MAGNETORESISTIVE SENSOR ELEMENT HAVING COMPENSATED TEMPERATURE COEFFICIENT OF SENSITIVITY AND METHOD FOR MANUFACTURING SAID ELEMENT

Abstract

A magnetoresistive sensor element including: a reference layer having a pinned reference magnetization; a sense layer having a free sense magnetization comprising a stable vortex configuration reversibly movable in accordance to an external magnetic field to be measured; a tunnel barrier layer between the reference layer and the sense layer; wherein the sense layer includes a first ferromagnetic sense portion in contact with the tunnel barrier layer and a second ferromagnetic sense portion in contact with the first ferromagnetic sense portion; the second ferromagnetic sense portion including a dilution element in a proportion such that a temperature dependence of a magnetic susceptibility of the sense layer substantially compensates a temperature dependence of a tunnel magnetoresistance of the magnetoresistive sensor element. Also, a method for manufacturing the magnetoresistive sensor element.

Claims

1-11. (canceled)

12. A magnetoresistive sensor element comprising: a reference layer having a pinned reference magnetization; a sense layer having a free sense magnetization comprising a stable vortex configuration having a core reversibly movable in accordance to an external magnetic field to be measured; a tunnel barrier layer between the reference layer and the sense layer, the tunnel barrier layer comprising an insulating material; wherein the sense layer comprises a first ferromagnetic sense portion in contact with the tunnel barrier layer and a second ferromagnetic sense portion in contact with the first ferromagnetic sense portion; and wherein the second ferromagnetic sense portion comprises a dilution element in a proportion such that a temperature dependence of a magnetic susceptibility of the sense layer substantially compensates a temperature dependence of a tunnel magnetoresistance (TMR) of the magnetoresistive sensor element.

13. The magnetoresistive sensor element, according to claim 12, wherein the dilution element comprises a transition metal element.

14. The magnetoresistive sensor element, according to claim 12, wherein the transition metal element comprises Ta, W or Ru.

15. The magnetoresistive sensor element, according to claim 12, wherein the first ferromagnetic sense portion comprises a CoFeB alloy.

16. The magnetoresistive sensor element, according to claim 12, wherein the second ferromagnetic sense portion comprises a NiFe alloy comprising the dilution element.

17. The magnetoresistive sensor element, according claim 12, wherein the second ferromagnetic sense portion comprises a plurality of ferromagnetic sub-layers comprising a ferromagnetic alloy and a plurality of dilution sub-layers comprising the dilution element.

18. The magnetoresistive sensor element, according to claim 17, wherein each dilution sub-layer is between 0.1 and 0.5 nm in thickness and each ferromagnetic sub-layer is between 0.5 and 5 nm in thickness.

19. The magnetoresistive sensor element, according to claim 17, wherein the ferromagnetic sub-layer comprises an NiFe, a CoFe, or a CoFeB alloy.

20. A magnetoresistive sensor for sensing a 1D external magnetic field, comprising a plurality of magnetoresistive sensor elements, each magnetoresistive sensor element comprising: a reference layer having a pinned reference magnetization; a sense layer having a free sense magnetization comprising a stable vortex configuration having a core reversibly movable in accordance to an external magnetic field to be measured; a tunnel barrier layer between the reference layer and the sense layer, the tunnel barrier layer comprising an insulating material; wherein the sense layer comprises a first ferromagnetic sense portion in contact with the tunnel barrier layer and a second ferromagnetic sense portion in contact with the first ferromagnetic sense portion; and wherein the second ferromagnetic sense portion comprises a dilution element in a proportion such that a temperature dependence of a magnetic susceptibility of the sense layer substantially compensates a temperature dependence of a tunnel magnetoresistance (TMR) of the magnetoresistive sensor element.

