Patent Description:
Magnetic sensors are widely used in cell phones and other mobile devices as electronic compass. For a two-dimensional external magnetic field in an X-Y plane, measurement of the X and Y components of the external magnetic field within the plane may be implemented by using two orthogonal sensors. For the measurement on the external magnetic field in a Z-axis direction, a separate single-axis planar magnetoresistive sensor arranged perpendicular to a two-axis planar sensor can be used. This solution requires assembling two different sensors, the X-Y two-axis magnetoresistive sensor and the Z-axis magnetoresistive sensor.

The external magnetic field in a Z-axis direction canal can further be measured by using a flux guide to convert an external magnetic field from the Z-axis direction into magnetic field components in the X- and Y-axis directions. For example, document <CIT> discloses a single-chip three-axis AMR sensor, which implements measurement of an external magnetic field in the Z-axis direction by placing a flux guide above in-plane sensors. In such solution, the external magnetic field in the Z-axis direction is not fully converted into the X- and Y-axis directions. Moreover, such sensor design needs to use a specific algorithm for calculating the external magnetic field in the Z-axis direction, which makes the sensor design more complicated.

Another solution for measuring an external magnetic field in a Z-axis direction includes using magnetic materials with perpendicular magnetic anisotropy. For example, document <CIT> discloses a magnetic sensor which measures a Z-axis component of an external magnetic field by using a perpendicular magnetic anisotropy material.

In the <CIT>, the present applicant discloses a magnetoresistive element for measuring a Z-axis component of an external magnetic field (or having an out-of-plane (OOP) sensitivity axis). The magnetoresistive element comprises a reference layer having a reference magnetization that is fixed and oriented substantially perpendicular to the plane of the reference layer. The magnetoresistive element further comprises a tunnel barrier layer and a sense layer having a free sense magnetization that has a vortex configuration in the absence of an external magnetic field. The vortex configuration is substantially parallel to the plane of the sense layer and has a vortex core magnetization direction along an out-of-plane axis substantially perpendicular to the plane of the sense layer.

However, the magnetoresistive element having an OOP sensitivity axis shows a non-negligible hysteresis that originates from the vortex core polarity switching which happens at high fields. The magnitude of the hysteresis and the vortex core switching field depends on the size of the vortex core, the geometry of the magnetoresistive element, and the magnetic material constituting the sensing layer. The magnitude of the hysteresis can reach <NUM> to <NUM> mT. The magnitude of the hysteresis can increase after the magnetoresistive element has been exposed to a high magnetic field (above the vortex core switching field). Consequently, the magnetoresistive element may have poor accuracy, reproducibility, and repeatability in operation.

Moreover, in the magnetoresistive element with the sense magnetization comprising a vortex configuration, different conductivity levels for different vortex core polarities may occur. In the case the reference layer comprises a synthetic antiferromagnet (SAF), the different core polarities will induce different conductivity levels. A larger or smaller conductivity of the magnetoresistive element is obtained when the vortex core and the reference magnetization are, respectively, in the same or opposite directions. A temperature increase results in a decrease in the core polarity switching field which, in turn, leads to a hysteresis appearing at smaller field exposure. This results in a reduced working range of the magnetoresistive element. In order to obtain a good accuracy and reproducibility of the electric response provided by the magnetoresistive element, the latter should be used in external magnetic fields that are below the vortex core switching field.

The present disclosure concerns a magnetoresistive element for sensing an external magnetic field, comprising a tunnel barrier layer sandwiched between a ferromagnetic reference layer having a pinned reference magnetization, and a ferromagnetic sense layer having a sense magnetization that can be freely oriented in the external magnetic field. The reference, tunnel barrier, and sense layers are stacked perpendicular to a layer plane thereof. The reference magnetization is oriented substantially perpendicular to the layer plane, and the sense magnetization is oriented substantially parallel to the layer plane. At least a portion of the magnetoresistive element has an annular cross-section in the layer plane forming an interior part and an exterior part of said at least a portion, the interior part having an inner diameter and the exterior part having an outer diameter. Said at least a portion comprises the sense layer, the sense magnetization having a closed flux path configuration being orientable either in the clockwise or counterclockwise direction. The inner diameter is larger than <NUM>, and a difference between the inner and outer diameters is greater than <NUM>.

The present disclosure further concerns a magnetoresistive sensor device comprising a plurality of the magnetoresistive elements, wherein the magnetoresistive elements are electrically connected in series or in parallel.

