MAGNETORESISTIVE ELEMENT AND MAGNETIC SENSOR DEVICE HAVING A HIGH SENSITIVITY AND LOW ZERO-FIELD OFFSET-SHIFT

A magnetoresistive element has a tunnel barrier layer included between a ferromagnetic reference layer having a fixed reference magnetization and a ferromagnetic sense layer having a free sense magnetization. The sense magnetization has a ferromagnetic material composition and a stable vortex configuration in the absence of an applied magnetic field. The ferromagnetic material composition varies across the thickness of the sense layer from a composition with higher magnetization near the tunnel barrier layer to a composition with lower magnetization away from the tunnel barrier layer, such that the sense magnetization and ferromagnetic exchange strength of the sense layer are higher near the tunnel barrier layer than away from the tunnel barrier layer.

TECHNICAL DOMAIN

The present invention concerns a magnetoresistive element adapted to sense an external magnetic field and having a wide linear response and a nominal performance that remains substantially unchanged after the magnetoresistive element has been subjected to high magnetic fields. The present disclosure further concerns a magnetic sensor device comprising a plurality of the magnetoresistive element.

RELATED ART

A conventional magnetoresistive sensor element typically comprises a ferromagnetic reference layer having a reference magnetization, a ferromagnetic sense layer having an averaged free sense magnetization and a tunnel barrier layer between the reference and sense ferromagnetic layers. The sense magnetization can be oriented in an external magnetic field while the reference magnetization remains substantially undisturbed. The external magnetic field can thus be sensed by measuring a resistance of the magnetoresistive sensor element. The resistance depends on the orientation and magnitude of the averaged sense magnetization relative to the reference magnetization.

The sense magnetization can comprise a stable vortex configuration. In the vortex configuration, the magnetization curls in a circular path along the edge of the sense layer and around a core reversibly movable in accordance with the external magnetic field. The vortex configuration provides a linear and non-hysteretic behavior in a large magnitude range of the external magnetic field, for practical size of the magnetoresistive sensor element and thickness of the sense layer. The vortex configuration is thus advantageous for magnetic sensor applications.

Vortex-based magnetoresistive sensors typically operate at low fields, for example external magnetic fields below 100 mT. The performance of vortex-based magnetoresistive sensors is often modified after being exposed to high magnetic fields, because such high fields can sufficiently saturate the sensor free layer magnetization that the vortex configuration no longer exists. This vortex annihilation or “expulsion” can occur for example in magnetic fields greater than 200 mT often used during magnetic reliability testing. When a vortex-based magnetoresistive sensors is subjected to such high magnetic fields, the details of the sensor magnetic configuration at low fields can be modified, and they therefore tend to suffer from zero-field offset shifts that reduces their accuracy in low field measurements.

SUMMARY

The present disclosure concerns a magnetoresistive element which comprises a tunnel barrier layer included between a ferromagnetic reference layer having a fixed reference magnetization and a ferromagnetic sense layer having a free sense magnetization. The sense magnetization comprises a ferromagnetic material composition and a stable vortex configuration in the absence of an applied magnetic field. The ferromagnetic material composition varies across the thickness of the sense layer in such a way that the sense magnetization and ferromagnetic exchange strength of the sense layer are higher near the tunnel barrier layer than away from the tunnel barrier layer.

The present disclosure further concerns a magnetic sensor device comprising a plurality of the magnetoresistive element.

With respect to what is known in the art, the magnetoresistive element disclosed herein has reduced zero-field offset-shift while having a high sensitivity. The magnetic sensor device can be exposed to high magnetic fields without significant change in its nominal performance.

EXAMPLES OF EMBODIMENTS

With reference toFIG.1, a magnetoresistive element2is represented according to an embodiment. The magnetoresistive element2comprises a tunnel barrier layer22included between a ferromagnetic reference layer21having a fixed reference magnetization210and a ferromagnetic sense layer23having a free sense magnetization230. The sense magnetization230can have its average magnetization oriented in an external magnetic field60while the reference magnetization210remains substantially undisturbed. The external magnetic field60can thus be sensed by measuring a resistance of the magnetoresistive sensor element2. The resistance depends on the average orientation of the sense magnetization230relative to the reference magnetization210.

In a preferred configuration, the sense magnetization230comprises a stable vortex configuration in the absence of an applied magnetic field. The vortex configuration consists in a magnetization which follows a circular path along the edge of the sense layer23and around a core231, the position of the core being reversibly movable in accordance with the external magnetic field60. For a given lateral dimension of the magnetoresistive sensor element2, the thickness of the sense layer23is chosen such that the sense layer23has a stable vortex configuration magnetization in the absence of an applied magnetic field.

In the example ofFIG.1, the reference magnetization210is substantially longitudinally oriented in the plane of the reference layer21. The magnetoresistive element2may comprise a reference pinning layer25configured to exchange couple the reference layer21. In the presence of the reference pinning layer25, the orientation of the reference magnetization210is determined by the exchange coupling (generating an exchange-bias) between the reference pinning layer25and the reference layer21. The reference pinning layer25can comprise an antiferromagnet (AFM). The reference layer21can comprise a synthetic antiferromagnet (SAF).