21. The magnetoresistive sensor according to claim 20, arranged in a Wheatstone full-bridge configuration.

22. Method for manufacturing the magnetoresistive sensor element, wherein the magnetoresistive sensor element comprises: a reference layer having a pinned reference magnetization; a sense layer having a free sense magnetization comprising a stable vortex configuration having a core reversibly movable in accordance to an external magnetic field to be measured; a tunnel barrier layer between the reference layer and the sense layer, the tunnel barrier layer comprising an insulating material; wherein the sense layer comprises a first ferromagnetic sense portion in contact with the tunnel barrier layer and a second ferromagnetic sense portion in contact with the first ferromagnetic sense portion; and wherein the second ferromagnetic sense portion comprises a dilution element in a proportion such that a temperature dependence of a magnetic susceptibility of the sense layer substantially compensates a temperature dependence of a tunnel magnetoresistance (TMR) of the magnetoresistive sensor element, the method comprising, for a plurality of temperatures and concentrations of the dilution element: measuring an electrical conductance of the magnetoresistive sensor element as a function of the external magnetic field for a plurality of temperatures, such as to obtain a plurality of measured conductance curve for each temperature; calculating a value of TMR of the magnetoresistive sensor element from the plurality of conductance curves, such as to determine the temperature dependence of TMR; and measuring a magnetization of the sense layer, such as to determine the temperature dependence of the magnetic susceptibility of the sense layer; and determining the proportion of the transition metal element for which the temperature dependence of TMR substantially compensates the temperature dependence of the magnetic susceptibility.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The invention will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which:

[0021] FIG. 1 shows a cross-section view of a magnetoresistive sensor element comprising a sense layer;

[0022] FIGS. 2a and 2b illustrate a top view of the sense layer having a sense magnetization comprising a vortex configuration movable in an external magnetic field;

[0023] FIG. 3 shows the full hysteresis response to the external magnetic field on the sense magnetization;

[0024] FIGS. 4a and 4b show the electrical conductance of the magnetoresistive sensor element as a function of the external magnetic field, when the sense magnetization is parallel (FIG. 4a) and antiparallel (FIG. 4b) to the reference magnetization;

[0025] FIG. 5 reports the ratio of the sense magnetization over the saturated sense magnetization as a function of the external field, measured for several temperatures in the magnetoresistive sensor element;

[0026] FIGS. 6a to 6c report the electrical conductance of the magnetoresistive sensor element as a function of the external magnetic field for increasing temperatures;

[0027] FIG. 7 shows a detail of the sense layer 21 comprising a first ferromagnetic sense portion and a second ferromagnetic sense portion, according to an embodiment;

[0028] FIG. 8 shows the saturation magnetization as a function of temperature for different dilution of the sense magnetization;

[0029] FIG. 9 reports experimentally measured saturation magnetization as a function of temperature for the sense layer, the second ferromagnetic sense portion comprising NiFe alloy for different content of a transition metal element;

[0030] FIG. 10 reports TCS values for various dilution achieved by adding Ta in the NiFe alloy of the second ferromagnetic sense portion;

[0031] FIG. 11 shows a detail of the second ferromagnetic sense portion, according to an embodiment, and

[0032] FIG. 12 presents a Wheastone bridge configuration in which four magnetoresistive elements are used to create a magnetoresistive sensor, according to an embodiment.

DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS

[0033] Referring to FIG. 1, a magnetoresistive sensor element 2 according to an embodiment comprises a ferromagnetic reference layer 23 having a pinned reference magnetization 230, a ferromagnetic sense layer 21 having a sense magnetization 210 that is movable in accordance to the external magnetic field 60, and a tunnel barrier layer 22 between the sense and reference ferromagnetic layers 21, 23. The sense magnetization 210 comprises a stable vortex configuration having a core 213 reversibly movable in accordance to the external magnetic field 60 (see FIGS. 2a and 2b).

[0034] The ferromagnetic layers can be made of a Fe based alloy, such as CoFe, NiFe or CoFeB. The reference layer can be pinned by an antiferromagnetic layer 24 by magnetic exchange bias coupling. The antiferromagnetic layer 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 23 can comprise one or a plurality of ferromagnetic layers or, as illustrated in FIG. 1, the reference layer 23 can comprise a synthetic antiferromagnet (SAF) comprising at least a first ferromagnetic layer 231 separated from a second ferromagnetic layer 232 by an antiparallel coupling layer 233 comprising of: Ru, Ir or Cu or a combination of these elements. A second reference magnetization 235 of the ferromagnetic layer 232 adjacent to the antiferromagnetic layer 24 is pinned and a first reference magnetization 234 of the first ferromagnetic layer 231 is coupled antiparallel to the second reference magnetization 235 by the antiparallel coupling layer 233. The tunnel barrier 22 can comprise an insulating material. Suitable insulating materials include oxides, such as aluminum oxide (e.g., Al.sub.2O.sub.3) and magnesium oxide (e.g., MgO). A thickness of the tunnel barrier layer 22 can be in the nm range, such as from about 1 nm to about 3 nm.

