Patent Description:
Currently, magnetic sensors are widely used in cell phones and other mobile devices as electronic compass. For a two-dimensional magnetic field in an X-Y plane, measurement of the X and Y components of the magnetic field within the plane may be implemented by using two orthogonal sensors, but for the measurement on the magnetic field in a Z-axis direction there are many difficulties. The following solutions are typically utilized.

One solution includes a separate single-axis planar magnetoresistive sensor installed perpendicular to a two-axis planar sensor. This solution requires assembling two different sensors, the X-Y two-axis magnetoresistive sensor and the Z-axis magnetoresistive sensor.

Another solution includes a flux guide to convert a 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 a magnetic field in the Z-axis direction by placing a flux guide above in-plane sensors. In such solution, the magnetic field in the Z-axis direction is not fully converted into the X- and Y-axis directions. In addition, such sensor design needs to use a specific algorithm for calculating the magnetic field in the Z-axis direction, which makes the sensor design more complicated.

Yet another solution includes micro-machining a substrate to form an inclined plane, onto which a sensor that partially senses the magnetic field in the Z-axis direction is deposited. Such a process is very complicated, has a low spatial efficiency, and may cause some shadowing effects in the deposition of the sensor, which may degrade the performance of the sensor.

Yet another solution includes using magnetic materials with perpendicular magnetic anisotropy for measuring the magnetic field in the Z-axis direction. 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. The perpendicular magnetic anisotropy material has a high coercivity, and low magnetoresistance.

Reference: <NPL>, discusses magnetization dynamics of a soft layer with in-plane anisotropy and under a spin transfer torque from a perpendicularly magnetized reference layer.

Document <CIT> discloses a sensor including first and second magnetoresistive sensor elements configured to produce respective first and second output signals in response to an external magnetic field. The first and second magnetoresistive sensor elements form a gradient unit, each of the magnetoresistive sensor elements includes a sense layer having a vortex magnetization pattern.

The present invention according to appended claim <NUM> concerns a magnetoresistive element comprising a reference layer having a fixed reference magnetization, a sense layer having a free sense magnetization and a tunnel barrier layer between the reference and sense layers. The magnetoresistive element is configured to measure an external magnetic field oriented substantially perpendicular to the plane of the layers. The reference magnetization is oriented substantially perpendicular to the plane of the reference layer. The sense magnetization comprising 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. The magnetoresistive element has a lateral dimension being smaller than <NUM>, the sense layer has a thickness being greater than <NUM> and a sense magnetization below <NUM> kA/m at room temperature.

The present invention according to appended claim <NUM> further concerns a method for operating the magnetoresistive element, comprising:.

The magnetoresistive element disclosed herein can measure an external magnetic field along an out-of-plane axis substantially perpendicular to the plane of the sense layer. The vortex configuration of the magnetoresistive element can have an expulsion field greater than <NUM> mT or <NUM> mT.

The magnetoresistive element has low hysteresis, less than <NUM>µV/V for external magnetic field magnitudes up to the expulsion field, and high linearity, i.e., less than <NUM>% or <NUM>% error.

<FIG> illustrates a magnetoresistive element <NUM>, according to an embodiment. The magnetoresistive element comprises 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>. The reference layer <NUM> can have perpendicular magnetic anisotropy (PMA) such that the reference magnetization <NUM> is oriented substantially perpendicular to the plane of the reference layer <NUM>.

The magnetoresistive element <NUM> can measure an external magnetic field <NUM> being oriented substantially perpendicular to the plane of the reference and sense layers <NUM>, <NUM>.

The sense layer <NUM> has a sense magnetization <NUM> direction distribution with a vortex configuration, whereby the vortex magnetization curls in a circular path along the edge of the sense layer <NUM> and around a vortex core <NUM>. The vortex magnetization direction may be arranged in a clockwise direction and may also be arranged in a counterclockwise direction. During normal sensor operation, the magnetization of the vortex core <NUM> can vary in accordance with the external magnetic field <NUM> in a direction substantially perpendicular to the plane of the sense layer <NUM> (or direction ±z). Referring to <FIG>, the magnetization of the vortex core <NUM> can be oriented in an upward direction (i.e., toward the direction +z) or in a downward direction (i.e., toward an opposite direction -z). The size of the vortex core increases or decreases in the direction +z or -z when the magnitude of the external magnetic field <NUM> increases or decreases, respectively. However, during normal sensor operation the vortex core magnetization direction ±z, or vortex core magnetization polarity, is fixed.

