Patent Description:
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 <NUM>. This results in increasing the saturation field Hsat of 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 <CIT> 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 <CIT> 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, a third ferromagnetic layer above the fixed 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. The TMR film in the TMR head may be subjected to heat treatment in a magnetic field using a heat treating furnace for four hours at an applied magnetic flux density of <NUM> T and at a heat treatment temperature of <NUM>° C in order to optimize the MR ratio.

Document <CIT> 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 <CIT> 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 <CIT> 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.

The present disclosure concerns a method for obtaining a magnetoresistive element for a two-dimensional magnetic field sensor according to claim <NUM>, the magnetoresistive element 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 provides a ferromagnetic exchange coupling between the second and third coupled sublayers. The insert layer comprises Ta and has a thickness of <NUM>. The method comprises forming the ferromagnetic reference layer, the tunnel barrier layer, and the ferromagnetic sense layer. The reference pinned layer comprises a CoFe alloy and has a thickness of <NUM>, the tunnel barrier layer comprises Mg, the first coupled sublayer is made of a CoFe alloy, <NUM> in thickness, the second coupled sublayer is made of a CoFeB alloy and has a thickness of <NUM>, and the third coupled sublayer is made of a CoFeB alloy and has a thicknesses between <NUM> and <NUM>. The method further comprises thermally treating the magnetoresistive element at <NUM> during <NUM> under an applied magnetic field of about 1T.

The magnetoresistive element obtained by the method has high saturation field Hsat and 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.

<FIG> shows a schematic view of a magnetoresistive element <NUM>. The magnetoresistive element <NUM> comprises a ferromagnetic reference layer <NUM>, a ferromagnetic sense layer <NUM> and a tunnel barrier layer <NUM> between the reference and sense ferromagnetic layers <NUM>, <NUM>. The reference layer <NUM> has a fixed reference magnetization <NUM> while the sense layer <NUM> has a sense magnetization <NUM> that can be freely oriented relative to the reference magnetization <NUM> in the presence of an external magnetic field <NUM>. In other words, when the magnetoresistive element <NUM> is in the presence of the external magnetic field <NUM>, the reference magnetization <NUM> remains substantially fixed while the sense magnetization <NUM> is deflected in the direction of the external magnetic field <NUM>. The magnetoresistive element <NUM> can further comprise a capping layer <NUM> on the side of the sense layer <NUM> and a seed layer <NUM> on the side of the reference layer <NUM>.

The capping layer <NUM> can comprise a layer of TaN, Ru or Ta. The capping layer <NUM> can 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 layer <NUM> comprises a multilayer including a <NUM> layer of TaN, a <NUM> layer of Ru, a <NUM> layer of TaN, a <NUM> layer of Ru, a <NUM> layer of Ta and a <NUM> layer of Ru. The seed layer <NUM> can comprise any one of Ta, tungsten (W), molybdenum (Mo), titanium (Ti), hafnium (Hf) or magnesium (Mg).

The tunnel barrier layer <NUM> can comprise, or be formed of, an insulating material. Suitable insulating materials include oxides, such as aluminum oxide (e.g., Al<NUM>O<NUM>) and magnesium oxide (e.g., MgO). A thickness of the tunnel barrier layer <NUM> can be in the nm range, such as from about <NUM> to about <NUM>. An optimal thickness of the tunnel barrier <NUM> can be obtained by inserting a plurality (double or multilayer) of MgO (or another suitable oxide or insulating material) layers. The tunnel barrier layer <NUM> can be configured to provide high TMR, for example above <NUM>%.

The reference layer <NUM> and the sense layer <NUM> can 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 field <NUM> that is required to reverse the magnetization <NUM> after 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 magnetization <NUM> and the sense magnetization <NUM> can be orientable substantially within the plane of the reference layer <NUM> and sense layer <NUM> (in-plane, as illustrated in <FIG>) or can be orientable in a plane substantially perpendicular to the plane of the reference layer <NUM> and sense layer <NUM> (out-of-plane, not shown).