[0035] FIG. 7 shows a detail of the sense layer 21, according to an embodiment. The sense layer 21 comprises a first ferromagnetic sense portion 211 in contact with the tunnel barrier layer 22 and a second ferromagnetic sense portion 212 in contact with the first ferromagnetic sense portion 211. The second ferromagnetic sense portion 212 comprises a dilution element in a proportion such that a temperature dependence of a magnetic susceptibility χ of the sense layer 21 substantially compensates a temperature dependence of a tunnel magnetoresistance TMR of the magnetoresistive sensor element 2. The second ferromagnetic sense portion 212 should be thick enough to allow for a vortex state (to be adapted to the device size).

[0036] The dilution element dilutes the sense magnetization 210 and decreases the Curie temperature Tc of the sense layer 21. FIG. 8 shows the saturation magnetization normalized at 0K M.sub.s(T)/M.sub.s(0) as a function of temperature T for undiluted sense magnetization (curve A) having a first Curie temperature Tc.sub.1, moderately diluted sense magnetization (curve B) having a second Curie temperature Tc.sub.2 smaller than the first Curie temperature Tc.sub.1, and highly diluted sense magnetization (curve C) having a third Curie temperature Tc.sub.3 smaller than the second Curie temperature Tc.sub.2.

[0037] Also shown in FIG. 8 is the tangent at a median point of the curves A to C within a working temperature range T.sub.WR. The working temperature range T.sub.WR is shown by the dotted box in FIG. 8 and corresponds to temperatures at which the magnetoresistive sensor element 2 is typically operated. The tangent for curves A to C show that decreasing the Curie temperature Tc of the sense layer 21 results in a faster drop in magnetization with increasing temperature T in the working temperature range T.sub.WR. The faster drop in magnetization with increasing temperature T results in a faster increase of the susceptibility χ with increasing temperature.

[0038] By adjusting the dilution of the sense magnetization 210 it is possible to substantially compensate the decrease of the TMR with the increase of the susceptibility x with increasing temperature. Adjusting the dilution of the sense magnetization 210 thus allows for controlling the TCS, for example making the TCS to be substantially null in the working temperature range T.sub.WR. Here, dilution of the sense magnetization 210 is achieved by adding a dilution element in the ferromagnetic material forming the second ferromagnetic sense portion 212.

[0039] In an embodiment, the dilution element is a transition metal element. For example, the second ferromagnetic sense portion 212 can comprise a NiFe alloy including a transition metal element. The transition element can include for instance Ta, W or Ru.

[0040] FIG. 9 reports the measured saturation magnetization normalized at room temperature M/M(RT) as a function of temperature T for the sense layer 21 in which the second ferromagnetic sense portion 212 comprises a NiFe alloy for different content of the dilution element included in the layer. Magnetization curves are reported for no dilution (curve A), a concentration of 10% vol. of Ta (curve C), a concentration of 10% vol. of W (curve D) and a concentration of 7% vol. of Ta (curve B). FIG. 10 shows that the inclusion of Ta in the NiFe alloy of the second ferromagnetic sense portion 212 leads to a faster decrease of the magnetization with temperature T. Adding W to the second ferromagnetic sense portion 212 yields a stronger decrease of the magnetization with temperature T than when Ta is added.

[0041] FIG. 10 reports TCS values for various dilution achieved by adding Ta in the NiFe alloy of the second ferromagnetic sense portion 212 having a vortex configuration of the sense magnetization 210 having a diameter of about 440 nm. The TCS is substantially compensated (TCS approaching a null value) for a concentration of about 8% vol. of Ta in the NiFe alloy.

[0042] In an embodiment shown in FIG. 11, the second ferromagnetic sense portion 212 comprises a plurality of ferromagnetic sub-layers 214 comprising a ferromagnetic alloy and a plurality of dilution sub-layers 215 the dilution element, for example comprising a transition metal element. The ferromagnetic sub-layer 214 can have a thickness that is 0.5 nm or above. For example, the ferromagnetic sub-layer 214 can have a thickness between 0.5 and 5 nm. The dilution sub-layers 215 can have a thickness between 0.1 and 0.5 nm.

[0043] The ferromagnetic sub-layer 214 can comprise an NiFe, a CoFe or a CoFeB alloy.

[0044] The first ferromagnetic sense portion 211 can comprise a CoFeB alloy.

