WO2021066743A1

WO2021066743A1 - Spin-orbit torque device and material for a spin-orbit torque device

Info

Publication number: WO2021066743A1
Application number: PCT/SG2020/050534
Authority: WO
Inventors: Jingsheng Chen; Liang Liu
Original assignee: National University Of Singapore
Priority date: 2019-10-02
Filing date: 2020-09-18
Publication date: 2021-04-08

Abstract

A spin-orbit torque device 100 is described. In an embodiment, the spin-orbit torque device 100 comprises: a functionally graded magnetic layer 104, the functionally graded magnetic layer 104 having a switchable magnetisation direction and a composition gradient adapted to create a broken inversion symmetry to interact with an electric current 114 to generate a spin torque for switching the switchable magnetisation direction of the functionally graded magnetic layer 104. In a particular embodiment, the functionally graded magnetic layer comprises a binary ferromagnetic alloy L1₀ FePt which has a perpendicular magnetic anisotropy.

Description

Spin-orbit torque device and material for a spin-orbit torque device

Technical Field

The present disclosure relates to a spin-orbit torque device and also a material for a spin- orbit torque device.

Background

Electrical manipulation of magnetisation using spin-orbit torque (SOT) has generated massive interest due to its potential applications in spintronics devices such as magnetic random access memory (MRAM). Generation of SOTs is believed to require either a magnetic layer having a broken inversion symmetry which leads to the inverse spin galvanic effect (iSGE), or a heterostructure comprising a layer having a strong spin-orbit coupling, such as a heavy metal layer, which leads to the spin Hall effect. Based on these, two types of electrically controlled SOT devices can be envisaged. A first type of SOT device comprising a multilayer heterostructure, and a second type of SOT device comprising a bulk non-centrosymmetric conductor or semiconductor.

The first type of SOT device, which includes a bilayer, a trilayer, or a periodic multilayer heterostructure, possesses broken structural inversion symmetry at an interface within the heterostructure. This broken inversion symmetry at the interface gives rise to an interface-like SOT when an electric current is applied. Examples of a magnetic material used for the first type of SOT device includes ferromagnetic metals, such as Co, Ni, Py, CoPt, and FePt, which have a centrosymmetric crystal structure in its singular phase. On the other hand, the second type of SOT device includes a non-centrosymmetric conductor/semiconductor which possesses a bulk (either global or local) broken inversion symmetry. This bulk broken inversion symmetry produces a bulk-like SOT when an electric current is applied. Examples of non-centrosymmetric conductor/semiconductor includes ferromagnetic semiconductors, such as (Ga,Mn)As, which have bulk inversion asymmetry, or antiferromagnetic metals, such as CuMnAs and Mn2Au, which have locally broken inversion symmetry.

In principle, the efficiency of the interface-like SOT decreases with increasing thickness of the ferromagnetic layer while that for the bulk-like SOT is independent of the thickness of the ferromagnetic layer. Therefore, for a multilayer heterostructure SOT device, a thin ferromagnetic layer is typically used. On the other hand, a bulk non-centrosymmetric conductor/semiconductor SOT device can include a much thicker magnetic layer. This is because current-induced spin polarization is uniformly generated in the bulk of the non-centrosymmetric conductor/semiconductor magnetic layer so that a spin torque is directly exerted on the local magnetic moment. Switching of the magnetisation in such a non-centrosymmetric conductor/semiconductor is therefore independent of its thickness.

However, there are issues in relation to each of these types of SOT devices. For the first type of SOT device which requires a thin ferromagnetic layer due to the broken inversion symmetry at the interface, it suffers from thermal instability as a result of the thin ferromagnetic layer because the thermal stability is proportional to the FM thickness. For the second type of SOT device, the SOT switching in non-centrosymmetric conductors/semiconductors such as the aforementioned GaMnAs, CuMnAs, and Mn₂Au induces a magnetic domain rotation in a surface plane of the magnetic layer. A readout process of such magnetic domain rotation utilizes the anisotropic magnetoresistance (AMR) effect, which is not conducive for ultra-high density storage.

It is therefore desirable to provide a spin-orbit torque device and a material for the spin- orbit torque device which address the aforementioned problems and/or provides a useful alternative. Further, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.

Summary

Aspects of the present application relate to a spin-orbit torque device and also a material for a spin-orbit torque device.

In accordance with a first aspect, there is provided a spin-orbit torque device comprising: a functionally graded magnetic layer, the functionally graded magnetic layer having a switchable magnetisation direction and a composition gradient adapted to create a broken inversion symmetry to interact with an electric current to generate a spin torque for switching the switchable magnetisation direction of the functionally graded magnetic layer.

By incorporating the functionally graded magnetic layer having a composition gradient adapted to create a broken inversion symmetry to generate a spin torque, the spin torque generated in the functionally graded magnetic layer possesses a bulk nature which is independent of a thickness of the functionally graded magnetic layer. This allows a thick magnetic layer to be used unlike in a conventional heavy metal/ferromagnet (HM/FM) bilayer heterostructure SOT device, thereby providing thermal stability for the spin-orbit torque (SOT) device without compromising on its efficiency. Moreover, an efficiency of the spin torque generated in this case can be tuned by engineering the composition gradient of the functionally graded magnetic layer. This provides an accessible avenue to customise the SOT device. Further, since the use of the composition gradient in the functionally graded magnetic layer creates broken inversion symmetry without requiring an interface, a single layer structure (i.e. using a single magnetic layer, and not a heterostructure with more than one material layer) can be used to form the SOT device. This advantageously simplifies a fabrication process of the SOT device.

The spin-orbit torque device may comprise: a tunnelling barrier layer formed on the functionally graded magnetic layer; and a pinned magnetic layer formed on the tunnelling barrier layer, the pinned magnetic layer having a fixed magnetisation direction, wherein the switchable magnetisation direction of the functionally graded magnetic layer is switched by the spin torque to provide a low resistance state and a high resistance state, the low resistance state being a state in which the switchable magnetisation direction is in the same direction as the fixed magnetisation direction and the high resistance state being a state in which the switchable magnetisation direction is in an opposite direction as the fixed magnetisation direction.

The spin-orbit torque device may comprise a write electrode arranged to inject a spin polarised current to create a skyrmion in the functionally graded magnetic layer.

The spin-orbit torque device may comprise: a tunnelling barrier layer formed on the functionally graded magnetic layer; and a pinned magnetic layer formed on the tunnelling barrier layer, the pinned magnetic layer having a fixed magnetisation direction, wherein at least a portion of the functionally graded magnetic layer, the tunnelling barrier layer and the pinned magnetic layer form a magnetic tunnel junction for detecting a first resistance state and a second resistance state of the spin-orbit torque device, the first resistance state being a state in which the skyrmion is present in the magnetic tunnel junction and the second resistance state being a state in which the skyrmion is absent in the magnetic tunnel junction, and wherein the presence and the absence of the skyrmion in the magnetic tunnel junction is controllable by a motion of the skyrmion in the functionally graded magnetic layer using the electric current.

