MAGNETORESISTIVE SENSOR ELEMENT FOR SENSING A TWO-DIMENSIONAL MAGNETIC FIELD WITH LOW HIGH-FIELD ERROR

A magnetoresistive element for a two-dimensional magnetic field sensor, including: a ferromagnetic reference layer having a fixed reference magnetization, a ferromagnetic sense layer having a sense magnetization that can be freely oriented relative to the reference magnetization in the presence of an external magnetic field, and a tunnel barrier layer between the reference and sense ferromagnetic layers; the reference layer including a reference coupling layer between a reference pinned layer and a reference coupled layer; the reference coupled layer including a first coupled sublayer in contact with the reference coupling layer, a second coupled sublayer, a third coupled sublayer and a insert layer between the second and third coupled sublayers; the insert layer comprising a transition metal and has a thickness between about 0.1 and about 0.5 nm, and the thickness of the reference coupled layer is between about 1 nm and about 5 nm.

FIELD

The present invention relates to a magnetoresistive sensor element for sensing a two-dimensional (2D) magnetic field with low high-field error. The present invention also relates to a magnetic field sensor comprising the magnetoresistive element.

BACKGROUND

A magnetic sensor element based on the tunnel magnetoresistance (TMR) effect can be used for 2D magnetic field detection. Such magnetic sensor element typically comprises a ferromagnetic reference layer having a fixed reference magnetization, a tunnel barrier layer and a ferromagnetic sense layer having a sense magnetization freely orientable relative to the reference magnetization in the presence of the magnetic field. The reference layer can comprise a synthetic antiferromagnetic (SAF) structure including a pinned first ferromagnetic reference layer in contact with an antiferromagnetic layer, a coupling spacer layer and a second ferromagnetic reference layer. In order to have a good accuracy, the magnetic sensor element should have low angular error at high magnetic fields.

Low angular error at high magnetic fields can be achieved by increasing the stiffness of the SAF structure. Increasing the stiffness of the SAF structure is usually achieved by decreasing the thicknesses of the first ferromagnetic reference layer and of the second ferromagnetic reference layer. For instance, the thicknesses of the first and second ferromagnetic layers can be decreased down to 1.0 nm. This results in increasing the saturation field Hsatof the magnetic sensor element. The magnetization of the SAF structure becomes more stable (rigid) at high applied magnetic fields. However, decreasing the thickness of the first and second first ferromagnetic reference layers is detrimental to magneto-transport properties of the magnetic sensor element. Such small thickness further results in losing the pinning with the AF layer and the TMR response of the magnetic sensor element becomes very low.

Document US2011134563 relates to a magnetoresistive head including a magnetically pinned layer, a free magnetic layer positioned above the magnetically pinned layer, and a tunnel barrier layer, wherein at least one of the magnetically pinned layer and the free magnetic layer has a layered structure, the layered structure including a crystal layer comprising one of: a CoFe magnetic layer or a CoFeB magnetic layer and an amorphous magnetic layer comprising CoFeB and an element selected from: Ta, Hf, Zr, and Nb, wherein the crystal layer is positioned closer to a tunnel barrier layer than the amorphous magnetic layer.

Document US2012257298 describes a TMR head including an AFM layer, a first ferromagnetic layer above the AFM layer, a second ferromagnetic layer above the first ferromagnetic layer, an AF coupling layer between the first and second ferromagnetic layers, a fixed layer above the second ferromagnetic layer, an insertion layer adjacent the fixed layer or in the fixed layer, a barrier layer above the fixed layer, and a free layer above the barrier layer.

Document US202006679 describes a material stack including a first magnetoresistance element with a first direction of response to an external magnetic field and a second magnetoresistance element with second direction of response to the external magnetic field, opposite to the first direction of response. The first magnetoresistance element can be disposed under or over the second magnetoresistance element. An insulating layer separates the first and second magnetoresistance elements.

Document U.S. Pat. No. 8,582,253 discloses a magnetic sensor having a high spin polarization reference layer stack above AFM layers. The reference layer stack comprises a first boron-free ferromagnetic layer above the AFM coupling layer; a magnetic coupling layer on and in contact with the first boron-free ferromagnetic layer; a second ferromagnetic layer comprising boron deposited on and contact with the magnetic coupling layer; and a boron-free third ferromagnetic layer on and in contact the second ferromagnetic layer.

Document US2015162525 discloses a magnetic memory device including a magnetic tunnel junction memory element that may include a reference magnetic layer, a tunnel barrier layer, and a free magnetic layer. The reference magnetic layer may include a first pinned layer, an exchange coupling layer, and a second pinned layer. The exchange coupling layer may be between the first and second pinned layers, and the second pinned layer may include a ferromagnetic layer and a non-magnetic layer. The second pinned layer may be between the first pinned layer and the tunnel barrier layer, and the tunnel barrier layer may be between the reference magnetic layer and the free magnetic layer.