In an embodiment, the sense layer23comprises, or is formed of, a ferromagnetic material. The chemical composition of the ferromagnetic material varies across the thickness of the sense layer23from a composition with higher magnetization near the tunnel barrier layer22to a composition with lower magnetization away from the tunnel barrier layer22.

A specific chemical composition of the ferromagnetic material provides a high sense magnetization230. Here, the expression “magnetization” is used indifferently for “saturation magnetization” or “spontaneous magnetization”, where saturation magnetization has its usual meaning of the maximum induced magnetic moment. A ferromagnetic material composition yielding a high sense magnetization230provides a high ferromagnetic exchange strength. The ferromagnetic exchange strength can be tuned by varying the sense magnetization230and thus, by varying the chemical composition of the ferromagnetic material.

The variation of the ferromagnetic material composition across the thickness of the sense layer23results in a variation of magnetic properties across the sense layer thickness. Here, the sense layer23is configured such that the sense magnetization230and ferromagnetic exchange strength of the sense layer23are higher near the tunnel barrier layer22than away from the tunnel barrier layer22.

In one aspect illustrated inFIG.2, the ferromagnetic material composition comprises a composition gradient across the thickness of the sense layer23. The composition gradient can be linear or nonlinear across the thickness of the sense layer23. Here, the sense layer23can be formed as comprising a single layer having the composition gradient.

In another aspect illustrated inFIG.3, the sense layer23comprises a multilayer structure. The multilayer structure can comprise a first sublayer232a, near the tunnel barrier layer22and having a chemical composition resulting in high magnetization. The multilayer structure can further comprise a second sublayer232b, away from the tunnel barrier layer22and having a chemical composition resulting in a lower magnetization.

The multilayer structure may comprise more than two sublayers. In the example ofFIG.4, the sense layer23comprises the first sublayer232anear the tunnel barrier layer22and having a chemical composition resulting in high magnetization. The sense layer23further comprises the second sublayer232b, away from the tunnel barrier layer22and having a chemical composition resulting in low magnetization. An intermediary sublayer232is included between the first and second sublayers232a,232b. The intermediary sublayer232can have a ferromagnetic material composition that is varied or being constant across the sublayer thickness. The ferromagnetic material of the intermediary sublayer232can have a chemical composition resulting in a magnetization that is lower than the magnetization of the ferromagnetic material in the first sublayer232aand higher than the magnetization in the second sublayer232b.

FIG.5shows the sense layer23wherein the multilayer structure comprises more than three sublayers232,232a,232b. The ferromagnetic material composition can be constant across the thickness of each sublayer232,232a,232b. In such configuration, the variation of the ferromagnetic material chemical composition between the regions of the sense layer23near the tunnel barrier layer22and away from the tunnel barrier layer22can be obtained by varying the chemical composition of the ferromagnetic material between the different sublayers232,232a,232b. Alternatively, the ferromagnetic material chemical composition can be varied across the thickness of each sublayer232,232a,232b, or at least across the thickness of one of the sublayers232,232a,232bcomprised in the multilayer structure.

Each of the first sublayer232aand the second sublayer232bcan have a constant composition across the thickness of the sublayer232a,232b. Here, the variation in the ferromagnetic material magnetization can be obtained by the first sublayer232ahaving a chemical composition resulting in higher magnetization, and the second sublayer232bhaving a chemical composition resulting in lower magnetization.

Alternatively, at least one of the first or second sublayers232a232b, or both the first and second sublayers232a232bcan have a ferromagnetic material composition being modulated across the sublayer thickness.

InFIGS.1to5, a first portion of the sense layer23near the tunnel barrier layer22is indicated by the numeral23aand a second portion23bof the sense layer23away from the tunnel barrier layer22is indicated by the numeral23b. In all the configurations, the ferromagnetic material composition varies across the thickness of the sense layer23from a composition resulting in higher magnetization near the tunnel barrier layer22(in the first portion23a), to a composition resulting in a lower magnetization away from the tunnel barrier layer22(in the second portion23b).

In some embodiments, the sense magnetization230in the first portion23aof the sense layer23is at least 30% higher than the sense magnetization230in the second portion23bof the sense layer23. The first and second portions23a,23bcan correspond to about a third of the thickness of the sense layer23. Depending on the configuration of the sense layer23(such as the exemplary configurations ofFIGS.1to5), the first and second portions23a,23bcan be comprised in a single layer or encompass one of more than one sublayer of a multilayer structure of the sense layer23.

In some embodiments, the reference and sense layers21,23can comprise, or can be 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 layer21can have a thickness between 2 nm and 10 nm. The reference and sense magnetizations210,230can have magnetic anisotropy substantially parallel to the plane of the layers21,23(in-plane, as shown inFIG.1) and/or substantially perpendicular to the plane of the layers21,23(out-of-plane).