The vortex configuration provides a linear and non-hysteretic behavior in a large magnitude range of the external magnetic field <NUM>, for practical size of the magnetoresistive element <NUM> and thickness of the sense layer <NUM>. The linear and non-hysteretic portion of the magnetization curve facilitates the measurement of small variations of the external magnetic field <NUM>. The vortex configuration is thus advantageous for magnetic sensor applications.

<FIG> shows a magnetization curve (or hysteresis response) of the magnetoresistive element <NUM> as a function of the external magnetic field <NUM> (indicated by the symbol Bextz). Here, "mz" corresponds to the averaged component along direction ±z of the sense magnetization (<Mz> or z-component of the sense magnetization Ms), normalized to the sense magnetization Ms (mz=<Mz>/Ms). In the case of a vortex configuration, the magnetization curve is characterized by a linear increase of the vortex core magnetization with the external magnetic field Bextz until the vortex expulsion field is reached at the Hexpl point. At this point the sense magnetization <NUM> becomes magnetically saturated (represented by the arrow pointing upwards (the direction +z). The vortex state in the sensing layer <NUM> is recovered when the external magnetic field Bextz is reduced below a nucleation field Hnucl. When the external magnetic field Bextz is decreased until the vortex expulsion field is reached at the Hexpl point (negative external magnetic field Bext) the sense magnetization <NUM> becomes magnetically saturated (represented by the arrow pointing downwards (the direction -z). The nucleation field Hnucl is the field at which vortex re-forms after vortex expulsion. As long as the magnitude of the external magnetic field Bextz is below the absolute value corresponding to the expulsion field (+/-Hexpl), the magnetization curve comprises a reversible linear portion corresponding to the variation of the vortex core <NUM> magnetization with the external magnetic field <NUM>. The vortex core magnetization polarity can be reversed (between direction z and -z) when the expulsion field Hexpl is exceeded.

<FIG> shows an enlarged view of a portion of the reversible linear portion of the magnetization curve of <FIG>. The magnetization curve to the external magnetic field Bextz is shifted towards higher magnetization values when the external magnetic field Bextz is decreased from the nucleation field Hnucl, compared to when the external magnetic field Bextz is increased from the nucleation field Hnucl. In other words, the reversible linear portion of the magnetization curve exhibits a hysteresis due to the different vortex core magnetization polarity, that is the direction of the vortex core <NUM> magnetization.

The vortex core magnetization polarity depends on the nucleation field +/-Hnucl at which the vortex re-forms after vortex expulsion. It is then possible to operate the magnetoresistive element <NUM> in only one of the branch of the magnetization curve, for example the branch A (see <FIG>) when the field is swept back to negative from the positive nucleation field +Hnucl (and the positive vortex expulsion field +Hexpl) or the branch B when the field is swept back to positive from the negative nucleation field - Hnucl (and the negative vortex expulsion field -Hexpl).

A method for operating the magnetoresistive element <NUM> can comprise the steps of selecting the direction z or -z of the vortex core magnetization (vortex core magnetization polarity) by applying an initialization magnetic field to the magnetoresistive element <NUM> until the vortex expulsion field Hexpl is reached and then, reducing the initialization magnetic field below the nucleation field Hnucl at which the vortex re-forms. The vortex core magnetization polarity is determined by the polarity of the vortex expulsion field Hexpl and the nucleation field Hnucl. The method further comprises the step of measuring an external magnetic field <NUM>.

After applying an initialization magnetic field, the method can further comprise a step of programming the magnetoresistive element <NUM> to program the orientation of the reference magnetization <NUM>. The programming step can be performed by applying a programming magnetic field adapted to orient the reference magnetization <NUM>. The programming step can further comprise heating the magnetoresistive element <NUM> to a temperature where the orientation of the reference magnetization <NUM> is facilitated, for example at a temperature where the reference magnetization <NUM> is unpinned. Heating the magnetoresistive element <NUM> can be performed by using resistive heating or laser heating. During the programming step, the vortex core magnetization polarity can be considered fixed.

It should be noted that the operation of the magnetoresistive element <NUM> is not limited to any specific portion of the branches A or B shown in <FIG>. Indeed, the magnetoresistive element <NUM> can be operated anywhere in the linear region of branch A or branch B (the latter with a vortex magnetization polarity reversed relative to the one in branch A).