SUMMARY

The present disclosure concerns a magnetoresistive element for a two-dimensional magnetic field sensor, comprising a ferromagnetic reference layer having a fixed reference magnetization, a ferromagnetic sense layer having a sense magnetization that can be freely oriented relative to the reference magnetization in the presence of an external magnetic field, and a tunnel barrier layer between the reference and sense ferromagnetic layers; the reference layer comprising a reference coupling layer between a reference pinned layer and a reference coupled layer; the reference coupled layer comprises a first coupled sublayer in contact with the reference coupling layer, a second coupled sublayer, a third coupled sublayer and a insert layer between the second and third coupled sublayers; the insert layer comprising a transition metal and has a thickness between about 0.1 and about 0.5 nm, and the thickness of the reference coupled layer is between about 1 nm and about 5 nm.

In an embodiment, the reference pinned layer comprises a CoFe alloy and has a thickness of 2 nm, the tunnel barrier layer comprises Mg, insert layer comprises Ta, the first coupled sublayer is made of a CoFe alloy, 0.5 nm in thickness, the second coupled sublayer is made of a CoFeB alloy and has a thickness of 0.75 nm, and the third coupled sublayer is made of a CoFeB alloy and has a thickness between 0.45 nm and 0.95 nm. The magnetoresistive element is thermally treated at 310° C. during 90 min under an applied magnetic field of about 1 T.

The present disclosure further concerns a magnetic field sensor for sensing a two-dimensional magnetic field, comprising the magnetoresistive element.

The magnetoresistive element disclosed herein has high saturation field Hsatand TMR response. The magnetoresistive element further has high stiffness of the SAF reference layer and improved thermal stability. The magnetoresistive element provides reduced angular error, even at high magnetic fields, and thus improved accuracy. The high saturation field Hsat, exchange stiffness (SAF coupling) and TMR response of the magnetoresistive element can be obtained without decreasing the magnetic layer thicknesses of the SAF structure.

DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS

FIG.1shows a schematic view of a magnetoresistive element2according to an embodiment. The magnetoresistive element2comprises a ferromagnetic reference layer21, a ferromagnetic sense layer23and a tunnel barrier layer22between the reference and sense ferromagnetic layers21,23. The reference layer21has a fixed reference magnetization210while the sense layer23has a sense magnetization230that can be freely oriented relative to the reference magnetization210in the presence of an external magnetic field60. In other words, when the magnetoresistive element2is in the presence of the external magnetic field60, the reference magnetization210remains substantially fixed while the sense magnetization230is deflected in the direction of the external magnetic field60. The magnetoresistive element2can further comprise a capping layer25on the side of the sense layer23and a seed layer27on the side of the reference layer21.

The capping layer25can comprise a layer of TaN, Ru or Ta. The capping layer25can comprises multilayers including any layer of tantalum nitride (TaN), ruthenium (Ru) or tantalum (Ta) or a combination of these layers. In a particular configuration, the capping layer25comprises a multilayer including a 80 nm layer of TaN, a 5 nm layer of Ru, a 2 nm layer of TaN, a 5 nm layer of Ru, a 2 nm layer of Ta and a 1 nm layer of Ru. The seed layer27can comprise any one of Ta, tungsten (W), molybdenum (Mo), titanium (Ti), hafnium (Hf) or magnesium (Mg).

The tunnel barrier layer22can comprise, or be formed of, 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. An optimal thickness of the tunnel barrier22can be obtained by inserting a plurality (double or multilayer) of MgO (or another suitable oxide or insulating material) layers. The tunnel barrier layer22can be configured to provide high TMR, for example above 80%.

The reference layer21and the sense layer23can include, or be formed of, a magnetic material and, in particular, a magnetic material of the ferromagnetic type. A ferromagnetic material can be characterized by a magnetization with a particular coercivity, which is indicative of a magnitude of the external magnetic field60that is required to reverse the magnetization230after it is driven to saturation in one direction. Suitable ferromagnetic materials include transition metals, rare earth elements, and their alloys, either with or without main group elements. For example, suitable ferromagnetic materials include iron (Fe), cobalt (Co), nickel (Ni), and their alloys, such as NiFe or CoFe alloys; alloys based on Ni, Fe, and boron (B) and alloys based on Co, Fe, and B. In some instances, alloys based on Ni and Fe (and optionally B) can have a smaller coercivity than alloys based on Co and Fe (and optionally B).

In particular, the reference magnetization210and the sense magnetization230can be orientable substantially within the plane of the reference layer21and sense layer23(in-plane, as illustrated inFIG.1) or can be orientable in a plane substantially perpendicular to the plane of the reference layer21and sense layer23(out-of-plane, not shown).