In accordance with a second aspect, there is provided a method for fabricating a spin- orbit torque device comprising: forming a functionally graded magnetic layer, the functionally graded magnetic layer having a switchable magnetisation direction and a composition gradient adapted to create a broken inversion symmetry to interact with an electric current to generate a spin torque for manipulating the switchable magnetisation direction of the functionally graded magnetic layer.

The method may comprise: forming a tunnelling barrier layer on the functionally graded magnetic layer; and forming a pinned magnetic layer on the tunnelling barrier layer, the pinned magnetic layer having a fixed magnetisation direction, wherein the switchable magnetisation direction of the functionally graded magnetic layer is switched by the spin torque to provide a low resistance state and a high resistance state, the low resistance state being a state in which the switchable magnetisation direction is in the same direction as the fixed magnetisation direction and the high resistance state being a state in which the switchable magnetisation direction is in an opposite direction as the fixed magnetisation direction.

The method may comprise forming a write electrode on the functionally graded magnetic layer, the write electrode being arranged to inject a spin polarised current to create a skyrmion in the functionally graded magnetic layer.

The method may comprise: forming a tunnelling barrier layer on the functionally graded magnetic layer; and forming a pinned magnetic layer on the tunnelling barrier layer, the pinned magnetic layer having a fixed magnetisation direction, wherein at least a portion of the functionally graded magnetic layer, the tunnelling barrier layer and the pinned magnetic layer form a magnetic tunnel junction for detecting a first resistance state and a second resistance state of the spin-orbit torque device, the first resistance state being a state in which the skyrmion is present in the magnetic tunnel junction and the second resistance state being a state in which the skyrmion is absent in the magnetic tunnel junction, and wherein the presence and the absence of the skyrmion in the magnetic tunnel junction is controllable by a motion of the skyrmion in the functionally graded magnetic layer using the electric current. In accordance with a third aspect, there is provided a functionally graded magnetic material for a spin-orbit torque device, the functionally graded magnetic material having a switchable magnetisation direction and a composition gradient adapted to create a broken inversion symmetry to interact with an electric current to generate a spin torque for switching the switchable magnetisation direction of the functionally graded magnetic material.

The functionally graded magnetic material may comprise a binary alloy consisting of elements A and B in the form of Ai-_XB_X, where x is a percentage concentration of element B in the binary alloy Ai-*B_X and is a number between 0 and 1 .

An elemental ratio A/B defined as (1-x)/x may vary across a thickness profile of the functionally graded magnetic material to create the composition gradient in the functionally graded magnetic material.

The elemental ratio A/B may increase across the thickness profile of the functionally graded magnetic material.

The elemental ratio A/B may vary linearly across the thickness profile of the functionally graded magnetic material.

A thickness of the functionally graded magnetic material may be is equal to or more than 2 nm.

In accordance with a fourth aspect, there is provided a method for generating a spin torque comprising: using a composition gradient in a functionally graded magnetic material to create a broken inversion symmetry to interact with an electric current to generate the spin torque.

The functionally graded magnetic material may comprise a binary alloy consisting of elements A and B in the form of Ai-_XB_X, where x is a percentage concentration of element B in the binary alloy Ai-*B_X and is a number between 0 and 1 , and the method may comprise: creating the composition gradient by varying an elemental ratio A/B across a thickness profile of the functionally graded magnetic material, the elemental ratio A/B being defined as (1-x)/x.

The method may comprise: increasing the elemental ratio A/B across the thickness profile of the functionally graded magnetic material. The method may comprise: varying the elemental ratio A/B varies linearly across the thickness profile of the functionally graded magnetic material.

The functionally graded magnetic material may comprise L1₀ FePt.

It should be appreciated that features relating to one aspect may be applicable to the other aspects. Embodiments therefore provide a spin-orbit torque device comprising a functionally graded magnetic layer having a switchable magnetisation direction and a composition gradient adapted to create a broken inversion symmetry to interact with an electric current to generate a spin torque for switching the switchable magnetisation direction. The spin torque generated using the composition gradient of the functionally graded magnetic layer possesses a bulk nature which is independent of a thickness of the functionally graded magnetic layer. A relatively thick (e.g. equal to or more than 2 nm thick) functionally graded magnetic layer can therefore be used to provide thermal stability for the SOT device without compromising on its efficiency. Moreover, an efficiency of the spin torque for this SOT device can be easily tuned by engineering the composition gradient of the functionally graded magnetic layer. Further, given that the spin torque is generated using the composition gradient of the functionally graded magnetic layer, an interface is no longer required for generating the spin torque. This allows single layer structure (i.e. using a single magnetic layer, and not a heterostructure with more than one material layer) to be used for the SOT device which advantageously simplifies a fabrication process of the SOT device.

Brief description of the drawings

Embodiments will now be described, by way of example only, with reference to the following drawings, in which:

Figure 1 shows a schematic structure of a spin-orbit torque (SOT) device which uses a functionally graded magnetic layer having a composition gradient in accordance with an embodiment;

Figure 2 shows a plot illustrating two types of composition gradients which can exist in the functionally graded magnetic layer of the SOT device of Figure 1 ; Figure 3 shows a schematic structure of a SOT device which uses skyrmions as storage bits for memory applications in accordance with an embodiment, where the skyrmions are generated using the functionally graded magnetic layer of Figure 1 ;

Figure 4 shows a flowchart showing steps of a method for fabricating the SOT device of Figure 1 in accordance with an embodiment;

Figures 5A, 5B and 5C show schematic diagrams illustrating three different types of inversion asymmetries, where Figure 5A illustrates structure inversion asymmetry for a bilayer heterostructure, Figure 5B illustrates bulk inversion asymmetry for a non- centrosymmetric crystal structure, and Figure 5C illustrates inversion asymmetry for a binary alloy having a composition gradient;

Figures 6A, 6B, 6C and 6D show diagrams illustrating structural and magnetic properties of L1₀ FePt for use as the binary alloy of Figure 5C in accordance with an embodiment, where Figure 6A shows a schematic of a crystal structure of L1₀ FePt, Figure 6B shows a plot of high-resolution X-ray diffraction spectrum of a 20 nm thick L1o FePt film on TiN/MgO, Figure 6C shows a scanning transmission electron microscopy (STEM) image of the L1₀ FePt film, and Figure 6D shows plots of out-of-plane (OP) and in-plane (IP) magnetic hysteresis loops of the L1₀ FePt film ;

Figures 7A, 7B and 7C show cross-section STEM images illustrating a composition gradient in a 60 nm-thick L1 o FePt film in accordance with an embodiment, where Figure 7A shows a high-angle annular dark-field (HAADF) STEM image of the L1o FePt film, Figure 7B shows a zoom-in STEM image at a bottom of the L1o FePt film as indicated in Figure 7A, and Figure 7C shows a zoom-in STEM image at a top of the L1₀ FePt film as indicated in Figure 7A;