[0045] In one particular example, the first ferromagnetic sense portion 211 comprises a CoFeB alloy and the second ferromagnetic sense portion 212 comprises a plurality of ferromagnetic sub-layers 214 comprising a ferromagnetic NiFe alloy and a plurality of dilution sub-layers 215 comprising Ta. Here, the first ferromagnetic sense portion 211 can have a thickness of about 2.4 nm, the ferromagnetic sub-layers 214 can have a thickness of about 1.2 nm and the dilution sub-layers 215 can have a thickness of about 0.1 nm.

[0046] According to an embodiment, a method for manufacturing the magnetoresistive sensor element 2 comprises, for a plurality of temperatures and concentrations of the dilution element, performing the steps of:

[0047] measuring an electrical conductance G of the magnetoresistive sensor element 2 as a function of the external magnetic field H.sub.ext for a plurality of temperatures T, such as to obtain a plurality of measured conductance curves for each temperature T;

[0048] calculating a value of TMR of the magnetoresistive sensor element 2 from the plurality of measured conductance curves, such as to determine the temperature dependence of TMR; and

[0049] measuring the magnetization of the sense layer 21, such as to determine the temperature dependence of the magnetic susceptibility χ of the sense layer 21.

[0050] The electrical conductance G of the magnetoresistive sensor element 2 can be measured by passing a read current 31 (see FIG. 1) through the magnetoresistive sensor element 2. Examples of measured conductance curve for three different temperatures T are shown if FIGS. 6a-6c.

[0051] From the preformed steps, the method further comprises a step of determining the proportion of the diluting element for which the temperature dependence of TMR substantially compensates the temperature dependence of the magnetic susceptibility χ.

[0052] To compensate the TCS, the change of magnetization Ms(T) with temperature T should follow:

Ms(T)=ATMR(T)/(2+TMR(T))  (3)

where A is a constant and TMR(T) if the temperature dependence of TMR.

[0053] A magnetoresistive sensor for sensing a 1D external magnetic field H.sub.ext can comprises a plurality of the magnetoresistive sensor element 2. In one embodiment illustrated in FIG. 12, the magnetoresistive sensor 20 is arranged in a Wheatstone full-bridge configuration. This can improve sensor thermal stability and linearity. Inside such a full bridge, the diagonal elements 2 have the same response curve (FIG. 4a, for example) while in each half-bridge there is one sensing element with response curve 4a and the other with the reversed response curve (FIG. 4b). This is achieved by having opposite directions of the reference layer 230 for the two sensing elements 2 having response curve like the one of FIG. 4a and FIG. 4b, respectively.

[0054] In the case where the magnetoresistive sensor 20 is biased with 1 V (V.sub.in=1V), the temperature dependence of TMR can be calculated from the electrical output V.sub.out of the magnetoresistive sensor 20 at the the saturation magnetization FB.sub.out in the hysteresis curve (see for example FIG. 3), by using:

TMR=2FB.sub.out/(FB.sub.out−2), where FB.sub.out=max(FB.sub.out)−min(FB.sub.out)  (4),

where max(FB.sub.out) and min(FB.sub.out) is the saturation magnetization in the hysteresis curve.

[0055] The change of magnetization Ms(T) with temperature T can be measured independently on the sense layer 21 using a magnetometer.

REFERENCE NUMBERS AND SYMBOLS

[0056] 2 magnetoresistive sensor element [0057] 20 magnetoresistive sensor [0058] 21 sense layer [0059] 210 sense magnetization [0060] 211 first ferromagnetic sense portion [0061] 212 second ferromagnetic sense portion [0062] 213 core [0063] 214 ferromagnetic sub-layer [0064] 215 dilution sub-layer [0065] 22 tunnel barrier layer [0066] 23 reference layer [0067] 230 reference magnetization [0068] 231 first ferromagnetic layer [0069] 232 second ferromagnetic layer [0070] 233 antiparallel coupling layer [0071] 234 first reference magnetization [0072] 235 second reference magnetization [0073] 24 antiferromagnetic layer [0074] 31 read current [0075] 60 external magnetic field [0076] G electrical conductance [0077] H.sub.ext external magnetic field [0078] H.sub.expl expulsion field [0079] H.sub.nucl nucleation field [0080] Ms saturation magnetization [0081] S sensitivity [0082] T temperature [0083] Tc Curie temperature [0084] TCS temperature coefficient of sensitivity [0085] TMR tunnel magnetoresistance [0086] T.sub.WR working temperature range [0087] χ magnetic susceptibility