FIG.2shows a schematic view of the sense layer23according to a particular configuration. The sense layer23comprises a first ferromagnetic sense sublayer231in contact with the tunnel barrier layer22, a second ferromagnetic sense sublayer232, and a non-magnetic sense sublayer233between the first and second ferromagnetic sense sublayers231,232. The first ferromagnetic sense sublayer231can comprise a CoFeB alloy, for example a (CoxFe 1−x)80B20 alloy, where x=0.1 to 0.9, the non-magnetic sense sublayer233can comprise a layer comprising Ta, W, Mo, Ti, Hf, Mg or Al or a combination of any of these elements, and the second ferromagnetic sense sublayer232can comprise a soft magnetic material having a low planar anisotropy value. For example, the second ferromagnetic sense sublayer232can comprise a NiFe alloy or a NiFe alloy comprising B or Cr. In one particular but not limiting aspect, the structure of the sense layer21is as follows: CoFeB1.5/Ta0.3/NiFe4, where the first ferromagnetic sense sublayer231comprises a CoFeB alloy 1.5 nm in thickness, the non-magnetic sense sublayer233comprises a Ta layer 0.3 nm in thickness, the second ferromagnetic sense sublayer232comprises a NiFe alloy 4 nm in thickness. Such sense layer23can have very low anisotropy field Hk.

The magnetoresistive element2can comprise an antiferromagnetic layer24exchange coupling the reference layer21such as to pin the reference magnetization210at a low temperature threshold and free it at a high temperature threshold. Suitable antiferromagnetic materials can include 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 or CrPdM); alloys based on Ni and Mn (e.g., NiMn) or oxides such as NiO. Suitable materials for the antiferromagnetic layer24can further include oxide layers, such as NiO. In a possible configuration, the antiferromagnetic layer24have a thickness of about 4 nm to about 30 nm. Alternatively, the antiferromagnetic layer24can comprise a multilayer wherein each layer has a thickness between 1 and 10 nm or between 1 and 2 nm. In another arrangement, the antiferromagnetic layer24can comprise a tri-layer arrangement including, for example, a central antiferromagnetic layer sandwiched between two antiferromagnetic layer having lower blocking temperature Tb than the central antiferromagnetic layer. Such tri-layer arrangement ease switching the reference magnetization210when programming the reference layer21. The antiferromagnetic layer24can be separated from the seed layer27by an underlayer26, where the underlayer26can comprise Ru, Cu or their nitrides. The underlayer26can have a thickness between about 1 nm and about 5 nm.

In an embodiment shown inFIG.3, the reference layer21comprises a synthetic antiferromagnetic (SAF) structure comprising a reference coupling layer213between a reference pinned layer211and a reference coupled layer212. The reference pinned layer211is pinned by the antiferromagnetic layer24. The reference coupled layer212can be coupled to the reference pinned layer211by an RKKY coupling mechanism through the reference coupling layer213. The reference coupling layer213can comprise a non-magnetic layer of Ru, Ir or copper (Cu) or a combination thereof.

In one aspect, the reference coupled layer212comprises a first coupled sublayer214in contact with the reference coupling layer213, a second coupled sublayer215and a third coupled sublayer217. The reference coupled layer212can further comprise an insert layer216between the second and third coupled sublayers215,217.

The insert layer216comprises a transition metal. The insert layer216comprises Ta, Ti, W, Mo, Hf, Mg or aluminium (Al) or a combination of any of these elements. Alternatively, the insert layer216can comprise Ni, chromium (Cr), vanadium (V) or silicon (Si) or a combination of any of these elements. The insert layer216may be amorphous or quasi-amorphous or nanocrystalline.

The insert layer216can have a thickness between about 0.1 and about 0.5 nm. Such thickness of the insert layer216allows for ferromagnetic exchange coupling and thus, maintaining the alignment of the magnetization of the second coupled sublayer215and the third coupled sublayer217parallel to each other. The insert layer216further allows for increasing the TMR of the magnetoresistive element2. For example, a TMR increase from about 90%, without the insert layer216, to about 120% can be achieved. A high TMR is results in a better SNR ratio of the magnetoresistive element2response and decreases dispersion of the magnetoresistive element2response.

The thin insert layer216allows to preserve or even improve the smoothness of the reference coupled layer212at interface of the reference coupling layer213. The thin insert layer216increases the RKKY coupling between the reference pinned layer211and the reference coupled layer212(through the reference coupling layer213) and thus, increases the stiffness of the reference pinned layer211and the reference coupled layer212. A high RKKY coupling results in the reference magnetization210being less likely to be tilted by the external magnetic field60. Therefore, a high RKKY coupling between the reference pinned layer211and the reference coupled layer212allows for decreasing angular errors, including at high magnitude of the external magnetic field60and thus broadening the high-field operation margin of the magnetoresistive element2. A high RKKY coupling further improves thermal stability of the magnetoresistive element2. In one aspect, the RKKY coupling constant energy (JRKKYparameter) of the ferromagnetic reference layer21is about 1 erg/cm2.