<FIG> shows a schematic view of the sense layer <NUM> according to a particular configuration. The sense layer <NUM> comprises a first ferromagnetic sense sublayer <NUM> in contact with the tunnel barrier layer <NUM>, a second ferromagnetic sense sublayer <NUM>, and a non-magnetic sense sublayer <NUM> between the first and second ferromagnetic sense sublayers <NUM>, <NUM>. The first ferromagnetic sense sublayer <NUM> can comprise a CoFeB alloy, for example a (CoxFe<NUM>-x)<NUM>B<NUM> alloy, where x = <NUM> to <NUM>, the non-magnetic sense sublayer <NUM> can 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 sublayer <NUM> can comprise a soft magnetic material having a low planar anisotropy value. For example, the second ferromagnetic sense sublayer <NUM> can comprise a NiFe alloy or a NiFe alloy comprising B or Cr. In one particular but not limiting aspect, the structure of the sense layer <NUM> is as follows: CoFeB<NUM>/Ta<NUM>/NiFe<NUM>, where the first ferromagnetic sense sublayer <NUM> comprises a CoFeB alloy <NUM> in thickness, the non-magnetic sense sublayer <NUM> comprises a Ta layer <NUM> in thickness, the second ferromagnetic sense sublayer <NUM> comprises a NiFe alloy <NUM> in thickness. Such sense layer <NUM> can have very low anisotropy field Hk.

The magnetoresistive element <NUM> can comprise an antiferromagnetic layer <NUM> exchange coupling the reference layer <NUM> such as to pin the reference magnetization <NUM> at 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 layer <NUM> can further include oxide layers, such as NiO. In a possible configuration, the antiferromagnetic layer <NUM> have a thickness of about <NUM> to about <NUM>. Alternatively, the antiferromagnetic layer <NUM> can comprise a multilayer wherein each layer has a thickness between <NUM> and <NUM> or between <NUM> and <NUM>. In another arrangement, the antiferromagnetic layer <NUM> can 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 magnetization <NUM> when programming the reference layer <NUM>. The antiferromagnetic layer <NUM> can be separated from the seed layer <NUM> by an underlayer <NUM>, where the underlayer <NUM> can comprise Ru, Cu or their nitrides. The underlayer <NUM> can have a thickness between about <NUM> and about <NUM>.

In <FIG>, the reference layer <NUM> comprises a synthetic antiferromagnetic (SAF) structure comprising a reference coupling layer <NUM> between a reference pinned layer <NUM> and a reference coupled layer <NUM>. The reference pinned layer <NUM> is pinned by the antiferromagnetic layer <NUM>. The reference coupled layer <NUM> can be coupled to the reference pinned layer <NUM> by an RKKY coupling mechanism through the reference coupling layer <NUM>. The reference coupling layer <NUM> can comprise a non-magnetic layer of Ru, Ir or copper (Cu) or a combination thereof.

In one aspect, the reference coupled layer <NUM> comprises a first coupled sublayer <NUM> in contact with the reference coupling layer <NUM>, a second coupled sublayer <NUM> and a third coupled sublayer <NUM>. The reference coupled layer <NUM> further comprises an insert layer <NUM> between the second and third coupled sublayers <NUM>, <NUM>.

The insert layer <NUM> comprises a transition metal. The insert layer <NUM> comprises Ta, Ti, W, Mo, Hf, Mg or aluminium (Al) or a combination of any of these elements. Alternatively, the insert layer <NUM> can comprise Ni, chromium (Cr), vanadium (V) or silicon (Si) or a combination of any of these elements. The insert layer <NUM> may be amorphous or quasi-amorphous or nanocrystalline.

