Patent Description:
<FIG> shows a cross-section view of a magnetoresistive element <NUM> comprising a ferromagnetic reference layer <NUM> having a reference magnetization <NUM>, a ferromagnetic sense layer <NUM> having a free sense magnetization <NUM> and a tunnel barrier layer <NUM> between the sense and reference ferromagnetic layers <NUM>, <NUM>. The sense magnetization <NUM> can be oriented in an external magnetic field <NUM> while the reference magnetization <NUM> remain substantially undisturbed. The external magnetic field <NUM> can thus be sensed by measuring a resistance of the magnetoresistive sensor element <NUM>. The resistance depends on the relative orientation of the sense magnetization and the reference magnetization.

A low measured resistance (RP) is measured in the magnetoresistive element <NUM> when the sense magnetization <NUM> is oriented parallel to the reference magnetization <NUM>. A high resistance (RAP) is measured in the magnetoresistive element <NUM> when the sense magnetization <NUM> is oriented antiparallel to the reference magnetization <NUM>. The difference between the value of the high and low resistance (RAP - RP) is also known as the tunnel magnetoresistance (TMR).

The sense magnetization <NUM> can comprise a stable vortex configuration providing a linear and non-hysteretic behavior in a large magnitude range of an external magnetic field. Such magnetoresistive element is thus advantageous for 1D magnetic sensor applications and can be easily configured to adjust its sensitivity.

<FIG> shows a hysteresis response (or magnetization curve) to the external magnetic field <NUM> (Hext, in arbitrary unit) on the sense magnetization <NUM> (M, in arbitrary unit). The full hysteresis loop of a vortex sense magnetization <NUM> is characterized by a linear increase of magnetization M with the applied magnetic field Hext until the vortex expulsion field is reached at the Hexpl point. At this point the sense magnetization <NUM> becomes magnetically saturated. To recover the vortex state in the sensing layer <NUM>, one needs to reduce the magnetic field below the nucleation field Hnucl. As long as the applied magnetic field is within the magnitudes corresponding to the expulsion field (+/-Hexpl) of the vortex in the sense magnetization <NUM>, the hysteresis response to the external magnetic field <NUM> (Hext) comprises a reversible linear portion corresponding to the movement of a core of the vortex with the external magnetic field Hext. The values and the slope of the linear part of hysteresis loop are strongly dependent on the size 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 Hext.

The vortex is characterized by its susceptibility χ, which corresponds to the slope of the linear region of the M(H) loop: <MAT>.

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

A drawback of such magnetoresistive element is the temperature dependence of the TMR and the magnetic susceptibility χ of the sense layer <NUM> on the temperature T. When the temperature T increases, the sense magnetization <NUM> decreases which leads to an increase of the susceptibility χ. On the other hand, the TMR diminishes when the temperature T is increased. This dependance results in a limited accuracy of the magnetoresistive element response over working temperatures and limits potential applications of the magnetoresistive element. Typically, the temperature coefficient of TMR of a conventional magnetoresistive element is large and negative leading to overall negative temperature coefficient of sensitivity (TCS) in the magnetoresistive element.

The TCS can be controlled by using an electronic circuit which compensates change of the sensitivity S of the magnetoresistive sensor element <NUM> by changing the magnetoresistive element bias voltage with respect to the temperature change. This solution however requires trimming to adjust the TCS. Moreover, using an additional electronic circuit requires larger die size, making the process and development of the magnetoresistive sensor element <NUM> more complicated.

In European patent application <CIT> by the present applicant, a magnetoresistive element comprises a sense layer having a portion comprising a transition metal element in a proportion such that a temperature dependence of a magnetic susceptibility of the sense layer substantially compensates a temperature dependence of the TMR of the magnetoresistive element.

<CIT> discloses a TMR layer stack <NUM> comprising two free layers <NUM>-<NUM> and <NUM>-<NUM> having a vortex magnetization and a reference layer <NUM>.

The present disclosure concerns a magnetoresistive element comprising a reference layer having a pinned reference magnetization, a ferromagnetic sense layer having a free sense magnetization comprising a stable vortex configuration, and a tunnel barrier layer between the reference layer and the sense layer. The sense layer comprises a first sense layer portion in contact with the tunnel barrier layer and a second sense layer portion in contact with the first sense layer portion. The first sense layer portion is configured such that a magnetic coupling between the first and second sense layer portions is between ±<NUM>-<NUM> J/m<NUM> and ±<NUM>-<NUM> J/m<NUM>; and a perpendicular magnetic anisotropy (PMA) originating from the interface between the first sense layer portion and the tunnel barrier layer is between 8x10<NUM> A/m and 8x10<NUM> A/m, such as to shift positively the TCS of the magnetoresistive element and compensate the negative temperature coefficient of TMR of the magnetoresistive element.