The thin insert layer216further acts as a texture transition layer between the magnetic properties of the reference layer21(such as magnetic saturation field Hsatand SAF coupling exchange field Hex) and the electric properties of the tunnel barrier layer22(such as TMR).

The magnetoresistive element2comprising the transition metal containing thin insert layer216having a thickness between 0.1 and about 0.5 nm increases the magnetic saturation field Hsatby about 5% compared to the magnetic saturation field Hsatof the magnetoresistive element2without the thin insert layer216. The insert further allows to increase the TMR of the magnetoresistive element2by about 30%. The high TMR allows for reducing magnetic noise level in the magnetoresistive element2response and decreasing dispersion in the magnetoresistive element2response between different magnetoresistive elements2.

The SAF structure of the reference layer21can be compensated such that the macroscopic magnetization is null without applied field by adjusting the thickness of the reference pinned layer211and the reference coupled layer212.

In one aspect, the first coupled sublayer214comprises a Co or CoFe alloy. The second coupled sublayer215and the third coupled sublayer217can comprise Co, Fe, Ni, Cr, V, Si or B, or a combination of any of these elements.

The reference pinned layer211can comprise a CoFe alloy or Co or CoFe/CoFeB/CoFe or Co/CoFeB/Co multilayers or any other layers comprising Co, CoFe and CoFeB.

The thickness of the second coupled sublayer215can be below about 1 nm and the thickness of the third coupled sublayer217can be below about 1 nm or about 2 nm. The thickness of the first coupled sublayer214can be below about 1 nm.

In one aspect, the second coupled sublayer215has a thickness between 1 and 2 times the thickness of the third coupled sublayer217.

In an embodiment, the total thickness of the reference coupled layer212is between about 1 nm and about 5 nm. The total thickness of the reference coupled layer212can be between about 1 nm and about 3 nm and preferably between about 2 nm and about 3 nm.

The reference layer21disclosed herein has an enhanced SAF stiffness. The magnetoresistive element2has low angular errors, even at high magnetic fields, and improved thermal stability, while not affecting the other magnetic properties of the magnetoresistive element2, such as the SAF saturation field Hsatand even increasing TMR.

FIG.4is a graph reporting the relationship between the magnetic saturation field Hsatof the magnetoresistive element2and the thickness of the reference coupled layer212. The saturation field Hsatwas measured on the magnetoresistive element2comprising a reference pinned layer211comprising a CoFe alloy and having a thickness of about 2 nm and a tunnel barrier layer22comprising a deposited metallic Mg that is oxidized, for example by using a natural or plasma oxidation processes. The deposition and oxidation can be repeated two to four successive times. The magnetoresistive element2was thermally treated at 310° C. during 90 min under a applied magnetic field of about 1 T.

In a first configuration, the reference coupled layer212comprises a single layer made of a CoFeB alloy and about 1.9 nm in thickness (dot). In a second configuration, the reference coupled layer212is about 1.9 nm in thickness and comprises the first coupled sublayer214made of a CoFe alloy, about 0.5 nm in thickness, and the second coupled sublayer215made of a CoFeB alloy, about 1.4 nm in thickness of (triangle). In a third configuration, the reference coupled layer212comprises the first coupled sublayer214made of a CoFe alloy, about 0.5 nm in thickness, the second coupled sublayer215made of a CoFeB alloy, about 0.75 nm in thickness, the insert layer216made of Ta, about 0.2 nm in thickness, and the third coupled sublayer217made of a CoFeB alloy and having various thicknesses between about 0.45 nm and about 0.95 nm (stars).

Compared to the first and second configurations, in the third configuration, the magnetoresistive element2has a saturation field Hsatthat is higher by about 300 Oe (5%).

FIG.5is a graph reporting the relationship between the resistance-area product RA of the magnetoresistive element2and the thickness of the reference coupled layer212. The RA was measured for the magnetoresistive element2in the first, second and third configurations described above (respectively closed dot, triangle and star).FIG.5further reports the relationship between the TMR response of the magnetoresistive element2and the thickness of the reference coupled layer212for the first, second and third configurations of the magnetoresistive element2(respectively open dot, triangle and star).

As shown inFIG.5, the third configuration of the magnetoresistive element2yields higher TMR values compared to the TMR observed for the other configurations. For the third configuration with the third coupled sublayer217having a thickness of about 0.65 nm, the TMR is higher by about 30%. A third coupled sublayer217having a thickness between about 0.45 nm and about 0.95 nm yields a TMR increase from about 90%, without the insert layer216, to about 140%.

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