The insert layer <NUM> can have a thickness between about <NUM> and about <NUM>. Such thickness of the insert layer <NUM> allows for ferromagnetic exchange coupling and thus, maintaining the alignment of the magnetization of the second coupled sublayer <NUM> and the third coupled sublayer <NUM> parallel to each other. The insert layer <NUM> further allows for increasing the TMR of the magnetoresistive element <NUM>. For example, a TMR increase from about <NUM>%, without the insert layer <NUM>, to about <NUM>% can be achieved. A high TMR is results in a better SNR ratio of the magnetoresistive element <NUM> response and decreases dispersion of the magnetoresistive element <NUM> response.

The thin insert layer <NUM> allows to preserve or even improve the smoothness of the reference coupled layer <NUM> at interface of the reference coupling layer <NUM>. The thin insert layer <NUM> increases the RKKY coupling between the reference pinned layer <NUM> and the reference coupled layer <NUM> (through the reference coupling layer <NUM>) and thus, increases the stiffness of the reference pinned layer <NUM> and the reference coupled layer <NUM>. A high RKKY coupling results in the reference magnetization <NUM> being less likely to be tilted by the external magnetic field <NUM>. Therefore, a high RKKY coupling between the reference pinned layer <NUM> and the reference coupled layer <NUM> allows for decreasing angular errors, including at high magnitude of the external magnetic field <NUM> and thus broadening the high-field operation margin of the magnetoresistive element <NUM>. A high RKKY coupling further improves thermal stability of the magnetoresistive element <NUM>. In one aspect, the RKKY coupling constant energy (JRKKY parameter) of the ferromagnetic reference layer <NUM> is about <NUM> mJ/m<NUM> (<NUM> erg/cm<NUM>).

The thin insert layer <NUM> further acts as a texture transition layer between the magnetic properties of the reference layer <NUM> (such as magnetic saturation field Hsat and SAF coupling exchange field Hex) and the electric properties of the tunnel barrier layer <NUM> (such as TMR).

The magnetoresistive element <NUM> comprising the transition metal containing thin insert layer <NUM> having a thickness between <NUM> and about <NUM> increases the magnetic saturation field Hsat by about <NUM>% compared to the magnetic saturation field Hsat of the magnetoresistive element <NUM> without the thin insert layer <NUM>. The insert further allows to increase the TMR of the magnetoresistive element <NUM> by about <NUM>%. The high TMR allows for reducing magnetic noise level in the magnetoresistive element <NUM> response and decreasing dispersion in the magnetoresistive element <NUM> response between different magnetoresistive elements <NUM>.

The SAF structure of the reference layer <NUM> can be compensated such that the macroscopic magnetization is null without applied field by adjusting the thickness of the reference pinned layer <NUM> and the reference coupled layer <NUM>.

In one aspect, the first coupled sublayer <NUM> comprises a Co or CoFe alloy. The second coupled sublayer <NUM> and the third coupled sublayer <NUM> can comprise Co, Fe, Ni, Cr, V, Si or B, or a combination of any of these elements.

The reference pinned layer <NUM> can 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 sublayer <NUM> can be below about <NUM> and the thickness of the third coupled sublayer <NUM> can be below about <NUM> or about <NUM>. The thickness of the first coupled sublayer <NUM> can be below about <NUM>.

In one aspect, the second coupled sublayer <NUM> has a thickness between <NUM> and <NUM> times the thickness of the third coupled sublayer <NUM>.

The total thickness of the reference coupled layer <NUM> is between about <NUM> and about <NUM>. The total thickness of the reference coupled layer <NUM> can be between about <NUM> and about <NUM> and preferably between about <NUM> and about <NUM>.

The reference layer <NUM> disclosed herein has an enhanced SAF stiffness. The magnetoresistive element <NUM> has low angular errors, even at high magnetic fields, and improved thermal stability, while not affecting the other magnetic properties of the magnetoresistive element <NUM>, such as the SAF saturation field Hsat and even increasing TMR.

<FIG> is a graph reporting the relationship between the magnetic saturation field Hsat of the magnetoresistive element <NUM> and the thickness of the reference coupled layer <NUM>. Note that <NUM> Oe = <NUM> A/m. The saturation field Hsat was measured on the magnetoresistive element <NUM> comprising a reference pinned layer <NUM> comprising a CoFe alloy and having a thickness of about <NUM> and a tunnel barrier layer <NUM> comprising 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 element <NUM> was thermally treated at <NUM> during <NUM> under a applied magnetic field of about 1T.