The magnetoresistive element disclosed herein can achieve a high degree of TCS compensation by a relatively modest modification of the layers in the magnetoresistive element. By contrast, compensation of TCS only by dilution of the sense layer magnetization requires a high concentration of nonmagnetic impurities. This results in nonlinear temperature dependence of magnetic susceptibility which is transferred to nonlinearity of the TSC of the magnetoresistive element.

<FIG> illustrates a magnetoresistive element <NUM> according to an embodiment. The magnetoresistive element <NUM> comprises a ferromagnetic reference layer <NUM> having a pinned reference magnetization <NUM>, a ferromagnetic sense layer <NUM> having a sense magnetization <NUM>, and a tunnel barrier layer <NUM> between the sense and reference ferromagnetic layers <NUM>, <NUM>. The sense magnetization <NUM> is configured to be orientable in an external magnetic field <NUM> while the reference magnetization <NUM> remains substantially undisturbed. The external magnetic field <NUM> can thus be sensed by measuring a resistance of the magnetoresistive element <NUM>. The resistance depends on the orientation of the sense magnetization <NUM> relative to the reference magnetization <NUM>.

The ferromagnetic layers can be made of a Fe based alloy, such as CoFe, NiFe or CoFeB. The reference layer <NUM> can be pinned by an antiferromagnetic layer <NUM> by magnetic exchange bias coupling. The antiferromagnetic layer <NUM> can comprise an alloy 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); and alloys based on Ni and Mn (e.g., NiMn). The reference layer <NUM> can comprise one or a plurality of ferromagnetic layers. In the example illustrated in <FIG>, the reference layer <NUM> comprises a synthetic antiferromagnet (SAF) structure comprising at least a first ferromagnetic layer <NUM> and a second ferromagnetic layer <NUM> that are separated and coupled antiferromagnetically through an intervening non-magnetic layer <NUM>. The non-magnetic layer is typically a Ru layer but can also comprise any of: Ru, Ir or Cu or a combination of these elements. In a SAF structure, a first reference magnetization <NUM> of the first ferromagnetic layer <NUM> is coupled antiparallel to a second reference magnetization <NUM> by the non-magnetic layer <NUM>. In the case the magnetoresistive element <NUM> comprise an antiferromagnetic layer <NUM>, the second reference magnetization <NUM> of the ferromagnetic layer <NUM> adjacent to the antiferromagnetic layer <NUM> is pinned by the antiferromagnetic layer <NUM>. The thickness of the reference layer <NUM> will depend on the material selected. In one example, the reference layer <NUM> may have a thickness from <NUM> to <NUM>.

The tunnel barrier <NUM> can comprise 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>.

According to some embodiments, the sense layer <NUM> comprises a first sense layer portion <NUM> in contact with the tunnel barrier layer <NUM> and a second sense layer portion <NUM> in contact with the first sense layer portion <NUM>.

The second sense layer portion <NUM> can be configured to allow for a vortex state. For instance, the sense magnetization <NUM> can comprise a stable vortex configuration. The second sense layer portion <NUM> can comprise a soft ferromagnetic alloy. For example, the second sense layer portion <NUM> can comprise an Ni alloy or NiFe alloy possibly comprising a few wt% of Cr, Si, B or V. In one aspect, the second sense layer portion <NUM> has a thickness that is greater than <NUM>. The second sense layer portion <NUM> can be up to <NUM> in thickness.

In an embodiment, the first sense layer portion <NUM> is configured such that a magnetic coupling between the first and second sense layer portions <NUM>, <NUM> is between <NUM>-<NUM> J/m<NUM> and <NUM>-<NUM> J/m<NUM>, where the sign of the exchange bias coupling can be positive or negative. The first sense layer portion <NUM> can be further configured such that a PMA field originating from the interface between the first sense layer portion <NUM> and the tunnel barrier layer <NUM> is between 8x10<NUM> A/m and 8x10<NUM> A/m.

Adjusting the exchange bias coupling between the first and second sense layer portions <NUM>, <NUM> to the desired exchange bias coupling values between ±<NUM>-<NUM> J/m<NUM> and ±<NUM>-<NUM> J/m<NUM> and adjusting the PMA field to the desired PMA field values between 8x10<NUM> A/m and 8x10<NUM> A/m modifies the magnetic susceptibility of the first sense layer portion <NUM> and makes its temperature coefficient more positive. The positive temperature coefficient of the first sense layer portion <NUM> shifts positively the TCS of the magnetoresistive element <NUM>. The positive shift of the magnetoresistive element <NUM> TCS can be such as to compensate the negative temperature coefficient of TMR of the magnetoresistive element <NUM>. For instance, the TCS of the magnetoresistive element <NUM> can be shifted more positively by about <NUM> to <NUM> ppm/°C, depending on the diameter of the magnetoresistive element <NUM> and on material type of the second sense layer portion <NUM>.

