Field

[0001] The present invention concerns a magnetoresistive element having a magnetostriction that is adjustable and a magnetic device comprising the magnetoresistive element. Description of related art

[0002] Magnetic tunnel junctions are used in a wide variety of applications namely including MRAM, HDD read head and magnetic sensors.

[0003] Magnetic tunnel junctions are conventionally made of an insulating barrier, or tunnel barrier, comprising MgO or AIO. The tunnel barrier is sandwiched between two ferromagnetic layers, such as a reference or storage layer and a sense layer. The ferromagnetic layers are usually made of a Fe based alloy, such as CoFe or CoFeB.

[0004] One of the ferromagnetic layers, usually a reference layer or a storage layer, can be pinned by an antiferromagnetic layer by magnetic exchange bias coupling. The antiferromagnetic layer can comprise a Co or Fe based alloy, such as CoFe.

[0005] The reference or storage layer and the sense layer can comprise a SAF structure. Such SAF structure comprises two ferromagnetic layers that are sandwiching a non-magnetic layer, for example a Ru layer. The nonmagnetic layer magnetically couples the two magnetic layers due to the RKKY coupling. The two ferromagnetic layers usually comprise an Fe based alloy, such as CoFe or CoFeB.

[0006] The ferromagnetic layers used for the reference or storage layer and sense layer, or for the SAF structures have typically a positive magnetostriction constant that is above 10 ppm. Such positive

magnetostriction constant can be problematic since different metal levels or oxide/nitride layers constitutive of the layers can induce mechanical stress on the magnetic tunnel junction. Due to the magnetostriction effect, such stress change the magnetic properties of the magnetic layers.

[0007] The change in the properties are detrimental for the functioning of the device using such magnetic tunnel junctions. For example, in the case of a MRAM device, it can results in high error rate when the bits are written. In the case of sensor devices, the mechanical stress can induce a decrease of sensitivity.

[0008] Moreover, since the mechanical stresses are usually not well controlled, it results in a wide dispersion of properties among devices on a wafer, or among different wafers, resulting in a poor yield.

[0009] On the other hand, magnetic layers with negative or low magnetostriction cannot be used directly because they do not provide good electrical or magnetic properties for the magnetic tunnel junction (low TMR, low RKKY coupling, low exchange bias).

[0010] US20081 13220 discloses methods and apparatus for magnetic tunnel junctions (MTJs) employing synthetic antiferromagnet (SAF) free layers. The MTJ comprises a pinned ferromagnetic (FM) layer , the SAF and a tunneling barrier therebetween. The SAF has a first higher spin

polarization FM layer proximate the tunneling barrier and a second FM layer desirably separated from the first FM layer by a coupling layer with magnetostriction adapted to compensate the magnetostriction of the first FM layer. Such compensation reduces the net magnetostriction of the SAF to near zero even with high spin polarization proximate the tunneling barrier . Higher magnetoresistance ratios (MRs) are obtained without adverse affect on other MTJ properties; NiFe combinations are desirable for the first and second free FM layers, with more Fe in the first free layer and less Fe in the second free layer. CoFeB and NiFeCo are also useful in the free layers. Summary

[0011] The present disclosure concerns a magnetoresistive element comprising: a storage layer having a first storage magnetostriction; a sense layer having a first sense magnetostriction; a barrier layer between and in contact with the storage and sense layer; wherein the magnetoresistive element further comprises a compensating ferromagnetic layer having a second magnetostriction different from the first storage magnetostriction and/or sense magnetostriction, and adapted to compensate the first storage magnetostriction and/or the first sense magnetostriction so that a net magnetostriction of the storage layer and/or sense layer is adjustable between -10 ppm et +10 ppm or more negative than -10 ppm by adjusting a thickness of the compensating ferromagnetic layer.

[0012] The present disclosure further concerns a magnetic device comprising the magnetoresistive element. The magnetic device can comprise a MRAM based device, a sensor device, a HDD read head device or any other magnetic device suing the magnetoresistive element.

[0013] An advantage of the magnetoresistive element disclosed herein is that the net magnetostriction is between -10 ppm et +10 ppm, the magnetic properties of the magnetoresistive element do not depend on the stress experienced on the of the magnetoresistive element and/or on a device comprising the magnetoresistive element. The magnetoresistive element and a device comprising the magnetoresistive element has improved magnetic properties and lower properties dispersions.