The tunnel barrier22can comprise an insulating material. Suitable insulating materials include oxides, such as aluminum oxide (e.g., Al2O3) and magnesium oxide (e.g., MgO). A thickness of the tunnel barrier layer22can be in the nm range, such as from about 1 nm to about 3 nm.

In a preferred embodiment, the sense layer23comprises, or is formed of, a ferromagnetic material containing a CoFe, NiFe or CoFeB based alloy, alone in in combination.

In an embodiment, the ferromagnetic material comprises a mixture containing CoFe and NiFe -based alloy where the concentration of CoFe relative to NiFe is higher near the tunnel barrier layer22and lower away from the tunnel barrier layer22. In one aspect, the sense layer23can be configured such that the ferromagnetic material composition varies across the thickness of the sense layer23from a concentration of the CoFe based alloy higher than the concentration of NiFe based alloy near the tunnel barrier layer22, to a concentration of the CoFe based alloy lower than the concentration of NiFe based alloy away from the tunnel barrier layer22. In another aspect, the concentration of CoFe relative to NiFe can be higher in the first portion23aand lower in the second portion23b. In one aspect, the ferromagnetic material composition in the first portion23acan comprise at least 95% by volume of a CoFe-based alloy and the second portion23bcan comprise at least 95% by volume of a NiFe-based alloy. In a further aspect, the first portion23acan have a thickness of about 75% of the thickness of the sense layer23, the second portion23bhaving a thickness of about 25% of the thickness of the sense layer23. Different thickness ratios of the first portion23ato the second portion23bcan also be contemplated.

Referring again toFIG.2, the magnetoresistive element2can comprises an interface layer24between the sense layer23and the tunnel barrier layer22. The interface layer24is configured to increase the tunneling magnetoresistance (TMR) of the magnetoresistive element2. More particularly, the interface layer24can comprise a CoFe or CoFeB alloy and have a thickness between 1 nm and 3 nm. Preferably, the tunnel barrier layer22comprises MgO.

FIG.6reports experimental data measured for a magnetoresistive element having a lateral dimension (diameter) of 450 nm and comprising a MgO tunnel barrier layer22, a reference layer21comprising a SAF structure and an in-plane reference magnetization210. The reference magnetization210is pinned by the antiferromagnetic layer25. The measurements were performed for a sense layer23having a constant ferromagnetic material composition across the thickness of the sense layer23and having 50% by volume of CoFe and NiFe based alloy (A) or having 75% by volume of CoFe and NiFe based alloy (B). The measurements were also performed for a sense layer23having a varying ferromagnetic material composition across the thickness of the sense layer23and having 75% by volume of CoFe (C) near the tunnel barrier layer22.

FIG.6shows that the sensitivity decreases with the total sense magnetization230, when the volume percentage of the CoFe based alloy is increased relative to the volume percentage of the NiFe based alloy. For a sense layer23having a ferromagnetic material composition constant across the layer thickness and comprising 75% by volume of the CoFe based alloy, a sensitivity of about 1.75 mV/V/mT and an offset shift centered around 5 mV/V are measured. In the case of the sense layer23having a ferromagnetic material composition varying across the layer thickness and where the first portion23acomprises a ferromagnetic material composition of about 75% by volume of CoFe, a sensitivity similar to the one measured for the ferromagnetic material composition constant across the layer thickness and comprising 75% by volume of the CoFe based alloy. However, a much lower offset shift (centered around 0 mV/V) is measured.

The sense layer23having a composition of the ferromagnetic material that varies across the thickness of the sense layer23can be obtained by using various fabrication methods. For example, the sense layer23can be formed using continuous alloying, formation of multilayers, alloying or layering with non-magnetic materials, or a combination of layering and alloying, such as alloying or layering with non-magnetic materials. One way to reduce the sense magnetization230and the ferromagnetic exchange strength is dilution of the ferromagnetic material by nonmagnetic transition metals. A composition gradient of the ferromagnetic material across the thickness of the sense layer23can be obtained in a single-phase ferromagnetic material with gradient of dilution across the thickness. A preferred fabrication method of the sense layer23includes layering and co-deposition.

Deposition methods may include chemical vapor deposition (e.g., CVD, MOCVD, and the like), molecular beam epitaxy (MBE), pulsed laser deposition (PLD), and sputtering (e.g., RF sputtering and DC sputtering).

In an embodiment illustrated inFIG.7, a magnetic sensor device50comprises a plurality of the magnetoresistive element2. Fig. shows the magnetic sensor device50arranged in a full-bridge configuration, however other configurations of the magnetic sensor device50can be contemplated, for example a a half-bridge.

The magnetoresistive element2disclosed herein has reduced zero-field offset-shift while having a good sensitivity. The magnetoresistive element2can be used advantageously in a magnetic sensor device having a reduced zero-field offset-shift, even after the magnetic sensor device has been exposed to high fields. For example, the magnetic sensor device can be exposed to high magnetic fields, such as magnetic fields greater than 200 mT used during magnetic reliability testing, without significant change in its nominal performance.

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