In any case, the magnetoresistive element <NUM> should measure the external magnetic field <NUM> below the vortex expulsion field +/-Hexpl. The vortex magnetization polarity is fixed during sensor operation and is independent of vortex chirality (clockwise or counterclockwise).

The obtention of a vortex configuration in the sense layer <NUM> depends on a number of factors, including materials properties of the sense layer <NUM>. Generally, the vortex configuration is favored at zero applied field by increasing the aspect ratio of the thickness on the diameter of the sense layer <NUM>. The aspect ratio is still typically much less than <NUM> (for example <NUM> to <NUM>). Moreover, the values and the slope of the linear part of the magnetization curve of <FIG> are strongly dependent on the size of the sense layer <NUM>.

In particular, the vortex configuration can be characterized by its susceptibility χ, which corresponds to the slope of the linear region of the magnetization curve: <MAT>.

The sensitivity S of the magnetoresistive element <NUM> is then proportional to the product between the susceptibility χ and the tunnel magnetoresistance (TMR) of the magnetoresistive sensor element <NUM>: <MAT>.

<FIG> reports magnetization curves to the z-aligned external magnetic field Bextz on the magnetization of the sense layer <NUM>, for several thickness of the sense layer <NUM>, namely for thicknesses of the sense layer <NUM> between <NUM> and <NUM>. The magnetoresistive element <NUM> has a lateral dimension D of about <NUM>. <FIG> shows an increase in the slope of the magnetization curve, and thus the susceptibility χ, with increasing thickness of the sense layer <NUM>. For a given TMR value, increasing the thicknesses of the sense layer <NUM> results in an increase of the sensitivity S of the magnetoresistive element <NUM>. This is in contrast to the case of a vortex configuration in the the plane of the sense layer <NUM> and having a vortex core magnetization that is reversibly movable substantially parallel to the plane of the sense layer <NUM>.

<FIG> reports magnetization curves to the z-aligned external magnetic field Bextz of the sense layer <NUM> for several thickness of the sense layer <NUM>, namely for thicknesses of the sense layer <NUM> between <NUM> and <NUM>. The magnetoresistive element <NUM> has a lateral dimension of about <NUM>. <FIG> show that the susceptibility χ increases, and for a given TMR value the sensitivity S of the magnetoresistive element <NUM> increases, with decreasing smaller lateral dimension D of the magnetoresistive element <NUM>. For given thicknesses of the layers <NUM>, <NUM>, the value the sensitivity S increases with increasing the aspect ratio t/D of the thickness t to the diameter (lateral dimension) D of the magnetoresistive element <NUM>.

The sensitivity S of the magnetoresistive element <NUM> is plotted as a function of the thickness of the sense layer <NUM> in <FIG>. The sensitivity S was simulated for lateral dimensions D of the magnetoresistive element <NUM> of <NUM>, <NUM> and <NUM>, and for magnetizations of the sense layer <NUM> of <NUM> and <NUM> kA/m. The TMR value of the magnetoresistive element <NUM> was assumed to be <NUM>%. <FIG> shows that the higher values of sensitivity S are obtained for a lateral dimension of <NUM> and a magnetization of <NUM> kA/m. For a given TMR value, decreasing the sense magnetization <NUM> of the sense layer <NUM> results in an increase of the sensitivity S of the magnetoresistive element <NUM>. In the whole text of this application, the expression "sense magnetization" is used indifferently for "saturation sense magnetization" or "spontaneous sense magnetization", where saturation magnetization has its usual meaning of the maximum induced magnetic moment.

In term of design rules for the magnetoresistive element <NUM> that can measure an external magnetic field along an out-of-plane axis substantially perpendicular to the plane of the sense layer and having high working field range, low hysteresis, high linearity and sufficient sensitivity, the results shown above suggest providing the magnetoresistive element <NUM> where the reference layer <NUM> has a reference magnetization <NUM> oriented substantially perpendicular to the plane of the reference layer <NUM>, and where the sense layer <NUM> has a sense magnetization <NUM> comprising a vortex configuration in the absence of an external magnetic field <NUM>. The vortex configuration should be substantially parallel to the plane of the sense layer <NUM> and have a vortex core <NUM> magnetization along an out-of-plane axis <NUM> substantially perpendicular to the plane of the sense layer <NUM>.