Figures 8A and 8B show overall Energy Dispersive X-ray (EDX) intensities across a thickness profile of the L1o FePt films in relation to Figures 6B-6D and 7A-7C, where Figure 8A shows the overall EDX intensity for the 20 nm-thick L1o FePt film of Figures 6B-6D and Figure 8B shows the overall EDX intensity for the 60 nm-thick L1o FePt film of Figures 7A-7C;

Figure 9 shows a plot of composition gradient of L1₀ FePt films as a function of a thickness of the L1 o FePt films for three different thicknesses, namely 20 nm, 30 nm and 60 nm, in accordance with an embodiment; Figure 10 shows a plot of Hall resistance as a function of current density to illustrate current-induced magnetisation switching in the 20 nm-thick L1o FePt film of Figures 6B- 6D;

Figure 11 shows plots of current-induced longitudinal and transverse spin-orbit torque efficiencies as a function of a thickness of L1o FePt films in accordance with an embodiment; and

Figures 12A and 12B show plots of current-induced spin-orbit torque efficiencies as a function of a composition gradient of L1₀ FePt films in accordance with an embodiment, where Figure 12A shows a plot of longitudinal spin-orbit efficiency as a function of the composition gradient of the L1₀ FePt films, and Figure 12B shows a plot of transverse spin-orbit efficiency as a function of the composition gradient of the L1₀ FePt films.

Detailed description

Exemplary embodiments relate to a spin-orbit torque (SOT) device and a material for the SOT device.

In the present embodiments, by using a composition gradient in a functionally graded magnetic material, the composition gradient creates a broken inversion symmetry in the functionally graded magnetic material to interact with an electric current to generate a spin torque. The spin torque generated in this way possesses a bulk nature which is independent of a thickness of the functionally graded magnetic layer. This allows a thick magnetic layer to be used which provides better thermal stability for a SOT device, as compared to using a conventional heavy metal/ferromagnet (HM/FM) bilayer heterostructure where the ferromagnetic layer is required to be relatively thin (< 2 nm) in order to maintain a reasonable SOT efficiency. Further, since the use of the composition gradient in the functionally graded magnetic layer creates broken inversion symmetry without requiring an interface, a single layer structure (i.e. using a single magnetic layer, and not a heterostructure with more than one material layer) can be used to form the SOT device. This advantageously simplifies a fabrication process of the SOT device.

In the present disclosure, two embodiments of the SOT device are first described in relation to Figures 1 to 3. Figure 4 describes a method for fabricating the SOT devices of Figures 1 and 3, and Figures 5A to 5C illustrate broken inversion symmetries (or inversion asymmetries) formed by various ways. Figures 6A to 12B are related to experimental results for demonstrating composition gradient created inversion asymmetry in a binary alloy L1₀ FePt, in accordance with an exemplary embodiment.

Embodiments of SOT devices

The present disclosure describes a spin-orbit torque device comprising a functionally graded magnetic layer having a switchable magnetisation direction and a composition gradient. The composition gradient of the functionally graded magnetic layer is adapted to create a broken inversion symmetry to interact with an electric current to generate a spin torque for switching the switchable magnetisation direction of the functionally graded magnetic layer. In the embodiments described below, the functionally graded magnetic layer comprises a binary ferromagnetic alloy L1₀ FePt which has a perpendicular magnetic anisotropy. By having a perpendicular magnetic anisotropy, it is understood that the switchable magnetisation direction of the functionally graded magnetic layer is perpendicular to its planar surface (i.e. out-of-plane to the functionally graded magnetic layer). In other words, the switchable magnetisation direction is in a direction perpendicular to a longitudinal axis of the functionally graded magnetic layer, where the longitudinal axis is parallel to the planar surface of the functionally graded magnetic layer. The planar surface is parallel to, for example, a planar interface formed between the functionally graded magnetic layer and a substrate on which the functionally graded magnetic layer is formed.

Different embodiments for the SOT device comprising a functionally graded magnetic layer having a composition gradient are described below.

Example 1. A SOT magnetic memory device including a magnetic tunnel junction element

Figure 1 shows a schematic diagram of a SOT device 100 which comprises a magnetic tunnel junction (MTJ) 102 for the application as a spin-orbit torque magnetic random access memory (SOT-MRAM). As shown in Figure 1 , the MTJ element 102 includes a functionally graded magnetic layer 104, a tunnelling barrier layer 106, a pinned magnetic layer 108, and an antiferromagnetic (AFM) layer 110. The functionally graded magnetic layer 104 has a composition gradient across a thickness profile of the magnetic layer 104. In other words, a composition of the functionally graded magnetic layer 104 varies across its thickness profile. Examples of composition gradients of the functionally graded magnetic layer 104 are discussed below in relation to Figure 2. The functionally graded magnetic layer 104 also has a switchable magnetisation direction. In the present embodiment, the functionally graded magnetic layer 104 comprises a binary ferromagnetic alloy L1₀ FePt which has a perpendicular magnetic anisotropy. In this case, the magnetisation direction of the functionally graded magnetic layer 104 is free to switch between the “up” and “down” magnetisation directions. The tunnelling barrier layer 106 comprises an insulating oxide which is not electrically conductive. In the present embodiment, the tunnelling barrier layer 106 comprises MgO. The pinned magnetic layer 108 comprises a magnetic layer which has a fixed magnetisation direction. In the present embodiment, the pinned magnetic layer 108 comprises CoFe. The fixed magentisation direction of the pinned magnetic layer 108 is strengthened through the exchange bias which occurs between the pinned magnetic layer 108 and the antiferromagnetic layer 110. In the present embodiment, the antiferromagnetic layer 110 comprises an antiferromagnet lrMn3. As shown in Figure 1 , the MTJ element 102 is formed on a substrate 112. The substrate 112 serves to support the MTJ element 102. The substrate 112 of the present embodiment includes a single crystalline MgO (001 ) substrate with a 5 nm TiN buffer layer. Further, Figure 1 shows that there are two interfaces associated with the functionally graded magnetic layer 104. A first interface 116 is formed between the functionally graded magnetic layer 104 and the substrate 112, and a second interface 118 is formed between the functionally graded magnetic layer 104 and the tunnelling barrier layer 106. These interfaces 116, 118 are defined here as they will be helpful in the descriptions below in relation to composition gradients across the thickness profile of the functionally graded magnetic layer 104.

