with a tunnel barrier layer between them. The detected magnetization comprises a ferromagnetic material composition and a stable vortex configuration in the absence of an applied magnetic field. In the magnetic sensor device, the ferromagnetic material composition is varied over the thickness of the detection layer, from a composition having high magnetization near the tunnel barrier layer to a composition having lower magnetization as it moves away from the tunnel barrier layer, such that the detection magnetization and ferromagnetic exchange intensity of the detection layer are higher near the tunnel barrier layer than farther away from the tunnel barrier layer. The detection magnetization of the first portion of the detection layer near the tunnel barrier layer is at least 30% higher than the detection magnetization of the second portion of the detection layer away from the tunnel barrier layer. The concentration of CoFe relative to NiFe is higher in the first part and lower in the second part. A magnetic sensor device characterized in that the first portion has a thickness of approximately 75% of the thickness of the detection layer.

12. The magnetic sensor device according to claim 11, arranged in a half-bridge or full-bridge configuration.

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