Device using unconventional unidirectional magnetoresistance to read up and down states of a magnetic layer with perpendicular magnetic anisotropy

Abstract

Disclosed herein is a device using unconventional unidirectional magnetoresistance in layered heterostructures. The device consists of coupled layers of a material having out-of-plane spin polarization and a ferromagnet having perpendicular magnetic anisotropy (PMA) to read the up and down states of the PMA magnetic layer using longitudinal resistance measurements.

Description

Attorney Docket: 8350.2024-271WODEVICE USING U CONVENTIONAL UNIDIRECTIONAL MAGNETORESISTANCE TO READ UP AND DOWN STATES OF A MAGNETIC LAYER WITH PERPENDICULAR MAGNETIC ANISOTROPYRelated Applications

[0001] This application claims the benefit of U.S. Provisional Patent Application No.63 / 648,965, filed May 17, 2024, the contents of which are herebyincorporated herein in their entirety.Government Interest

[0002] This invention was made with the support of the United States Government under contracts N00014-23-1-2751, awarded by the Office of Naval Research (ONR) and 2011876 and 2208057, awarded by the National Science Foundation (NSF). The U.S. government has certain rights in the invention.Background

[0003] Since the discovery of the giant magnetoresistance and its tunneling counterpart, which led to the development of magnetic data storage devices, other magnetoresistance effects have been observed in magnetic heterostructures and have been employed to electrically read magnetic states.

[0004] Distinct from other forms of magnetoresistance, unidirectional magnetoresistance (UMR) features linear dependence on both the appliedAttorney Docket: 8350.2024-271WOcharge density and the magnetization direction of the magnetic layer. As such, it can be used to monitor the magnetic state of a magnetic system. The UMR is finite only when the spin polarization has a component collinear with the magnetization. In conventional spin-source materials, such as heavy metals and topological insulators, a charge current applied in the film plane can only generate a spin accumulation polarized in-plane and transverse to the charge current. Consequently, the UMR response in prevailing heterostructures cannot be employed to read the up and down states of a ferromagnet (FM) with perpendicular magnetic anisotropy (PMA). However, FMs with PMA are highly desired for magnetic memory and spin-logic devices to realize ultra-fast operation, thermally stable nanometer sized magnetic- bits, and achieving attojoule-class logic gates.

[0005] In this regard, the key to achieving a non-volatile writing operation is toutilize the spin-orbit torque (SOT) driven magnetic switching, wherein an out- of-plane magnetization can be efficiently manipulated by the SOT originating from the nonequilibrium spin accumulation generated in a nearby spinsource layer. However, SOT-based planar magnetic memory devices considered so far mainly rely on a three-terminal configuration, wherein a magnetic tunnel junction (MTJ) is integrated on top of spin-source material to read the magnetic state through tunnel-magnetoresistance effect, featuring operational separation between the read and write circuits, which critically affect the footprint of the individual memory nodes. At present, an SOT-Attorney Docket: 8350.2024-271WObased two-terminal magnetic memory device, like commercially available MTJ-based magnetic memory devices, is critically missing due to the lack of identified magnetoresistance phenomena to distinguish the up and down magnetic states of a PMA magnet in widely used bilayer heterostructures for SOT magnetic switching devices. A two-terminal SOT switching device has been previously demonstrated by employing current-in-plane giant magnetoresistance effect in trilayer heterostructures and by optimizing the distribution of charge current flowing through an ultrathin SOT layerintegrated with an MTJ stack, but such a device based on a purely bilayer heterostructure is still missing.Summary of the Invention