As shown in Figure 1 , the functionally graded magnetic layer 104 is arranged to receive an electric current 114. The broken inversion symmetry (or inversion asymmetry) created as a result of the composition gradient in the functionally graded magnetic layer 104 interacts with the electric current 114 to generate a spin torque which acts on the switchable magnetisation direction of the functionally graded magnetic layer 104 for switching its magnetisation direction. To deterministically switch the switchable magnetisation direction of the functionally graded magnetic layer 104, the electric current 114 is applied in the presence of an in-plane external magnetic field. As shown in Figure 1 , the fixed magnetisation direction of the pinned magnetic layer 108 is in the “up” direction, antiferromagnetically coupled to the AFM layer 110 which has a magnetisation in the “down” direction. A readout signal for the MTJ element 102 is the tunnelling magnetoresistance (TMR). For the MTJ element 102, switching of the magnetisation direction of the functionally graded magnetic layer 104 provides two different resistance states which can be used for memory applications. The two different switchable magnetisation directions 120 (i.e. “up” and “down” directions) of the functionally graded magnetic layer 104 which provide for these two different resistance states are shown in Figure 1 . The two resistance states are parallel (P) state and anti-parallel (AP) state. For the P state, the magnetisation directions of the functionally graded magnetic layer 104 and the pinned magnetic layer 108 are parallel (i.e. in a same direction), and the TMR readout from the MTJ element 102 is low (e.g. represented as R low). For the AP state, the magnetisation directions of the functionally graded magnetic layer 104 and the pinned magnetic layer 108 are anti-parallel (or opposite to each other), and the TMR readout of the MTJ element 102 is high (e.g. represented as R high). The tunnelling magnetoresistance (TMR) ratio is defined as TMR ratio = (R high - R_low)/R_low. The two resistance states of the SOT device 100 can function as binary states of a storage bit for magnetic memory applications. Though not shown in Figure 1 , one way to switch the switchable magnetisation direction of the functionally graded magnetic layer 104 is by using the spin torque generated by the electric current 114 to propagate domain walls in the functionally graded magnetic layer 104. For example, a magnetic domain in the functionally graded magnetic layer 104 can be defined by two domain walls each being on an opposite side of the magnetic domain. When the electric current 114 is applied in the presence of an external in-plane magnetic field, the domain walls on opposite sides of the magnetic domain are propagated in opposite directions to expand or shrink the magnetic domain. By expanding or shrinking the magnetic domain in the functionally graded magnetic layer 104, the switchable magnetisation direction of the functionally graded magnetic layer 104 can be changed to realise the two different resistance states as read off by the MTJ element 102.

Compared to SOT-MRAMs of existing art that typically use a heavy metal/ferromagnet (FIM/FM) bilayer structure which requires a thin ferromagnet, a thickness restriction on the functionally graded magnetic layer 104 of the SOT device 100 is eliminated by the use of the binary ferromagnetic alloy L1o FePt with a bulk-like inversion asymmetry due to its composition gradient. Figure 2 shows a plot 200 illustrating two types of composition gradients 202, 204 which can exist in the functionally graded magnetic layer 104 of the SOT device 100 of Figure 1 . In the present embodiment, the functionally graded magnetic layer 104 comprises the binary ferromagnetic alloy L1₀ FePt as discussed above. In general, a binary alloy, such as the FePt, is composed of two elements A and B and can be expressed in the form Ai- _XB_X, where x is a percentage concentration of element B in the binary alloy Ai-_XB_X and is a number between 0 and 1 . An elemental ratio A/B can be defined as (1 -x)/x. Two types of composition gradients may exist in the binary alloy Ai-_XB_X. A first one is represented by the plot 202 which shows an increasing A/B elemental ratio across a thickness profile of the binary alloy Ai-_XB_X, while a second one is represented by the plot 204 which shows a decreasing A/B elemental ratio across the thickness profile of the binary alloy Ai__xB_x. It is this variation in the elemental ratio across the thickness profile of the binary alloy Ai_ _XB_X (i.e. the thickness profile of the functionally graded magnetic layer 104 in this embodiment) that creates the composition gradient in the functionally graded magnetic layer 104. Further, it can be seen in Figure 2 that the elemental ratio A/B varies linearly across the thickness profile of the functionally graded magnetic layer 104 for both plots 202, 204. In relation to Figure 1 , the thickness profile of the functionally graded magnetic layer 104 is defined in the z-direction where z = 0 is at the first interface 116 between the functionally graded magnetic layer 104 and the substrate 112. In other words, having the elemental ratio A/B increases across the thickness profile of the functionally graded magnetic layer 104 can be understood to mean that a concentration of the element A with respect to a concentration of the element B increases from the first interface 116 towards the second interface 118 in the functionally graded magnetic layer 104. It is noted that the spin Flail angles for these two different composition gradients (i.e. two different inversion asymmetries), which are associated with charge-to-spin conversion efficiencies, have opposite signs.

Example 2. A SOT magnetic memory device including a chiral spin structure

This inversion asymmetry can also play an important role in the designs of chiral spin structures.

Figure 3 shows a schematic structure of a SOT device 300 for illustrating use of skyrmions as storage bits for memory applications, where the skyrmions are generated using a functionally graded magnetic layer 302, such as the functionally graded magnetic layer 104 shown in Figure 1 . An example of a skyrmion 304 is shown at a top of Figure 3. The combination of broken inversion symmetry and spin-orbit coupling of the functionally graded magnetic layer 302 induces the Dzyaloshinskii-Moriya interaction (DMI), which is one of the elements for generating skyrmions. The DMI is related to an interaction between neighbouring spins in the functionally graded magnetic layer. A diameter of a skyrmion can be controlled to range from several nanometers to several hundred nanometers by engineering a strength of the DMI. The strength of the DMI can be controlled by the broken inversion symmetry (e.g. in relation to a degree of the composition gradient of the functionally graded magnetic layer) and a strength of the spin orbit coupling of the chemical elements which made up the functionally graded magnetic layer (e.g. a heavy element has a larger spin-orbit coupling than a light element).

In particular, a motion of the skyrmions can be controlled by the spin-orbit torque generated from an interaction between an electric current 306 flowing through the functionally graded magnetic layer 302 and the inversion asymmetry created by the composition gradient in the functionally graded magnetic layer 302. The motion of the skyrmions is similar to the motion of the domain walls of the functionally graded magnetic layer 302, except that the skyrmions propagate in a same direction whereas in the case of domain wall propagation as described in relation to Figure 1 , the domain walls on opposite sides of the magnetic domain propagate in opposite directions in the presence of an external in-plane magnetic field (e.g. H_x = 1000 Oe).

As shown in Figure 3, the spin-orbit torque (SOT) device 300 includes a track which comprises the functionally graded magnetic layer 302 formed on a substrate 308. The functionally graded magnetic layer 302 comprises L1₀ FePt, and the substrate 308 includes a single crystalline MgO (001 ) substrate with a 5 nm TiN buffer layer. As shown in Figure 3, a plurality of skyrmions 304 reside on the track. These skyrmions 304 can be used as storage bits for memory applications, where a presence and an absence of the skyrmions each represents a binary state of a storage bit in the SOT device 300. For example, the logic states 310 “1 ” and “0” are represented by the presence and the absence of skyrmions. This is further described in relation to an operation of the SOT device 300 below. The SOT device 300 further comprises a write electrode 312 which is arranged to inject a spin poloarised current {Iwnte) 313 to create a skyrmion in the track. The skyrmion can be created at a location beneath the write electrode. The write electrode 312 includes any electrically conductive material. In the present embodiment, the write electrode 312 comprises aluminum (Al). The SOT device 300 further comprises a magnetic tunnel element (MTJ) for detecting the presence or the absence of a skyrmion on the track. The MTJ element for the SOT device 300 comprises at least a portion of the functionally graded magnetic layer 302, a tunnelling barrier layer 314, a pinned magnetic layer 316, and an antiferromagnetic (AFM) layer 318. The tunnelling barrier layer 314 comprises an insulating oxide that is not electrically conductive. In the present embodiment, the tunnelling barrier layer 314 comprises MgO. The pinned magnetic layer 316 comprises a magnetic layer which has a fixed magnetisation direction. In the present embodiment, the pinned magnetic layer 316 comprises CoFe. Similar to the MTJ element of Figure 1 , the fixed magnetisation direction of the pinned magnetic layer 316 is strengthened through the exchange bias which occurs between the pinned magnetic layer 316 and the antiferromagnetic layer 318. In the present embodiment, the antiferromagnetic layer 318 comprises an antiferromagnet lrMn₃.