The first sense layer portion <NUM> is configured such that a magnetic coupling between the first and second sense layer portions <NUM>, <NUM> is between <NUM>-<NUM> J/m<NUM> and 2x10-<NUM> J/m<NUM>, or 2x10-<NUM> J/m<NUM> and <NUM>-<NUM> J/m<NUM>, or 2x10-<NUM> J/m<NUM> and 2x10-<NUM> J/m<NUM>. The magnetic coupling between the first and second sense layer portions <NUM>, <NUM> is typically four to five time less than the one in a sense layer of a conventional magnetoresistive element <NUM>.

In an embodiment, the magnetoresistive element <NUM> further comprises a mediation layer <NUM> between the first and second sense layer portions <NUM>, <NUM>. The mediation layer <NUM> is configured to reduce the magnetic coupling between the first sense layer portion <NUM> and the second sense layer portion <NUM>. More particularly, the mediation layer <NUM> can be configured to adjust the exchange bias coupling between the first and second sense layer portions <NUM>, <NUM> to the desired exchange bias coupling values between ±<NUM>-<NUM> J/m<NUM> and ±<NUM>-<NUM> J/m<NUM>, and adjust the PMA field to the desired PMA field values between 8x10<NUM> A/m and 8x10<NUM> A/m. The mediation layer <NUM> can comprise a nonmagnetic layer, preferably a nonmagnetic layer.

In one aspect, the thickness and composition of the mediation layer <NUM> can be adjusted to arrive at the desired exchange bias coupling values between ±<NUM>-<NUM> J/m<NUM> and ±<NUM>-<NUM> J/m<NUM> and to the desired PMA field values between 8x10<NUM> A/m and 8x10<NUM> A/m. In an example, the mediation layer <NUM> can compriseor can be made of any one or a combination of: Nb, Ti, Ru, W, Ta, Ir, Mo or Cu. In another example, a thickness of the mediation layer <NUM> can range between <NUM> and <NUM>.

It was also shown that the exchange bias coupling between the first and second sense layer portions <NUM>, <NUM> between ±<NUM>-<NUM> J/m<NUM> and ±<NUM>-<NUM> J/m<NUM> is strong enough to allow the transfer of the vortex state from the second sense layer portion <NUM> to the first sense layer portion <NUM>. Therefore, the hysteresis response (or magnetization curve) of the magnetoresistive element <NUM> with the mediation layer <NUM> can be similar to the one of the magnetoresistive element <NUM> without the mediation layer <NUM>.

The effective TMR and magnetic susceptibility in the first sense layer portion <NUM> decreases with a decrease in the magnitude of the PMA field. Since the PMA field decreases quickly with the temperature, it provides a compensation effect to the TCS of the first sense layer portion <NUM>. This effect is more important when the first sense layer portion <NUM> is thin. For example, the first sense layer portion <NUM> can be at least about four times thinner than the second sense layer <NUM>.

In another embodiment, the first sense layer portion <NUM> comprises a CoxFeyBz alloy with x and y varying independently from <NUM> to <NUM> vol% and z varying from <NUM> to <NUM> vol%. The thickness the first sense layer portion <NUM> can be further adjusted to arrive at the desired exchange bias coupling values between ±<NUM>-<NUM> J/m<NUM> and ±<NUM>-<NUM> J/m<NUM> and to the desired PMA field values between 8x10<NUM> A/m and 8x10<NUM> A/m. For example, but without limitation, the first sense layer portion <NUM> has a thickness which can be from <NUM> to <NUM>. It should be noted that both PMA field and exchange bias coupling will be decreased by substantially the same amount when the thickness of the first sense layer portion <NUM> increases.

In an embodiment shown in <FIG>, the first sense layer portion <NUM> can comprises a plurality of ferromagnetic sense sublayers <NUM>, each sense sublayer <NUM> containing CoxFeyBz alloy wherein x, y, z can vary from one sense sublayers <NUM> to another. The sense sublayers <NUM> can have a thickness between <NUM> and <NUM>. The first sense layer portion <NUM> can further comprise nonmagnetic spacer sublayers <NUM> between the sense sublayers <NUM>. The spacer sublayers <NUM> can comprise a metallic nanolayer and have a thickness smaller than <NUM>.