[0006] Disclosed herein is a device using unconventional UMR in bilayer heterostructures consisting of a first layer of a material having out-of-plane spin polarization and a second layer of a ferromagnetic material having perpendicular magnetic anisotropy coupled to the first layer and arranged in a bilayer structure, which enables the electrical reading of the up and down states of the PMA magnetic layer through longitudinal resistance measurements in layered heterostructures. In one embodiment, the first layer may be tungsten ditelluride (WTe2~) and the second layer may be chromium germanium telluride (Cr2Ge2Te6or CGT)~ WTe2is a type-ll WeylAttorney Docket: 8350.2024-271WOsemimetal whose Tdphase has a crystal structure with broken mirror symmetry in the ac-plane, as shown in FIG. 1, which allows for the generation of nonequilibrium spin accumulation with a substantial out-of-plane spin polarization (crz) driven by a charge current applied along the crystallographic a-axis (see left panel of FIG. 2) owing to the spin-galvanic effect. The interactions between azin WTe2and the magnetization in CGT gives rise to a UMR (see right panel of FIG. 2) that can be used to distinguish between the up and down magnetic states through longitudinal resistance measurements, that is, lower or higher resistance when the magnetization direction of the second layer (m) and the spin polarization (crz) the first layer are parallel and antiparallel respectively.Brief Description of the Drawings

[0007] By way of example, specific exemplary embodiments of the disclosed system and method will now be described, with reference to the accompanying drawings, in which:

[0008] FIG. 1 is schematic representation of the crystal structure of WTe2with thea-axis and b-axis labelled.

[0009] FIG. 2: The left panel is a schematic showing the generation of nonequilibrium spin accumulation with an out-of-plane spin polarization (crz), when a charge current ( / ) is applied along the a-axis of WTe2. The rightAttorney Docket: 8350.2024-271WOpanel is a schematic depicting the concept of unconventional UMR in heterostructures, that is, a change in longitudinal resistance (RUMR) depending on the relative orientation of out-of-plane magnetization (mx) and spin polarization (crz).

[0010] FIG. 3 is a side cross sectional view of a first exemplary device having 2 layers of W7e2.

[0011] FIG. 4 is an optical image of the device of FIG. 3 with the a-axis of WTe2aligned along the current electrodes.

[0012] FIG. 5 is a STEM image of a cross section of a second exemplary device having 3 layers of WTe2.

[0013] FIG. 6 is a graph showing the four-point longitudinal resistance as a function of gate voltage measured for the device of FIG. 3, showing hysteresis in the longitudinal resistance.

[0014] FIG. 7 is a graph showing the raw longitudinal resistance signal measured as a function of out-of-plane magnetic field (i.e., an unconventional UMR signal) when a positive charge current of 100 pA is applied along the a-axis in the device of FIG. 3 at 10 K.

[0015] FIG. 8 (top) is a graph showing the measured two-point longitudinalresistance when positive (orange curve) and negative (blue curve) charge currents are applied along the a-axis of WTe2in the device of FIG.3. FIG. 8 (bottom) is a graph showing the antisymmetric part and the symmetric partAttorney Docket: 8350.2024-271WOof the two-point longitudinal resistance plotted as the yellow and purple curves, respectively.

[0016] FIG. 9 is a schematic of a device in accordance with this disclosure showingthe measurement configuration to read the out-of-plane magnetic state of CGT using the two-point longitudinal resistance.Detailed Description

[0017] Electrical readout of magnetic states is a key to realize novel spintronics devices for efficient computing and data storage. Unidirectional magnetoresistance (UMR) in a bilayer heterostructure, consisting of a spinsource material and a magnetic layer, produces a change in the longitudinal resistance RUMR) upon the reversal of magnetization, which typically originates from the interaction of nonequilibrium spin accumulation and magnetization at the interface. Because of the linear dependence of UMR on applied charge current and magnetization, it can be used to electrically read the magnetization state. A change in the magnetoresistance phenomenologically obeys RUMROC (m ■ <J)J, where J is the applied charge current density, <T is the spin polarization generated by the inverse spin- galvanic effect (a.k.a., spin-Edelstein effect) in the spin-source material, and m specifies the magnetization direction of the magnetic layer.

[0018] However, in conventional spin-source materials, the spin-polarization of an electric-field induced nonequilibrium spin accumulation is restricted to be inAttorney Docket: 8350.2024-271WOthe film plane and hence the ensuing UMR can only respond to the in-plane component of the magnetization. Conversely, magnets with perpendicular magnetic anisotropy (PMA) are highly desired for magnetic memory and spinlogic devices while the electrical read-out of PMA magnets through UMR is critically missing. Topological semimetals with low-crystal symmetries, such as WTe2, are a unique spin-source material platform wherein out-of-plane spin polarization can be efficiently generated through the inverse spin- galvanic effects, which can enable electrical read-out of PMA magnetsthrough UMR.