The magnetoresistive element disclosed herein has no hysteresis. The magnetoresistive element can work in external magnetic fields above the vortex core switching field, thus has an extended magnetic field measuring working range.

Exemplar embodiments are disclosed in the description and illustrated by the drawings in which:.

<FIG> illustrates a magnetoresistive element <NUM>, according to an embodiment. The magnetoresistive element <NUM> can comprise a magnetic tunnel junction. In particular, the magnetoresistive element <NUM> can comprise a reference layer <NUM> having a fixed reference magnetization <NUM>, a sense layer <NUM> having a free sense magnetization <NUM> and a tunnel barrier layer <NUM> between the reference layer <NUM> and the sense layer <NUM>. The sense magnetization <NUM> comprises a vortex configuration substantially parallel to the plane of the sense layer <NUM> in the absence of an external magnetic field <NUM>.

As shown in <FIG>, the reference layer <NUM>, tunnel barrier layer <NUM> and sense layer <NUM> are stacked perpendicular to a layer plane PL in which the layers <NUM>, <NUM>, <NUM> extend. Preferably, the reference layer <NUM> has a perpendicular magnetic anisotropy (PMA) such that the reference magnetization <NUM> is oriented substantially perpendicular to the layer plane PL.

In an embodiment, at least a portion of the magnetoresistive element <NUM> has an annular cross-section in the layer plane PL. The portion of the magnetoresistive element <NUM> thus has an interior part <NUM> and an exterior part <NUM>. The interior part <NUM> can be void or comprise an insulating material. The magnetoresistive element <NUM> has an inner diameter Dint and outer diameter Dout. In one aspect, the portion comprises the sense layer <NUM>. In this configuration, the tunnel barrier <NUM> and the reference layer <NUM> do not have an annular cross-section. The interior part <NUM> extending only along the sense layer <NUM> thickness is shown in <FIG> by the dashed line. Due to the annular shape of the sense layer <NUM>, the sense magnetization <NUM> has a closed flux path configuration that is orientable either in the clockwise or counterclockwise direction. The closed flux path configuration of the sense magnetization <NUM> can be considered having a vortex configuration without vortex core.

Preferably, the sense magnetization <NUM> has a magnetic vector having a fixed orthogonal direction z that is substantially perpendicular to the layer plane PL. Thus, the closed flux path configuration of the sense magnetization <NUM> can measure a Z-axis component of an external magnetic field, i.e., a component perpendicular to the layer plane PL. In other words, the magnetoresistive element <NUM> has an OOP sensitivity axis. In normal operation of the magnetoresistive element <NUM>, the magnetic vector of the sense magnetization <NUM> varies in a first orthogonal direction +z or a second orthogonal direction -z, opposed to the first orthogonal direction +z, depending on the direction and magnitude of the external magnetic field <NUM> that is substantially perpendicular to the layer plane PL.

In one aspect, the portion of the magnetoresistive element <NUM> further comprises the sense layer <NUM> and the reference layer <NUM>. In this configuration, the reference layer <NUM> does not have an annular cross-section. In another aspect, the portion of the magnetoresistive element <NUM> can comprise the sense layer <NUM>, the tunnel barrier layer <NUM> and the reference layer <NUM>.

The performances of the magnetoresistive element <NUM> depend on its geometry and more particularly, the inner diameter Dint, the outer diameter Dout, and the thickness of the magnetoresistive element <NUM>.

<FIG> reports the field dependence of the magnetoresistance MRH, measured on the magnetoresistive element <NUM> for different inner and outer diameters Dint, Dout. More particularly, <FIG> reports the magnetic sensitivity as a function of the ratio of the inner diameter Dint to the outer diameter Dout. It can be seen that the magnetic sensitivity increases when the ratio of the inner diameter Dint to the outer diameter Dout is greater than <NUM>.

Moreover, electrical short circuit can be observed in the magnetoresistive element <NUM> (between the reference layer and the sense layer) when the inner diameter Dint is smaller than <NUM>.

It has been found that that optimum performances of the magnetoresistive element <NUM> are obtained when the inner diameter Dint is larger than <NUM>, and a difference between the inner and outer diameters Dint, Dout is greater than <NUM>.

In one aspect, the outer diameter Dout is between <NUM> and <NUM>. The inner diameter Dint is larger than <NUM> and the difference between the inner and outer diameters Dint, Dout is greater than <NUM>. The outer diameter Dout equal or smaller the <NUM> ensures that the sense magnetization <NUM> has a vortex configuration in the absence of an external magnetic field.