In a write operation using the SOT device 300, a skyrmion is written onto the track by injecting the spin polarised current {Iwnte) 313 (e.g. a current pulse) via the write electrode 312. Particularly, although skyrmions can be formed in the functionally graded magnetic layer 302, these skyrmions are not well arranged in the SOT device 300. The injected spin polarised current {Iwnte) 313 therefore assists to nucleate or create skyrmions at specific sites of the SOT device. A motion of the injected skyrmion can be controlled by the applied electric current ( v_e) 306 via the spin-orbit torque generated when the applied electric current ( 306 interacts with local spin moments of the skyrmion. For example, the applied electric current {l_dn_ve) 306 can be used to translate injected skyrmions across the functionally graded magnetic layer 302 so that different logic states 310 can be read off using the MTJ element. The logic states 310 (i.e. the presence or the absence of skyrmions) can be detected by a tunnelling magnetoresistance (TMR) read-out of the MTJ element, in a similar manner as previously described for the SOT device 100 of Figure 1. Particularly, the presence and the absence of a skyrmion in the MTJ element (e.g. the presence or the absence of a skyrmion beneath the tunnelling barrier layer 314) provide two different resistance states which can function as binary states for magnetic memory applications. As shown in Figure 3, the TMR of the MTJ element of the SOT device 300 can be measured by a voltmeter or a multi-meter 320.

Method for fabricating the SOT devices 100, 300 of Figures 1 and 3 As shown in Figures 1 and 3, the SOT devices 100, 300 have similar layered structures. In particular, each of the SOT devices 100, 300 comprises a functionally graded magnetic layer 104, 302 formed on a substrate. The SOT devices 100, 300 also each comprises a magnetic junction element (MTJ) which includes the functionally graded magnetic layer 104, 302, the tunnelling barrier layer 106, 314, the pinned magnetic layer 108, 316, and the antiferromagnetic (AFM) layer 110, 318. For the SOT device 300 of Figure 3, the write electrode 312 is additionally formed on the functionally graded magnetic layer 302. Formation of the write electrode 312 may involve an additional deposition step and/or lithography step as compared to a fabrication method for the SOT device 100 as shown in Figure 4 below.

Figure 4 shows a flowchart showing steps of a method 400 for fabricating the SOT device 100 of Figure 1 . Standard cleaning steps have been excluded from the method 400 for clarity and succinctness. As discussed above, given the similarity in the device structures of the SOT devices 100 and 300, the method 400 as described below is also applicable for the fabrication of the SOT device 300 of Figure 3.

In a step 402, the functionally graded magnetic layer 104 is formed on the substrate 112 as shown in Figure 1. The functionally graded magnetic layer 104 has a switchable magnetisation direction and a composition gradient adapted to create a broken inversion symmetry to interact with an electric current to generate a spin torque for manipulating the switchable magnetisation direction of the functionally graded magnetic layer. In the present embodiment, the functionally graded magnetic layer 104 comprises L1₀ FePt which can be epitaxially deposited on the substrate 112 by sputtering.

In a step 404, the tunnel barrier layer 106 is formed on the functionally graded magnetic layer 104. In the present embodiment, the tunnel barrier layer 106 comprises MgO which can also be epitaxially deposited on the functionally graded magnetic layer 104 using sputtering.

In a step 406, the pinned magnetic layer 108 is formed on the tunnel barrier layer 106. The pinned magnetic layer 108 has a fixed magnetisation direction. In the present embodiment, the pinned magnetic layer 108 comprises CoFe which can be epitaxially deposited on the tunnel barrier layer 106 using sputtering. In a step 408, the antiferromagnetic layer 110 is formed on the pinned magnetic layer 108. The presence of the antiferromagnetic layer 110 strengthened the fixed magnetisation direction of the pinned magnetic layer 108 via the exchange bias which occurs between the pinned magnetic layer 108 and the antiferromagnetic layer 110. In the present embodiment, the antiferromagnetic layer 110 comprises lrMn3 which can also be epitaxially deposited on the pinned magnetic layer 108 using sputtering.

In the present embodiment where the functionally graded magnetic layer 104, the tunnelling barrier layer 106, the pinned magnetic layer 108 and the antiferromagnetic layer 110 are deposited by sputtering, these steps 402-408 as aforementioned described may be performed consecutively one after another within a single deposition process using the sputtering equipment. This advantageously improves an interfacial quality between the different layers since these interfaces are not exposed to external environment between the deposition process steps 402-408.

Once the MTJ layered structure as aforementioned described is formed on the substrate 112, the layered structure is patterned to form the SOT device 100 using lithography techniques. For a typical lithography process, the layered structure is coated with a photoresist. Then a mask aligner is used to focus, align, and expose the photoresist for forming a device pattern on the layered structure. The exposed regions of the photoresist are washed away by a developer solution and the device pattern is transferred to the layered structure. The regions without the photoresist are then exposed to an etching process, where one or more layers of the layered structure can be etched away to create the desired device structure. The remaining photoresist is then removed. It will be appreciated by the skilled person in the art that the lithography process as described can be performed one or more times for fabricating the SOT device 100.

With reference to the method 400 as described above, to fabricate the SOT device 300, the layered structure can also be patterned to form a pillar 322 as shown in the SOT device 300 of Figure 3. The pillar 322 is part of the MTJ element of the SOT device 300, and it comprises the tunnelling barrier layer 314, the pinned magnetic layer 316, and the antiferromagnetic (AFM) layer 318. Similar lithography technique as described above can be used for patterning the pillar 322. Once the pillar 322 is formed, the write electrode 312 can be deposited. The write electrode 312 can be deposited, for example, by sputtering. To form the pattern of the write electrode 312, similar lithography technique as described above can be used. Alternatively, a shadow mask can be used during the deposition process of the material (e.g. Al) for the write electrode 312, which allows direct deposition of the write electrode 312 on the functionally graded magnetic layer 302 of the SOT device 300.