[0019] Disclosed herein is a storage device which uses unconventional UMR in heterostructures of a topological semimetal (e.g., WTe2) and a PMA ferromagnetic insulator (e.g. CGT), which allows the electrical reading of the up and down magnetic states of the CGT layer by measuring the longitudinal resistance. The unconventional UMR originates from the interplay of crystal symmetry-breaking in WTe2and magnetic exchange interaction across the WT e2I CGT interface. The combination of the ability of reading thedirection of the magnetization and the ability of WT e2to obtain magnetic field free switching of the PMA magnets can be used to achieve two-terminal magnetic memory devices based on purely bilayer heterostructures that operate solely on the spin-orbit torque and UMR for developing nextgeneration non-volatile and low-power consumption data storagetechnologies.Attorney Docket: 8350.2024-271WO

[0020] The device employs atomically thin flakes of V / Te2(few layers) and CGT (10- 15 nm). Techniques of mechanical dry transfer and standard devicefabrication are used to assemble van der Waals (vdW) WTe2I CGT heterostructures and their devices, as will be explained later. An exemplary device having two layers of WT e2is shown in a side cross sectional view in FIG. 3. FIG. 4 is an optical micrograph of the device of FIG. 3 showing the a- axis of the WT e2aligned along the current electrodes, with the CGT, WT e2, graphite and hBN flakes outlined and labelled. The Pt electrodes are outlined with dashed black lines. The bilayer WTe2flake used in the device of FIG. 3 is shown in the inset to FIG.4.

[0021] A second exemplary device having 3 layers of WTe2is shown in a cross- sectional scanning transmission electron microscopy (STEM) image in FIG. 5.The STEM cross-sectional image of the device of FIG.5 shows the device comprising a tri-layer of WTe2and 13.2 nm CGT, wherein an atomicallyclean and sharp WTe2 / CGT interface is clearly visible.

[0022] In devices in accordance with this disclosure, which consist of an atomically thin W / e2layer and an insulating magnet, the electric transport in theWT e2layer is tunable by applying an electrostatic gate voltage, whichadjusts the electron chemical potential in the W / e2layer. The measured longitudinal resistance (Rxx) for the device of FIG. 3 as a function of top gate voltage (l^) at 10 K is shown in FIG. 6, wherein Rxxincreases with increasing Vg, indicating that the bilayer WTe2is likely hole-doped due to the chargeAttorney Docket: 8350.2024-271WOtransfer across the interface. The hysteresis loop is also indicative of ferroelectric behavior in WTe2

[0023] The realization of unconventional UMR in WTe2I CGT heterostructures can be used to electrically read the out-of-plane magnetization of CGT. As shown in FIG. 7, when a positive charge current is applied along the crystallographic a-axis of WTe2in the device of FIG. 3, there is a clear step in resistance, proportional to mz, in the measured raw longitudinal resistance (Rxxw)as afunction of out-of-plane magnetic field (Bz), indicating the presence of unconventional UMR. When a positive charge current of 100 pA is applied along the a-axis in the device of FIG. 3 at 10 K. The orange and yellow lines are fit to the saturated region at a positive and negative magnetic field side respectively to subtract the background not associated with UMR.

[0024] The top graph in FIG. 8 shows the measured 2 -point longitudinal resistance Rxx), after subtracting a trivial background at saturated magnetization regions under positive (orange curve) and negative (blue curve) charge currents. The bottom graph shows the antisymmetric part and symmetric part of the longitudinal resistance plotted as the yellow and purple curves, respectively. As expected, UMR response, due to its unidirectional nature, is negligible in the symmetric part of Rxx. The small peaks in Rxxmmare due to a linear magnetoresistance effect due to band modulation induced by exchange interactions from the magnetization in CGT.Attorney Docket: 8350.2024-271WO

[0025] Magnetoresistance behavior is expected to be completely different when the current is applied along the b-axis (i.e., UMR due to out-of-plane magnetization should vanish due to the absence of <JZ). The device should show no sign of UMR response when the current is applied along the b-axis.This clearly suggests that the unconventional UMR in these devices is due to the presence of oz, which is only allowed when a charge current is applied along the low-symmetry axis (i.e., the a-axis, in W / Te2).