In another aspect, the aspect ratio of the thickness to diameter of the sense layer <NUM> can be between <NUM>,<NUM> and <NUM>. The aspect ratio between <NUM>,<NUM> and <NUM> also ensures that the sense magnetization <NUM> has a vortex configuration in the absence of an external magnetic field.

In some embodiments, the resistance-area-product RA of the magnetoresistive element <NUM> can be higher than <NUM> ohm·µm<NUM>.

<FIG> show different possible geometries of the magnetoresistive element <NUM> within the scope of the disclosure. More particularly, <FIG> shows the magnetoresistive element <NUM> where the interior part <NUM> has a substantially circular shape. <FIG> shows the magnetoresistive element <NUM> where the interior part <NUM> has an elliptical shape. In the geometries of <FIG>, the interior part <NUM> is substantially concentric with the exterior part <NUM>. <FIG> shows the magnetoresistive element <NUM> where the interior part <NUM> is nonconcentric with the exterior part <NUM>.

The reference and sense layers <NUM>, <NUM> can comprise, or 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 sense layer <NUM> should have a thickness that is greater than <NUM>. The reference and sense layers <NUM>, <NUM> can comprise a multilayer structure where each layer can include a ferromagnetic material such as a Co, Fe or Ni based alloy and preferentially a CoFe, NiFe or CoFeB based alloy, and nonmagnetic layers such as Ta, Ti, W, Ru, Ir.

In a possible configuration of the magnetoresistive element <NUM> shown in <FIG>, the reference layer <NUM> can comprise a synthetic antiferromagnetic (SAF) structure including a first reference sublayer <NUM> in contact with the tunnel barrier layer <NUM> and a second reference sublayer <NUM> separated from the first reference sublayer <NUM> by a coupling layer <NUM>, wherein the coupling layer <NUM> antiferromagnetically couple the first reference sublayer <NUM> to the second reference sublayer <NUM>. Each of the first and second reference sublayer <NUM>, <NUM> has a PMA such that a reference magnetization <NUM> is oriented substantially perpendicular to the plane of the first and second reference sublayer <NUM>, <NUM> and in opposite directions.

In some embodiments not represented, the sense layer <NUM> comprises a SAF sense layer. In particular, the sense layer <NUM> comprises a first sense ferromagnetic layer having a first sense magnetization, and a second sense ferromagnetic layer having a second sense magnetization. The magnetization of the two sense ferromagnetic layers can be coupled in an anti-parallel direction due to the presence of an anti-parallel coupling layer.

<FIG> illustrates a magnetoresistive sensor device <NUM> comprising a plurality of the magnetoresistive elements <NUM>, according to an embodiment. <FIG> shows only two magnetoresistive elements <NUM> but it should be understood that the magnetoresistive sensor device <NUM> can comprises more the two magnetoresistive elements <NUM>. The magnetoresistive elements <NUM> can be electrically connected in series or in parallel. In the example of <FIG>, the magnetoresistive elements <NUM> are electrically connected in series. In particular, one end of the magnetoresistive element <NUM> is connected to a first electrical connector <NUM> and the other end of the magnetoresistive element <NUM> is connected to a second electrical connector <NUM>, for example through a via <NUM>.

Claim 1:
Magnetoresistive element for sensing an external magnetic field, comprising:
a tunnel barrier layer (<NUM>) sandwiched between a ferromagnetic reference layer (<NUM>) having a pinned reference magnetization (<NUM>), and a ferromagnetic sense layer (<NUM>) having a sense magnetization (<NUM>) that can be freely oriented in the external magnetic field (<NUM>);
wherein the reference, tunnel barrier, and sense layers (<NUM>, <NUM>, <NUM>) are stacked perpendicular to a layer plane (PL) thereof;
wherein the reference magnetization (<NUM>) is oriented substantially perpendicular to the layer plane (PL), and the sense magnetization (<NUM>) is oriented substantially parallel to the layer plane (PL);
characterized in that
at least a portion of the magnetoresistive element (<NUM>) has an annular cross-section in the layer plane (PL) forming an interior part (<NUM>) and an exterior part (<NUM>) of said at least a portion , the interior part (<NUM>) having an inner diameter (Dint) and the exterior part (<NUM>) having an outer diameter (Dout);
wherein said at least a portion comprises the sense layer (<NUM>), and wherein the sense magnetization (<NUM>) has a closed flux path configuration being orientable either in the clockwise or counterclockwise direction; and
wherein the inner diameter (Dint) being larger than <NUM>, and a difference between the inner and outer diameters (Dint, Dout) is greater than <NUM>.