[0014] The adjusting the thickness of the compensating ferromagnetic layer can be done such that the net magnetostriction is negative (more negative than -10 ppm). This results in an stress-induced magnetic anisotropy on at least one of the sense layer and the storage layer such as to provide a stress-induced magnetic anisotropy.

Brief Description of the Drawings [0015] The disclosure 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:

Fig. 1 represents a magnetoresistive element, according to an embodiment;

Fig. 2 illustrates a the magnetoresistive element, according to another embodiment;

Fig. 3 shows the magnetoresistive element comprising a sense layer having a SAF structure, according to an embodiment;

Fig. 4 shows the magnetoresistive element of Fig. 3, according to another embodiment;

Fig. 5 represents the magnetoresistive element according to another embodiment wherein the storage layer comprises a SAF structure, according to an embodiment;

Fig. 6 illustrates a magnetoresistive element wherein both the storage layer and the sense layer comprise a SAF structure; and

Fig. 7 represents the evolution of a net magnetostriction of a portion of the magnetoresistive element.

Detailed Description of possible embodiments [0016] Fig. 1 represents a magnetoresistive element 1 according to an embodiment. The magnetoresistive element comprises a storage, or reference, layer 21 having a first storage magnetostriction RI, a sense layer 23 having a first sense magnetostriction s-, and a a barrier layer 22 between and in contact with the storage and sense layer 21 , 23. [0017] Each of the storage layer 21 and the sense layer 23 includes, or is formed of, a magnetic material and, in particular, a magnetic material of the ferromagnetic type. A ferromagnetic material can be characterized by a substantially planar magnetization with a particular coercivity, which is indicative of a magnitude of a magnetic field to reverse the magnetization after it is driven to saturation in one direction. In general, storage layer 21 and the sense layer 23 can include the same ferromagnetic material or different ferromagnetic materials. Suitable ferromagnetic materials include transition metals, rare earth elements, and their alloys, either with or without main group elements. For example, suitable ferromagnetic materials include iron (" Fe "), cobalt ("Co"), nickel (" Ni "), and their alloys, such as permalloy (or Ni80Fe20); alloys based on Ni, Fe, and boron (" B "); Co90Fe10; and alloys based on Co, Fe, and B. In some instances, alloys based on Ni and Fe (and optionally B) can have a smaller coercivity than alloys based on Co and Fe (and optionally B). [0018] Preferably, the storage layer 21 and/or the sense layer 23 comprises a Fe based alloy providing a TMR ratio greater than 20%.

[0019] The storage layer 21 can include a hard ferromagnetic material, namely one having a relatively high coercivity, such as greater than about 50 Oe. The sense layer 23 can include a soft ferromagnetic material, namely one having a relatively low coercivity, such as no greater than about 50 Oe. In such manner, a magnetization of the sense layer 23 can be readily varied under low-intensity magnetic fields during read operations, while a magnetization of the storage, or reference, layer 21 remains stable.

[0020] A thickness of each of the storage layer 21 and the sense layer 23 can be in the nanometer ("nm ") range, such as from about 0.5 nm to about 10 nm. A thickness of each of the storage layer 21 and the sense layer 23 is preferably from about 0.5 nm to about 5 nm and more and preferably between 1 nm and 2.5nm.

[0021] The barrier layer 22 includes, or is formed of, an insulating material. Suitable insulating materials include oxides, such as aluminum oxide (e.g., Al ₂ 0 ₃ ) and magnesium oxide (e.g., MgO). A thickness of the barrier layer 22 can be in the nm range, such as from about 0.5 nm to about 10 nm.