As shown in the embodiments of the SOT devices 100, 300 in Figures 1 and 3, an in plane electric current 114, 306 is to be applied to each of the functionally graded magnetic layers 104, 302. This likely necessitates formation of further electrical contact pads on the functionally graded magnetic layers 104, 302. The skilled person would appreciate that these electrical contact pads are not shown on the SOT devices 100, 300 in Figures 1 and 3 for clarity and succinctness. These electrical contact pads may be made of any conductive metal, for example, copper (Cu) or aluminium (Al).

Inversion asymmetry due to composition gradient

Figures 5A, 5B and 5C show schematic diagrams illustrating three different types of inversion asymmetries 500, 510, 520. Figures 5A and 5B are related to inversion asymmetries 500, 510 which have been used in SOT devices, while Figure 5C is related to a novel inversion asymmetry 520 created by composition gradient within a magnetic layer or magnetic material as described in the present disclosure.

Type I inversion asymmetry 500 is shown in Figure 5A. The Type I inversion asymmetry 500 is a structural inversion asymmetry as a result of the heterostructure 502, where an interface is created between two different material layers 504, 506. An example of a bilayer structure which creates the Type I structural inversion asymmetry is a Pt/Co bilayer. Figure 5B illustrates a Type II inversion asymmetry 510 which is a bulk inversion asymmetry due to a non-centrosymmetric crystal structure 512 of a bulk magnetic layer. In this case, in contrast to the Type I inversion asymmetry 500, the Type II bulk inversion asymmetry 510 due to the inherent non-centrosymmetric crystal structure 512 of the material and therefore does not require a structural interface for creating this inversion asymmetry. Figure 5C illustrates a Type III inversion asymmetry 520 which originates from a composition gradient within a functionally graded magnetic material, for example, a binary alloy Ai-_XB_X. The greyscale gradient as shown in Figure 5C represents an elemental distribution of the binary alloy Ai__xB_x for creating the composition gradient. The composition of the binary alloy Ai__xB_x varies across a thickness profile of the functionally graded magnetic material, with a higher concentration of element B at a bottom 522 of the material layer (i.e. a lower concentration of element A) and a lower concentration of element B at a top 524 (i.e. a higher concentration of element A) of the material layer.

Experimental data in relation to an exemplary binary alloy L1 n FePt having a composition gradient with inversion asymmetry

The following Figures 6A to 12B are related to experimental data of an exemplary binary alloy L1o FePt for demonstrating the use of Type III inversion asymmetry 520 in a SOT device, such as the SOT devices 100 and 300 as described in Figures 1 and 3, respectively. In the present experiments, a series of 20 nm to 60 nm thick epitaxial FePt films were deposited. These FePt films were each deposited on a single crystalline MgO (001) substrate with a 5 nm TiN buffer layer.

Figures 6A, 6B, 6C and 6D show diagrams illustrating structural and magnetic properties of L1o FePt in accordance with the exemplary embodiment.

Figure 6A shows a schematic of a crystal structure 600 of L1₀ FePt. The crystal structure 600 of L1o FePt is a face-centered tetragonal structure with a lattice constant c slightly smaller than the lattice constant a (c/a = 0.968). As shown in Figure 6A, Fe atoms 602 and Pt atoms 604 are alternately stacked along the c axis ([001] direction).

Figure 6B shows a plot of high-resolution X-ray diffraction spectrum 610 of a 20 nm-thick L1o FePt film. As shown in Figure 6B, the FePt (001) peak 612 in the Q-2Q X-ray diffraction (XRD) pattern confirms the L1o phase of the FePt film.

Figure 6C shows a cross-sectional scanning transmission electron microscopy (STEM) image 620 of the L1₀ FePt film in the [100]-[001 ] (i.e. x-z) plane. The brighter atoms 622 are Pt while the darker atoms 624 are Fe. A scale bar 626 for a 2 nm length is provided at a left bottom side of the image 620. As shown in Figure 6C, a well-defined atomically layered structure with an excellent chemical order can be seen. One advantage of using the L1o FePt is its high magnetocrystalline anisotropy (MCA) which enables a thermal stable grain down to a size of 3 nm. This enables its application in ultrahigh density magnetic memory.

Figure 6D shows plots 630 of out-of-plane (OP) and in-plane (IP) magnetic hysteresis loops of the 20 nm thick L1o FePt film. As shown in Figure 6D, the OP magnetic hysteresis loop 632 shows a good perpendicular magnetic anisotropy. There is negligible hysteresis observed for the IP magnetic hysteresis loop 634. The saturation magnetisation can be read off from Figure 6D as approximately 1050 emu/cc.

Figures 7A, 7B and 7C show cross-section STEM images 700, 710, 720 illustrating a composition gradient in a 60 nm thick L1o FePt film. The FePt film was deposited on a MgO substrate with a TiN buffer layer as indicated in the STEM image 700 of Figure 7A.

Figure 7A shows a high-angle annular dark-field (HAADF) STEM image 700 of the L1₀ FePt film. The STEM image 700 was used to inspect a crystallinity of the deposited FePt film. As shown in Figure 7A, a bottom region “b” 702 and a top region “c” 704 are indicated. Zoom-in images of these regions 702, 704 are shown in Figures 7B and 7C, respectively. Similar to the STEM image 620 as shown in Figure 6C, the brighter atoms as shown in Figures 7B and 7C are Pt, while the darker atoms as shown in Figures 7B and 7C are Fe.

Figure 7B shows a zoom-in STEM image 710 for the bottom region “b” 702, while Figure 7C shows a zoom-in STEM image 720 for the top region “c” 704. Both of these images 710, 720 indicate well-arranged atomically layered structures, demonstrating the L1₀ ordered structure of the 60 nm thick FePt film. As shown in these STEM images 710, 720, the FePt film is relatively uniform for these two regions and it is difficult to identify any inversion asymmetry. Nonetheless, when an elemental analysis was performed for these two different regions 702, 704, it is observed that the elemental compositions for these two regions 702, 704 are different. The elemental compositions of the FePt binary alloy are Fe4sPts2 and Fe_4iPts₉ for the bottom region “b” 702 and the top region “c” 704, respectively. This indicates a clear composition gradient in the 60 nm thick binary alloy FePt film.

Figures 8A and 8B show overall Energy Dispersive X-ray (EDX) intensities 800, 810 across thickness profiles of the 20 nm thick and 60 nm thick L1o FePt films used in relation to Figures 6B-6D and 7A-7C, respectively. An EDX intensity elemental mapping for a 30 nm thick FePt film was also measured although the data was not shown here. Line profiles of integrated intensities from elemental maps for Fe K line 802 and Pt L line 804 are shown in Figure 8A for the 20 nm thick FePt film, while line profiles of integrated intensities from elemental maps for Fe K line 812 and Pt L line 814 are shown in Figure 8B for the 60 nm thick FePt film. From Figures 8A and 8B, it is observed that each of the FePt films has a linear composition gradient. For example, Pt/Fe composition changes of about 7.1 % per 10 nm across a thickness profile for the 60 nm FePt can be calculated from the EDX intensity elemental mapping 810 of Figure 8B. As shown in Figure 8B, a top region of the 60 nm thick FePt film has less Fe than a bottom region of the FePt film (see e.g. Figure 7A which shows the bottom region “b” 702 and the top region “c” 704 of the FePt film). This composition gradient breaks the inversion symmetry of the FePt film.

