Patent ID: 12204005

EXAMPLES OF EMBODIMENTS

With reference toFIG.4, a cross-section view of a magnetoresistive sensor element2is shown according to an embodiment. The magnetoresistive sensor element2comprises a ferromagnetic reference layer21having a reference magnetization210, a ferromagnetic sense layer23having a free sense magnetization230and a tunnel barrier layer22between the reference and sense ferromagnetic layers21,23. The sense magnetization230can be oriented in an external magnetic field60while the reference magnetization210remains substantially undisturbed. The external magnetic field60can thus be sensed by measuring a resistance of the magnetoresistive sensor element2. The resistance depends on the orientation of the sense magnetization230relative to the reference magnetization210.

The sense magnetization230comprises a stable vortex configuration rotating in a circular path along the edge of the sense layer23and around a core231, reversibly movable in accordance to the external magnetic field60. For a given lateral dimension of the magnetoresistive sensor element2, the thickness of the sense layer23is chosen such that the sense layer23has a stable vortex configuration magnetisation in the absence of an applied magnetic field.

The reference magnetization210is substantially longitudinally oriented in the plane of the reference layer21. The orientation of the reference magnetization210is determined by the exchange coupling (generating an exchange-bias) between the reference pinning layer24and the reference layer21. The reference layer21can comprise a synthetic antiferromagnetic (SAF).

In one aspect, the reference and sense layers21,23comprise, or are 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 reference layer21can have a thickness between 2 nm and 7 nm. The reference and sense layers21,23can 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 non-magnetic layers such as Ta, Ti, W, Ru, Ir.

The reference and sense magnetizations210,230can have magnetic anisotropy substantially parallel to the plane of the layers21,23(in-plane, as shown inFIG.4) and/or substantially perpendicular to the plane of the layers21,23(out-of-plane).

The magnetoresistive sensor element2further comprises a reference pinning layer24pining the reference magnetization210by exchange-bias at a first threshold temperature Tb1. The expression “threshold temperature” can correspond to a blocking temperature, such as a Neel temperature, or another threshold temperature of the reference pinning layer24. The reference pinning layer24unpins, or decouples, the reference magnetization210when the temperature above the first threshold temperature Tb1.

The magnetoresistive sensor element2further comprises a sense pinning layer25pining the sense magnetization230by exchange-bias at a second threshold temperature Tb2lower that the first threshold temperature Tb1.

In one aspect, the sense pinning layer25can be configured such that the strength of exchange-bias generated in the sense layer23by the sense pinning layer25is less than that generated in the reference layer21by the reference pinning layer24. For example, the magnitude of the exchange-bias generated in the sense layer23by the sense pinning layer25is substantially between 2×10−8J/cm2and 4×10−8J/cm2(0.2 erg/cm2to 0.4 erg/cm2).

In some aspects, the thickness of the sense layer23can be selected such that the strength of exchange-bias generated in the sense layer23by the sense pinning layer25allows the sense magnetization230can be aligned with respect to the external magnetic field60to be measured in a magnetization changeable state.

In some aspects, the reference pinning layer24and the sense pinning layer25comprises, or is formed of, an antiferromagnetic material pinning the reference magnetization210and respectively the sense magnetization230, through exchange coupling. In particular, the reference pinning layer24and the sense pinning layer25comprise, or are formed of, a magnetic material of the antiferromagnetic type, including 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); and alloys based on Ni and Mn (e.g., NiMn) or alloys based on chromium (“Cr”), NiO or FeO.

In some aspects, the thickness of the reference pinning layer24and of the sense pinning layer25can be between 4 nm and 15 nm.

The tunnel barrier layer22comprises, or is 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 10 nm. Large TMR for example of up to 200% can be obtained for the magnetic tunnel junction2comprising a crystalline MgO-based tunnel barrier layer22.

FIG.5shows the vortex expulsion field Hexpland the vortex nucleation field Hnuclas a function of the thickness of the sense layer23. The values of the vortex expulsion field Hexpland of the vortex nucleation field Hnuclare calculated for a sense layer23comprising a NiFe alloy, a TMR of the magnetoresistive sensor element2of 140%, a reference magnetization210pinned in-plane, i.e., in the plane of the reference layer21. The calculations were performed for a strength of the exchange-bias generated in the sense layer23by the sense pinning layer25of 2×10−8J/cm2and 4×10−8J/cm2. The calculated vortex expulsion field Hexpland the vortex nucleation field Hnuclas a function of the thickness of the sense layer23in the absence of an exchange-bias generated by the sense pinning layer25are also shown.

FIG.5shows that the strength of the expulsion and nucleation fields Hnucl, Hexplare generally higher for thicknesses of the sense layer23smaller than about 40 nm, when the sense layer23is subjected to the exchange-bias generated by the sense pinning layer25and compared to the expulsion and nucleation fields Hnucl, Hexplin the absence of the exchange-bias generated by the sense pinning layer25. High values of the expulsion and nucleation fields Hnucl, Hexplcan be obtained for a thickness of the sense layer23between 15 nm and 80 nm. The higher strength of the expulsion and nucleation fields Hnucl, Hexplallows for increasing the stability of the vortex configuration. It further allows for wider linear response region of the magnetoresistive sensor element2and for a reduced change in the response of the magnetoresistive sensor element2when the latter is subjected to a high magnetic field used in magnetic permeability testing.