[0026] The microscopic origin of the out-of-plane oriented spins in WT e2can be attributed to the layered spin Edelstein effect, which is consistent with the screw-axis and glide-plane symmetries in WTe2. As the CGT is an insulating ferromagnet, considering only the first-adjacent WT e2layer is enough for a qualitatively modeling of the electron transport in Vl / Te2 / CGT heterostructure.

[0027] It is important to mention that the second-order longitudinal conductivity, which accounts for the UMR, vanishes if the electric field is applied along the y direction (i.e., the b-axis) (i.e. <JXX— 0). In contrast, axxis finite such that when the electric field has a finite projection along the x direction (i.e., the a- axis), a non-vanishing UMR should appear.

[0028] One of the outstanding challenges in the spintronics research is to realize a planar 2-terminal SOT-magnetic memory device, which requires that the out- of-plane magnetic state be electrically readable through 2-point measurements. As shown in FIG. 9, the UMR can be employed to electricallyAttorney Docket: 8350.2024-271WOread the magnetic state of CGT using a 2-point measurement configuration.In the exemplary device of FIG. 3, the UMR response is measured using a 2- point configuration, and the obtained Rxxas a function of Bzwith positive and negative current is shown in the top graph of FIG. 8. The extracted Rxxym(lower graph of FIG. 8) clearly shows that the perpendicular magnetization of CGT can be electrically detected by the unconventional UMR in 2-point measurements.

[0029] The fabrication of the device will now be described. WTe2and hexagonal boron nitride (hBN) crystals were prepared by previously published procedures. CGT single crystals were purchased from HQ Graphene.Mechanical exfoliation of CGT, hBN, graphite, and CGT was performed on separate silicon wafers with 300 nm of SiO2inside a glovebox filled with Ar gas. Flakes were selected through optical investigation through a microscope.WTe2flakes that have well-defined and straight edges were used because the a-axis tends to be along straight edges. Preferably, the WTe2flakes have a minimum thickness in the range of about .8 nm to 10s of nM.

[0030] Before the heterostructure fabrication, the Pt electrodes were defined on a separate Si / SiO2substrate using electron beam lithography (EBL) and sputtering deposition with a polymethyl methacrylate (PMMA) resist. The electrodes contacting the heterostructure were composed of Pt (6 nm) or Ti (1-3 nm) / Pt (6-7 nm). The Pt electrodes are then connected by Cr (5 nm) / Au (110 nm) electrodes for wire bonding pads, prepared using electron beamAttorney Docket: 8350.2024-271WOlithography (EBL) and electron beam deposition with a PMMA / methylmethacrylate (MMA) bilayer resist.

[0031] The heterostructure was fabricated using a custom transfer tool inside a glovebox filled with Ar gas. A transfer slide consisting of a polydimethylsiloxane (PDMS) slab and a thin film of polycarbonate (PC) was used for picking up hBN, CGT, and WT e2in that order and then putting the stack on the Pt electrodes. For devices with a top gate, an additional graphite flake is picked up in the beginning to contact the pre-patterned Pt electrodes.As for devices sandwiched between two hBN flakes, a graphite / hBN is first transferred to a Si / SiO2substrate. Pt electrodes composed of Ti (1 nm) / Pt (6 nm) were patterned on top of the hBN using electron beam lithography (EBL) and electron beam deposition with a PMMA / MMA bilayer resist. The Pt electrodes were connected to the Au electrodes by the same method mentioned previously. Finally, a stack consisting of WTe2I CGT I hBN was transferred on top of the Pt electrodes to complete the device. For all UMR devices, the Pt electrodes and the substrate were cleaned by atomic force microscopy in contact mode using / zmasch HQ:NSC15 / AI BS tips and a gentle oxygen plasma before the heterostructure was transferred to the electrodes to ensure the interface quality.