[0022] The magnetoresistive element 1 further comprises a

compensating ferromagnetic layer 25 included between an electrode 28 and the sense layer 23, such that the sense layer 23 is between the compensating ferromagnetic layer 25 and the barrier layer 22 with which it is in contact. The compensating ferromagnetic layer 25 has a second magnetostriction λ _2ι different from the first sense magnetostriction - The compensating ferromagnetic layer 25 is adapted to compensate the first sense magnetostriction -

[0023] The compensating ferromagnetic layer 25 can comprise a Ni or Co alloy containing less than 25%wt of Ta, Ti, Hf, Cr, Sc, Cu, Pt, Pd, Ag, Mo, Zr, W, Al, Si, Mg or any combinations of these elements. The compensating ferromagnetic layer 25 can also comprise pure Ni or pure Co. Here, pure Ni and pure Co can mean at least 99.9%wt Ni and at least 99.9% Co, respectively. The compensating ferromagnetic layer 25 has a thickness being typically between 0.5 nm and 10 nm.

[0024] The net magnetostriction _net of the sense layer 23 can be adjusted between -10 ppm et +10 ppm or to a more negative

magnetostriction than -10 ppm by adjusting a thickness of the

compensating ferromagnetic layer 25.

[0025] Fig. 2 illustrates a variant of the magnetoresistive element 1 where the compensating ferromagnetic layer 25 is included on the other side of the barrier layer 22 such that the storage layer 21 is between the compensating ferromagnetic layer 25 and the barrier layer 22 with which it is in contact. The second magnetostriction ₂ of the compensating ferromagnetic layer 25 differs from the first storage magnetostriction RI and the compensating ferromagnetic layer 25 is adapted to compensate the first storage magnetostriction RI .

[0026] In fact, the net magnetostriction _net of the storage and/or sense layer 21 , 23 can be adjusted between -10 ppm et +10 ppm or to a more negative magnetostriction than -10 ppm by adjusting the thickness of the compensating ferromagnetic layer 25 and/or the thickness of the storage and/or sense layer 21 , 23. [0027] Preferably, the second magnetostriction λ2 of the compensating ferromagnetic layer 25 is negative and the first storage and sense

magnetostriction XM , S - is positive.

[0028] Adjusting the thickness of the compensating ferromagnetic layer 25 such that the net magnetostriction _net is negative (more negative than - 10 ppm) results in providing an stress-induced magnetic anisotropy on at least one of the sense layer 23 and the storage layer 21 and provide a stress-induced magnetic anisotropy, as described in yet unpublished

European patent application number EP20150290013 by the present applicant.

[0029] In order to allow a structural transition between the layers having a negative magnetostriction (the compensating ferromagnetic layer 25) and the layers having a positive magnetostriction (the storage and/or sense layer 21 , 23), a transition layer 26 can be included between the layers having a negative magnetostriction and the layers having a positive magnetostriction. The transition layer 26 can comprise Ti, Hf, Ta, Nb, Cr or any combinations of these elements. The transition layer 26 should be thin enough so that a magnetic coupling still exists between the layers having a negative magnetostriction and the layers having a positive

magnetostriction. Preferably, the transition layer 26 has a thickness comprised between 0.1 nm and 1 nm.

[0030] In the embodiment of Fig. 1 , a transition layer 26 is included between compensating ferromagnetic layer 25 and the sense layer 23. In the embodiment of Fig. 2, the magnetoresistive element 1 comprises a transition layer 26 included between compensating ferromagnetic layer 25 and the storage layer 21 .

[0031 ] As illustrated in Fig. 2, the magnetoresistive element 1 can comprise a storage antiferromagnetic layer 24 magnetically exchange coupling the storage (or reference) layer 21 such as to pin the

magnetization of the storage layer 21 . In an embodiment, a ferromagnetic coupling layer 27 is included between the antiferromagnetic layer 24 and the compensating ferromagnetic layer 25. The ferromagnetic coupling layer 27 can comprise a Fe or Co based alloy and have a thickness between 0.2 nm and 5 nm and preferably between 0.5 nm and 1.5nm.

[0032] The ferromagnetic coupling layer 27 is adapted for providing a magnetic exchange coupling greater than 0.05 erg/cm ² . For example, the ferromagnetic coupling layer 27 can provide an exchange coupling that is greater than 0.05 erg/cm ² between the storage antiferromagnetic layer 24 and the storage layer 21.

[0033] A transition layer 26 can further be included between the compensating ferromagnetic layer 25 and the ferromagnetic coupling layer 27.