Moreover, in term of design rules for the magnetoresistive element <NUM> having high working field range (such as <NUM> mT or more), low hysteresis (such as less than <NUM>µV/V) for external magnetic field magnitudes up to the expulsion field, high linearity (such as less than <NUM>% or <NUM>% error) and sufficient sensitivity, the magnetoresistive element <NUM> should have small lateral dimension D (or high aspect ratio). According to the present invention, the magnetoresistive element has a lateral dimension D below <NUM>, preferably below <NUM>, or preferably below <NUM>, and a small sense magnetization <NUM> of the sense layer <NUM>, which is below <NUM> kA/m, possibly below <NUM> kA/m or possibly below <NUM> kA/M. Furthermore, the sense layer <NUM> has a thickness greater than <NUM>.

In one aspect, the thickness of the sense layer <NUM> can be thick (more than <NUM>) and the sense magnetization <NUM> of the sense layer <NUM> can have a value that corresponds to typical saturation magnetization values found in conventional magnetoresistive elements (for example <NUM> kA/m or larger, but less than <NUM> kA/m). In a first example (<FIG>), the magnetoresistive element <NUM> has a lateral dimension D of <NUM> and the sense layer <NUM> has a thickness of <NUM>. The ferromagnetic material forming the sense layer <NUM> can comprise a ferromagnetic alloy having a sense magnetization of <NUM> kA/m or larger (here <NUM> kA/m).

In an alternative configuration, not covered by the scope of the present invention, the low sense magnetization of the sense layer <NUM> can be obtained by decreasing the thickness (for example below <NUM>) and/or by a proper selection of the ferromagnetic material forming the sense layer <NUM> such as to obtain a low sense magnetization (for example lower than <NUM> kA/m). In a second example (<FIG>), the magnetoresistive element <NUM> has a lateral dimension D of <NUM>, the sense layer <NUM> has a thickness of <NUM> and has a sense magnetization of <NUM> kA/m. The sense layer <NUM> can be formed a ferromagnetic alloy having a low magnetization.

<FIG> and <FIG> show that a substantially identical magnetization curve is obtained for the magnetoresistive element <NUM> of the first and second examples. In both examples, the expulsion field Hexpl can be about <NUM> mT and the nucleation field Hnucl can be about <NUM> mT.

The magnetoresistive element <NUM> can have a sensitivity S of <NUM> mV/V or higher, for TMR = <NUM>%. The sensitivity S can be further increased by increasing TMR.

Referring again to <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 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 an embodiment represented in <FIG>, each of the first and second reference sublayer <NUM>, <NUM> includes a multilayer structure. In particular, each of the first and second reference sublayer <NUM>, <NUM> can include a plurality of alternating first metallic layers <NUM> and second metallic layers <NUM>. For example, the first metallic layer <NUM> can comprise an ultrathin Co layer and the second metallic layer <NUM> can comprise an ultrathin Pt layer. Although the second metallic layer <NUM> preferably comprises Pt, other metals that provide PMA can also be used.

In one aspect, the ultrathin Co layer <NUM> can have a thickness between <NUM> and <NUM>. The ultrathin Pt layer <NUM> can have a thickness between <NUM> and <NUM>. The coupling layer <NUM> can be a Ru layer. Although the coupling layer <NUM> preferably comprises Ru, other metals that generate RKKY coupling can also be used.

Claim 1:
Magnetoresistive element (<NUM>) comprising 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 magnetoresistive element (<NUM>) being configured to measure an external magnetic field (<NUM>) oriented substantially perpendicular to the plane of the layers (<NUM>, <NUM>);
wherein the reference magnetization (<NUM>) is oriented substantially perpendicular to the plane of the reference layer (<NUM>); and
wherein the sense magnetization (<NUM>) comprises a vortex configuration in the absence of an external magnetic field (<NUM>), the vortex configuration being substantially parallel to the plane of the sense layer (<NUM>) and having a vortex core (<NUM>) magnetization along an out-of-plane axis (<NUM>) substantially perpendicular to the plane of the sense layer (<NUM>);
characterized in that the magnetoresistive element (<NUM>) has a lateral dimension being smaller than <NUM>, the sense layer (<NUM>) has a thickness being greater than <NUM> and a sense magnetization (<NUM>) below <NUM> kA/m at room temperature.