Figure 9 shows a plot 900 of composition gradient of L1₀ FePt films as a function of thickness of the L1 o FePt films for three different thicknesses, namely 20 nm, 30 nm and 60 nm. As shown in Figure 9, the Pt/Fe composition gradient in these FePt films increases with a thickness of the FePt films. In particular, the Pt/Fe composition gradients 902, 904, 906 for the 20 nm, 30 nm, and 60 nm FePt films, are 0.31 %/nm, 0.53%/nm, and 0.71 %/nm, respectively. It is noted that the composition gradient in the FePt film can be controlled to be within a certain range by using a designated deposition process. By engineering the composition gradient using the deposition process, an efficiency of the spin torque generated using this functionally graded magnetic material can be tuned for specific SOT devices or applications.

The SOT characteristics of FePt single layer films were investigated using the current- induced switching and harmonic Flail voltage analysis. These are shown in Figures 10 to 12B.

Figure 10 shows a plot 1000 of Flail resistance as a function of current density to illustrate current-induced magnetisation switching in the 20 nm-thick L1o FePt film of Figures 6B- 6D. As shown in Figure 10, deterministic magnetisation switching 1002 can be observed by sweeping an electric current applied to the FePt film under a fixed in-plane magnetic field H_x = -1 ,000 Oe. This fixed magnetic field H_x = -1 ,000 Oe is in an opposite direction to the applied electric current. The polarity of the switching loop 1004 reverses after the direction of the magnetic field is reversed (i.e. H_x= +1 ,000 Oe). This indicates a typical SOT-induced switching behavior, similar to those obtained for conventional heavy metal/ferromagnet (HM/FM) bilayers used in existing SOT devices.

In order to estimate the spin torque efficiency in L1₀ FePt single layer films, Harmonic Hall measurements were conducted. The Harmonic Hall measurement technique is well established for studying the SOT in conventional HM/FM bilayer structures and therefore details of this measurement technique is not described here for succinctness. In the present Harmonic Hall measurements, a small a.c. excitation current was applied, and the in-phase first harmonic signal (V_w) and the out-of-phase second harmonic signal (½_w) were measured simultaneously using two lock-in amplifiers.

Figure 11 shows plots 1100 of current-induced longitudinal and transverse spin-orbit torque efficiencies as a function of thickness for a series of L1o FePt films. The current- induced longitudinal spin-orbit torque efficiency (jS_L) is represented by a plot 1102, while the current induced transverse spin-orbit torque efficiency (jS_T) is represented by a plot 1104. As shown in Figure 11 , the current-induced spin-torque efficiency /3i_ (i.e. the plot 1102) increases with increasing thickness of the FePt film from 6 nm to about 50 nm where it saturates. For the current-induced spin-torque efficiency jS_T (i.e. the plot 1104), JST increases continuously with increasing thickness of the FePt film from 6 nm to around 40 nm where it saturates. As shown in Figure 11 , a longitudinal spin-orbit efficiency (JSL) of more than 150 Oe/1 c 10⁷ A/cm² and a transverse spin-orbit efficiency (JST) of more than 125 Oe/1 x10⁷ A/cm² can be achieved for a FePt film thickness of about 40 nm. These spin-orbit efficiencies achieved are larger than that of most conventional HM/FM bilayer systems.

As discussed in relation to Figures 12A and 12B below, the trends shown for composition gradient dependence of current-induced spin-torque efficiencies are similar to those shown for thickness dependence of the current-induced spin-torque efficiencies as described above.

Figures 12A and 12B show plots of current-induced spin-orbit torque efficiencies as a function of composition gradient of L1₀ FePt films. Figure 12A shows a plot 1200 of longitudinal spin-orbit efficiency (jS_L) 1202 as a function of the composition gradient of the L1 o FePt films, while Figure 12B shows a plot 1210 of transverse spin-orbit efficiency (JST) 1212 as a function of the composition gradient of the L1o FePt films. As shown in both Figures 12A and 12B, linear correlations between the current-induced spin-orbit efficiencies (jS_L and jS_T) and the composition gradient are observed. Further, high spin torque efficiencies can be achieved using this Type III inversion asymmetry 520. For examples, as shown in Figures 12A and 12B, a longitudinal spin-orbit efficiency (jS_L) of more than 150 Oe/1 x10⁷ A/cm² and a transverse spin-orbit efficiency (jS_T) of more than 100 Oe/1 x10⁷ A/cm² can be achieved for a composition gradient of about 0.1 %/nm. These spin-orbit efficiencies achieved are larger than that of most conventional HM/FM bilayer systems. Alternative embodiments of the invention include: (i) the functionally graded magnetic layer 104, 302 comprising a ferrimagnetic alloy; (ii) the functionally graded magnetic layer 104, 302 comprising an in-plane magnetic anisotropy; (iii) the functionally graded magnetic layer 104, 302 comprising more than two elements, such as a tertiary or a quaternary alloy; (iv) the functionally graded magnetic layer 104, 302 comprising a binary alloy AB where A is a ferromagnetic element and can be one of Co, Fe and Ni, and B is a heavy metal or rare earth elements or their alloys, and can be one of Pt, Pd, Tb, Au, Ta, Ir, W, Gd, FePt and CoTb; (v) the composition gradient being non-linear across the thickness profile of the functionally graded magnetic layer 104, 302; and (vi) the functionally graded magnetic layer 104, 302, the tunnelling barrier layer 106, 314, the pinned magnetic layer 108, 316, and/or the antiferromagnetic layer 110, 318 are deposited using sputtering, e-beam evaporation, atomic layer deposition (ALD), singly or in combination.

Further, the antiferromagnetic layer 110, 318 is optional and may not be provided in an alternative embodiment of the SOT device 100 or the SOT device 300 as long as the pinned magnetic layer 108, 316 can maintain the fixed magnetisation direction for the operation of the SOT device 100, 300.

Still further, the different layers as described in the SOT devices 100, 300 may also be formed using other appropriate materials. For example, the tunnelling barrier layers 106, 314 may be formed by one or more layers of MgO and/or AIO_x, the pinned magnetic layer 108, 316 may be formed by one or more layers of CoFe and CoFeB, the antiferromagnetic layer 110, 318 may be formed by one or more layers of IrMn and PtMn, and the substrate 112, 308 may be form by one or more layers of MgO, SrTi0₃, KaTi0₃, glass (with CrRu buffer layer) and Si/Si0₂.

Although only certain embodiments of the present invention have been described in detail, many variations are possible in accordance with the appended claims. For example, features described in relation to one embodiment may be incorporated into one or more other embodiments and vice versa.