[0034] Fig. 3 shows the magnetoresistive element according to another embodiment wherein the sense layer comprises a SAF structure including a first ferromagnetic sense layer 231 in contact with the barrier layer 22, a second ferromagnetic sense layer 232, and a SAF sense coupling layer 233 between the first and second ferromagnetic sense layers 231 , 232. A compensating ferromagnetic layer 25 is included between the SAF sense coupling layer 233 and the first ferromagnetic sense layer 231. Another compensating ferromagnetic layer 25 is included between the SAF sense coupling layer 233 and the second ferromagnetic sense layer 232.

[0035] A transition layer 26 can be inserted between the compensating ferromagnetic layer 25 and the first ferromagnetic sense layer 231 and/or between the compensating ferromagnetic layer 25 and the second ferromagnetic sense layer 232. [0036] Fig. 4 shows the magnetoresistive element of Fig. 3, according to another embodiment. In the configuration of Fig. 4, the magnetoresistive element 1 comprises a ferromagnetic coupling layer 27 between the SAF sense coupling layer 233 and each of the compensating ferromagnetic layers 25. The ferromagnetic coupling layer 27 enhances the RKKY-type magnetic exchange coupling between the first ferromagnetic sense layer 231 and of the second ferromagnetic sense layer 232 to a value above 0.05 erg/cm ³ .

[0037] A transition layer 26 can be included between the ferromagnetic coupling layer 27 and each of the compensating ferromagnetic layers 25. [0038] Fig. 5 shows the magnetoresistive element according to another embodiment wherein the storage layer comprises a SAF structure including a first ferromagnetic storage layer 21 1 in contact with the barrier layer 22, a second ferromagnetic storage layer 212, and a SAF storage coupling layer 213 between the first and second ferromagnetic storage layers 21 1 , 212. A compensating ferromagnetic layer 25 is included between the SAF storage coupling layer 213 and the first ferromagnetic storage layer 21 1. Another compensating ferromagnetic layer 25 is included between the SAF storage coupling layer 213 and the second ferromagnetic storage layer 212.

[0039] A transition layer 26 can be inserted between the compensating ferromagnetic layer 25 and the first ferromagnetic storage layer 21 1 and/or between the compensating ferromagnetic layer 25 and the second ferromagnetic storage layer 212.

[0040] A ferromagnetic coupling layer 27 can be included between the SAF storage coupling layer 213 and each of the compensating

ferromagnetic layers 25. The ferromagnetic coupling layer 27 enhances the RKKY-type magnetic exchange coupling between the first ferromagnetic storage layer 21 1 and of the second ferromagnetic storage layer 212 to a value above 0.05 erg/cm ³ .

[0041] A transition layer 26 can be included between the ferromagnetic coupling layer 27 and each of the compensating ferromagnetic layers 25.

[0042] Fig. 6 illustrates a magnetoresistive element 1 wherein both the storage layer 21 and the sense layer 23 comprise a SAF structure, such that the magnetoresistive element 1 corresponds to a combination of the SAF sense structure of Fig. 4 and of the SAF storage structure of Fig. 5. [0043] Fig. 7 represents the evolution of the net magnetostriction _net of a portion of the magnetoresistive element comprising the sequence of layers Ta/Ni/Ta0.3/CoFeB 1 .5/MgO (corresponding to the sequence: electrode 28, compensating ferromagnetic layers 25, transition layer 26, sense layer 23 and barrier layer 22. The net magnetostriction _net continuously decreases as a function of the thickness of the compensating ferromagnetic layers 25 comprising Ni. The net magnetostriction _net can be cancelled when the thickness of the compensating ferromagnetic layers 25 is about 2.3 nm.

Reference Numbers and Symbols

1 magnetoresitive element

21 storage layer

21 1 first ferromagnetic storage layer

212 second ferromagnetic storage layer

213 SAF storage coupling layer

22 barrier layer

23 sense layer

231 first ferromagnetic sense layer

232 second ferromagnetic sense layer

233 SAF sense coupling layer

24 storage antiferromagnetic layer

25 compensating ferromagnetic layer

26 transition layer

27 ferromagnetic coupling layer

28 electrode RI first storage magnetostriction

first sense magnetostriction

2 second magnetostriction

net net magnetostriction