[0032] Disclosed herein is the use of an unconventional UMR in WTe2I CGT heterostructures, which allows the reading of the up and down states of the PMA magnetic thin film through longitudinal resistance measurements. TheAttorney Docket: 8350.2024-271WOUMR in WTe2I CGT heterostructures originates from the interplay between the interfacial exchange coupling and the interfacial Rashba spin-orbit coupling, in the presence of the low-symmetry crystal structure of WTe2.This provides a new pathway to realize spin-orbit torque (SOT) based two- terminal magnetic memory devices for the development of next-generation non-volatile and low-power consumption data storage technologies. The devices are not limited to the use of WTe2and CGT. The magnetoresistance phenomena should be observable by coupling WTe2(or other material system with out-of-plane spin polarization) to PMA magnets. It should be noted that the main ingredient for unconventional UMR (i.e., an material having out-of-plane spin polarization), is not only limited to low-symmetry of spin-source materials but can also arise in thin films and bulk crystals of magnetic systems.

[0033] The present invention illustrates various techniques and configurations that enable operation of the described device. As would be realized, many configurations of the components of the device are possible and are intended to be within the scope of the invention.

Claims

Attorney Docket: 8350.2024-271WOClaims:

1. A device comprising:a first layer of a material having out-of-plane spin polarization; and a second layer of a ferromagnetic material having perpendicular magnetic anisotropy coupled to the first layer;wherein a direction of a magnetic field in the second layer can be determined by a measurement of longitudinal resistance through the first layer.

2. The device of claim 1 wherein the first layer is composed of tungsten ditelluride (V Te2) and further wherein the second layer is composed of chromium germanium telluride (CGT).

3. The device of claim 1 further comprising:a first electrode located on one end of the first layer; anda second electrode located on an opposite end of a longitudinal axis of the first layer;wherein the resistance is measured between the first and second electrodes.

4. The device of claim 1 wherein the longitudinal resistance is measured when a current is applied along the longitudinal direction of the first layer.Attorney Docket: 8350.2024-271WO5. The device of claim 1 further comprising:an insulating layer covering the device.

6. The device of claim 5 wherein the insulating layer is composed of boron nitride (hBN).

7. The device of claim 1 wherein:the resistance is lower when the direction of the magnetization of the second layer is in a parallel configuration with the out-of-plane spin polarization of the first layer; andthe resistance is higher when the direction of the magnetization of the second layer is in an antiparallel configuration with the spin polarization of the first layer.

8. The device of claim 3 further comprising:a third electrode coupled to the first layer to apply a gate voltage, the gate voltage used to tune the longitudinal resistance.

9. The device of claim 7 wherein the out-of-plane spin polarization of the first layer is present when a current is applied along the longitudinal axis of the first layer.Attorney Docket: 8350.2024-271WO10. The device of claim 7 wherein the out-of-plane spin polarization of the first layer is absent when a current is applied perpendicular to the longitudinal axis of the first layer.

11. The device of claim 1 wherein the first layer comprises multiple layers of flakes of the material having out-of-plane spin polarization.

12. The device of claim 11 wherein the flakes have a minimum thickness of approximately .8 nM.

13. The device of claim 1 wherein the second layer has a thickness in the range of lOnm - 15nm.

14. The device of claim 3 wherein the electrodes are composed of Pt or a combination of Ti and Pt.

15. A device comprising:an Si I SiCh substrate;electrodes deposited on the substrate;one or more layers of flakes of a first material having out-of-plane spin polarization deposited on the substrate and in contact with the electrodes; anda layer of a ferromagnetic material having perpendicular magnetic anisotropy deposited on the one or more layers of flakes.Attorney Docket: 8350.2024-271WO16. The device of claim 15 wherein one or more layers of the first material are composed of tungsten ditelluride (WTe2) and further wherein the ferromagnetic material is composed of chromium germanium telluride (CGT).

17. The device of claim 15 further comprising:an insulating layer deposited over the device.

18. The device of claim 15 wherein the substrate is composed of Si, SiO2or boron nitride.

19. The device of claim 17 wherein the insulating layer is composed of boron nitride.

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