In a further embodiment, the second sense layer portion <NUM> can comprise a dilution element. The dilution element can be alloyed in the second sense layer portion <NUM> or the second sense layer portion <NUM> can comprise one or a plurality of dilution nanolayers. The dilution element or dilution nanolayer is configured to dilute the sense magnetization <NUM> and decreases the Curie temperature Tc of the sense layer <NUM>. Decreasing the Curie temperature Tc of the sense layer <NUM> results in a faster drop in magnetization with increasing temperature T in the working temperature range of the magnetoresistive element <NUM>. The decrease in magnetization with increasing temperature T results in an increase of the susceptibility χ with increasing temperature. By adjusting the dilution of the sense magnetization <NUM> it is then possible to compensate the decrease of the TMR with the increase of the susceptibility χ with increasing temperature. Adjusting the dilution of the sense magnetization <NUM> thus allows for further controlling the TCS, for instance shifting positively the TCS of the magnetoresistive element <NUM> and compensate the negative temperature coefficient of TMR of the magnetoresistive element <NUM>. The dilution element or dilution nanolayer can comprise a transition metal element, for instance Ta, W or Ru.

In an embodiment, the second sense layer portion <NUM> comprises the dilution element in a proportion less than <NUM> vol% (% by volume).

Compensation of the TCS by dilution of the second sense layer portion <NUM> can occurs independently from the compensation due to the first sense layer portion <NUM> and of the compensation due to the mediation layer <NUM>. Compensation of the TCS by dilution of the second sense layer portion <NUM> can thus be used in addition the compensation due to the first sense layer portion <NUM> and of the mediation layer <NUM> to fine tune the TCS of the magnetoresistive element <NUM>.

The inventors have shown that the TCS of the magnetoresistive element <NUM> disclosed herein can have a TCS about +<NUM> ppm of positive shift when the first sense layer portion <NUM> is separated from the second sense layer portion <NUM> by the mediation layer <NUM>. The mediation layer <NUM> provides the first sense layer portion <NUM> with a strong (for example between ±<NUM>-<NUM> J/m<NUM> and ±<NUM>-<NUM> J/m<NUM>) positive temperature coefficient of magnetic susceptibility. Thus, the positive temperature coefficient of the first sense layer portion <NUM> in the presence of the mediation layer <NUM> shifts positively the TCS of the magnetoresistive element <NUM> and can compensate the negative temperature coefficient originating from the TMR effect. This applies for both positive and negative magnitude of the exchange bias coupling between first and second sense layer portions <NUM>, <NUM>.

The inventors have also shown that the temperature coefficient of TMR in the magnetoresistive element <NUM> and the temperature dependence of the magnetic susceptibility of the second sense layer portion <NUM> are not substantially modified between the magnetoresistive element <NUM> comprising the mediation layer <NUM> and without the mediation layer <NUM>.

In fact, the magnetoresistive element <NUM> was characterized by using a physical property measurement system (PPMS). PPMS characterization has shown that there is no difference in temperature dependence of magnetic susceptibility of the second sense layer portion <NUM> and no significant change in temperature dependence of TMR compared to a conventional sense layer <NUM>. At the same time, one observes noticeable modification of temperature dependence of magnetic susceptibility in the first sense layer portion <NUM>. In fact, the first sense layer portion <NUM> acquires significantly higher positive temperature coefficient that is responsible for improved TCS of the vortex-based magnetoresistive element.

Claim 1:
Magnetoresistive element, comprising
a reference layer (<NUM>) having a pinned reference magnetization (<NUM>);
a ferromagnetic sense layer (<NUM>) having a free sense magnetization (<NUM>) comprising a stable vortex configuration; and
a tunnel barrier layer (<NUM>) between the reference layer (<NUM>) and the sense layer (<NUM>);
wherein the sense layer (<NUM>) comprises a first sense layer portion (<NUM>) in contact with the tunnel barrier layer (<NUM>) and a second sense layer portion (<NUM>) in contact with the first sense layer portion (<NUM>);
characterized in that
the first sense layer portion (<NUM>) is configured such that a magnetic coupling between the first and second sense layer portions (<NUM>, <NUM>) is between ±<NUM>-<NUM> J/m<NUM> and ±<NUM>-<NUM> J/m<NUM>; and a perpendicular magnetic anisotropy, PMA, originating from the interface between the first sense layer portion (<NUM>) and the tunnel barrier layer (<NUM>) is between 8x10<NUM> A/m and 8x10<NUM> A/m, such as to shift positively the temperature coefficient of sensitivity, TCS, of the magnetoresistive element (<NUM>) and compensate the negative temperature coefficient of TMR of the magnetoresistive element (<NUM>).