FIG.6reports the sensitivity S of the magnetoresistive element10as a function of the thickness of the sense layer23, for a range of the external magnetic field60of ±1.6×104A/m (±200 Oe) and for a strength of the exchange-bias generated by the sense pinning layer25of 2×10−8J/cm2(open circles) and 4×10−8J/cm2(open squares). The sensitivity S of a conventional magnetoresistive element not comprising a sense pinning layer25is also reported for an external magnetic field60of 1.6×104A/m (200 Oe) (plain squares). The magnetization vortex state results from an equilibrium between the vortex magnetostatic energy and exchange energy from the sense pinning layer25. Due to the competition between the magnetostatic energy at larger thickness of the sense layer23and increased exchange energy (or pinning field) for small thicknesses of the sense layer23, a maximum sensitivity S of the magnetoresistive element10can be obtained by adjusting the strength of the exchange-bias generated by the sense pinning layer25. Here, an exchange-bias of 4×10−8J/cm2yields a sensitivity S between 3 and 5 mV/V/mT for the sense layer23having a thickness between 15 nm and 80 nm. Such values of the sensitivity S are suitable for magnetic sensor applications.

FIG.7reports the linearity of the magnetoresistive element10response in term of linear error in % as a function of the thickness of the sense layer23, for a range of the external magnetic field60of ±1.6×104A/m (±200 Oe) and for a strength of the exchange-bias generated by the sense pinning layer25of 2×10−8J/cm2(open circles) and 4×10−8J/cm2(open squares). The linearity of the response obtained for a conventional magnetoresistive element not comprising a sense pinning layer25is also reported as a function of the thickness of the sense layer, for an external magnetic field60of 1.6×104A/m (200 Oe) (plain squares).FIG.7shows that the linear error for a range of the external magnetic field60of ±1.6×104A/m (±200 Oe) can be reduced for the sense layer23having a thickness less than 20 nm, compared to that of a magnetoresistive element without the sense pinning layer25.FIG.7shows that the linear error of the magnetoresistive element10response, for a range of the external magnetic field60of ±1.6×104A/m (200±Oe) is below 4% for the sense layer23having a thickness between 15 nm and 80 nm.

FIGS.8and9report the linearity of the magnetoresistive element10response in term of linear error in % as a function of the sensitivity S of the magnetoresistive element10for a strength of the exchange-bias generated by the sense pinning layer25of 2×10−8J/cm2(plain circles) and 4×10−8J/cm2(plain squares). InFIG.8, the linearity was calculated for a range of the external magnetic field60of ±1.6×104A/m (±200 Oe) and inFIG.9, the linearity was calculated for a range of the external magnetic field60of ±3.2×104A/m (±400 Oe). The linearity of the response obtained for a conventional magnetoresistive element not comprising a sense pinning layer25is also reported as a function of the sensitivity S of the conventional magnetoresistive element, for an external magnetic field60of ±1.6×104A/m (open circles) inFIG.8and for an external magnetic field60of ±3.2×104A/m (open circles) inFIG.9.

For an external magnetic field60range of ±1.6×104A/m and an exchange bias of 2×10−8J/cm2, a thickness of the sense layer23between 15 nm and 80 nm yields a linear error in the response of the magnetoresistive element10that is below 3.5%. For larger external magnetic field60range of ±3.2×104A/m and an exchange bias of 2×10−8J/cm2, a thickness of the sense layer23between 15 nm and 80 nm yields a linear error in the response of the magnetoresistive element10that is below 2%.

The thickness of the sense layer between 15 nm and 80 nm allows for obtaining strong exchange coupling of the sense magnetization in a portion of the layer near the sense pinning layer and the lesser exchange coupling of the sense magnetization in a portion of the layer far from the sense pinning layer such that vortex behave in a linear fashion in that farther portion, in the presence of the external magnetic field. The thickness of the sense layer between 15 nm and 80 nm allows for combining the effect if the exchange coupling of the sense magnetization (nominal performance being unchanged after the magnetoresistive element has been subjected to high magnetic fields) and obtaining a wide linear response.

A low sensitivity S of the magnetoresistive element10response (for example a sensitivity S smaller than 5%) can then be obtained by adjusting the strength of the exchange-bias generated by the sense pinning layer25and by adjusting the thickness of the sense layer23.

According to an embodiment, a method for manufacturing the magnetoresistive element10comprises the steps of:annealing the magnetoresistive element10with an applied external magnetic field sufficient to saturate the magnetization of the reference layer21, and at an annealing temperature higher than the first blocking temperature Tb1, which pins de reference layer21in a direction along the direction of the applied magnetic field; andannealing the magnetoresistive element10in the absence of an applied external magnetic field at an annealing temperature higher than the second blocking temperature Tb2and lower than the first blocking temperature Tb1, which pins the sense layer21in a magnetic vortex configuration.

Prior to the annealing steps, the method can comprise forming the magnetoresistive element10, including the steps of depositing the reference pinning layer24and the sense pinning layer25. The reference layer21can be deposited directly on the reference pinning layer24and the sense layer23can be deposited directly on the sense pinning layer25.

Forming the magnetoresistive element10can further comprise the step of depositing the tunnel barrier layer22, wherein the sense layer23is deposited on the tunnel barrier layer22. Depositing the tunnel barrier layer22can be performed by using an RF magnetron sputtering technique or any other suitable techniques.

In some aspects forming the magnetoresistive element10comprises depositing, in this order, the reference pinning layer24, reference layer23, tunnel barrier layer22, sense layer23and sense pinning layer25. The magnetoresistive element10can further comprise depositing, in this order, the sense pinning layer25, sense layer23, tunnel barrier layer22, reference layer23and reference pinning layer24.

In an embodiment, a magnetic sensor comprises a plurality of the magnetoresistive element2disclosed herein.

REFERENCE NUMBERS AND SYMBOLS

2magnetoresistive element21reference layer210reference magnetization22tunnel barrier layer23sense layer230sense magnetization231core24reference pinning layersense pinning layer60external magnetic fieldHextexternal magnetic fieldHexplexpulsion fieldHnuclnucleation fieldS sensitivityχ susceptibility