Claims

1 . A spin-orbit torque device comprising: a functionally graded magnetic layer, the functionally graded magnetic layer having a switchable magnetisation direction and a composition gradient adapted to create a broken inversion symmetry to interact with an electric current to generate a spin torque for switching the switchable magnetisation direction of the functionally graded magnetic layer.

2. The spin-orbit torque device of claim 1 , wherein the functionally graded magnetic layer comprises a binary alloy consisting of elements A and B in the form of Ai-_XB_X, where x is a percentage concentration of element B in the binary alloy Ai__xB_x and is a number between 0 and 1.

3. The spin-orbit torque device of claim 2, wherein an elemental ratio A/B defined as (1- x)/x varies across a thickness profile of the functionally graded magnetic layer to create the composition gradient in the functionally graded magnetic layer.

4. The spin-orbit torque device of claim 3, wherein the elemental ratio A/B increases across the thickness profile of the functionally graded magnetic layer.

5. The spin-orbit torque device of claim 3 or claim 4, wherein the elemental ratio A/B varies linearly across the thickness profile of the functionally graded magnetic layer.

6. The spin-orbit torque device of any preceding claim, wherein a thickness of the functionally graded magnetic layer is equal to or more than 2 nm.

7. The spin-orbit torque device of any preceding claim, wherein the functionally graded magnetic layer comprises L1₀ FePt.

8. The spin-orbit torque device of any preceding claim, further comprising: a tunnelling barrier layer formed on the functionally graded magnetic layer; and a pinned magnetic layer formed on the tunnelling barrier layer, the pinned magnetic layer having a fixed magnetisation direction, wherein the switchable magnetisation direction of the functionally graded magnetic layer is switched by the spin torque to provide a low resistance state and a high resistance state, the low resistance state being a state in which the switchable magnetisation direction is in the same direction as the fixed magnetisation direction and the high resistance state being a state in which the switchable magnetisation direction is in an opposite direction as the fixed magnetisation direction.

9. The spin-orbit torque device of any one of claims 1 to 7, further comprising a write electrode arranged to inject a spin polarised current to create a skyrmion in the functionally graded magnetic layer.

10. The spin-orbit torque device of claim 9, further comprising: a tunnelling barrier layer formed on the functionally graded magnetic layer; and a pinned magnetic layer formed on the tunnelling barrier layer, the pinned magnetic layer having a fixed magnetisation direction, wherein at least a portion of the functionally graded magnetic layer, the tunnelling barrier layer and the pinned magnetic layer form a magnetic tunnel junction for detecting a first resistance state and a second resistance state of the spin-orbit torque device, the first resistance state being a state in which the skyrmion is present in the magnetic tunnel junction and the second resistance state being a state in which the skyrmion is absent in the magnetic tunnel junction, and wherein the presence and the absence of the skyrmion in the magnetic tunnel junction is controllable by a motion of the skyrmion in the functionally graded magnetic layer using the electric current.

11 . A method for fabricating a spin-orbit torque device, the method comprising: forming a functionally graded magnetic layer, the functionally graded magnetic layer having a switchable magnetisation direction and a composition gradient adapted to create a broken inversion symmetry to interact with an electric current to generate a spin torque for manipulating the switchable magnetisation direction of the functionally graded magnetic layer.

12. The method of claim 11 , further comprising: forming a tunnelling barrier layer on the functionally graded magnetic layer; and forming a pinned magnetic layer on the tunnelling barrier layer, the pinned magnetic layer having a fixed magnetisation direction, wherein the switchable magnetisation direction of the functionally graded magnetic layer is switched by the spin torque to provide a low resistance state and a high resistance state, the low resistance state being a state in which the switchable magnetisation direction is in the same direction as the fixed magnetisation direction and the high resistance state being a state in which the switchable magnetisation direction is in an opposite direction as the fixed magnetisation direction.

13. The method of claim 11 , further comprising forming a write electrode on the functionally graded magnetic layer, the write electrode being arranged to inject a spin polarised current to create a skyrmion in the functionally graded magnetic layer.

14. The method of claim 13, further comprising: forming a tunnelling barrier layer on the functionally graded magnetic layer; and forming a pinned magnetic layer on the tunnelling barrier layer, the pinned magnetic layer having a fixed magnetisation direction, wherein at least a portion of the functionally graded magnetic layer, the tunnelling barrier layer and the pinned magnetic layer form a magnetic tunnel junction for detecting a first resistance state and a second resistance state of the spin-orbit torque device, the first resistance state being a state in which the skyrmion is present in the magnetic tunnel junction and the second resistance state being a state in which the skyrmion is absent in the magnetic tunnel junction, and wherein the presence and the absence of the skyrmion in the magnetic tunnel junction is controllable by a motion of the skyrmion in the functionally graded magnetic layer using the electric current.

15. A functionally graded magnetic material for a spin-orbit torque device, the functionally graded magnetic material having a switchable magnetisation direction and a composition gradient adapted to create a broken inversion symmetry to interact with an electric current to generate a spin torque for switching the switchable magnetisation direction of the functionally graded magnetic material.

16. The functionally graded magnetic material of claim 15 comprising a binary alloy consisting of elements A and B in the form of Ai__xB_x, where x is a percentage concentration of element B in the binary alloy Ai__xB_x and is a number between 0 and 1 .

17. The functionally graded magnetic material of claim 16, wherein an elemental ratio A/B defined as (1 -x)/x varies across a thickness profile of the functionally graded magnetic material to create the composition gradient in the functionally graded magnetic material.

18. The functionally graded magnetic material of claim 17, wherein the elemental ratio A/B increases across the thickness profile of the functionally graded magnetic material.

19. The functionally graded magnetic material of claim 17 or claim 18, wherein the elemental ratio A/B varies linearly across the thickness profile of the functionally graded magnetic material.

20. The functionally graded magnetic material of any one of claims 15 to 19, wherein a thickness of the functionally graded magnetic material is equal to or more than 2 nm.

21 . The functionally graded magnetic material of any one of claims 15 to 20, wherein the functionally graded magnetic material comprises L1₀ FePt.

22. A method for generating a spin torque, the method comprising: using a composition gradient in a functionally graded magnetic material to create a broken inversion symmetry to interact with an electric current to generate the spin torque.

23. The method of claim 22, wherein the functionally graded magnetic material comprises a binary alloy consisting of elements A and B in the form of Ai__xB_x, where x is a percentage concentration of element B in the binary alloy Ai__xB_x and is a number between 0 and 1 , the method further comprises: creating the composition gradient by varying an elemental ratio A/B across a thickness profile of the functionally graded magnetic material, the elemental ratio A/B being defined as (1-x)/x.

24. The method of claim 23, further comprising: increasing the elemental ratio A/B across the thickness profile of the functionally graded magnetic material.

25. The method of claim 23 or claim 24, further comprising: varying the elemental ratio A/B varies linearly across the thickness profile of the functionally graded magnetic material.

26. The method of any one of claims 22 to 25, wherein the functionally graded magnetic material comprises L1₀ FePt.