In a first configuration, the reference coupled layer <NUM> comprises a single layer made of a CoFeB alloy and about <NUM> in thickness (dot). In a second configuration, the reference coupled layer <NUM> is about <NUM> in thickness and comprises the first coupled sublayer <NUM> made of a CoFe alloy, about <NUM> in thickness, and the second coupled sublayer <NUM> made of a CoFeB alloy, about <NUM> in thickness of (triangle). In a third configuration which has been fabricated by the inventive method, the reference coupled layer <NUM> comprises the first coupled sublayer <NUM> made of a CoFe alloy, about <NUM> in thickness, the second coupled sublayer <NUM> made of a CoFeB alloy, about <NUM> in thickness, the insert layer <NUM> made of Ta, about <NUM> in thickness, and the third coupled sublayer <NUM> made of a CoFeB alloy and having various thicknesses between about <NUM> and about <NUM> (stars).

Compared to the first and second configurations, in the third configuration, the magnetoresistive element <NUM> fabricated according to the invention has a saturation field Hsat that is higher by about <NUM> kA/m (<NUM> Oe) (<NUM>%).

<FIG> is a graph reporting the relationship between the resistance-area product RA of the magnetoresistive element <NUM> and the thickness of the reference coupled layer <NUM>. The RA was measured for the magnetoresistive element <NUM> in the first, second and third configurations described above (respectively closed dot, triangle and star). <FIG> further reports the relationship between the TMR response of the magnetoresistive element <NUM> and the thickness of the reference coupled layer <NUM> for the first, second and third configurations of the magnetoresistive element <NUM> (respectively open dot, triangle and star).

Claim 1:
A method for obtaining a magnetoresistive element (<NUM>) for a two-dimensional magnetic field sensor, the magnetoresistive element (<NUM>) comprising:
a ferromagnetic reference layer (<NUM>) having a fixed reference magnetization (<NUM>), a ferromagnetic sense layer (<NUM>) having a sense magnetization (<NUM>) that can be freely oriented relative to the reference magnetization (<NUM>) in the presence of an external magnetic field, and a tunnel barrier layer (<NUM>) between the reference and sense ferromagnetic layers (<NUM>, <NUM>);
the reference layer (<NUM>) comprising a reference coupling layer (<NUM>) between a reference pinned layer (<NUM>) and a reference coupled layer (<NUM>);
the reference coupled layer (<NUM>) comprising a first coupled sublayer (<NUM>) in contact with the reference coupling layer (<NUM>), a second coupled sublayer (<NUM>) in contact with the first coupled sublayer (<NUM>), a third coupled sublayer (<NUM>) and an insert layer (<NUM>) between the second and third coupled sublayers (<NUM>, <NUM>), the insert layer (<NUM>) providing a ferromagnetic exchange coupling between the second and third coupled sublayers (<NUM>, <NUM>); and
the insert layer (<NUM>) comprising Ta and having a thickness of <NUM>;
wherein the method comprises:
forming the ferromagnetic reference layer (<NUM>), the tunnel barrier layer (<NUM>), and the ferromagnetic sense layer (<NUM>);
wherein the reference pinned layer (<NUM>) comprises a CoFe alloy and has a thickness of <NUM>, the tunnel barrier layer (<NUM>) comprises oxidized Mg, the first coupled sublayer (<NUM>) is made of a CoFe alloy and has a thickness of <NUM>, the second coupled sublayer (<NUM>) is made of a CoFeB alloy and has a thickness of <NUM>, and the third coupled sublayer (<NUM>) is made of a CoFeB alloy and has a thicknesses between <NUM> and <NUM>; and
thermally treating the magnetoresistive element (<NUM>) at <NUM> for <NUM> minutes under an applied magnetic field of about 1T.