Field-free deterministic switching of all–van der Waals spin-orbit torque system above room temperature

Two-dimensional van der Waals (vdW) magnetic materials hold promise for the development of high-density, energy-efficient spintronic devices for memory and computation. Recent breakthroughs in material discoveries and spin-orbit torque control of vdW ferromagnets have opened a path for integration of vdW magnets in commercial spintronic devices. However, a solution for field-free electric control of perpendicular magnetic anisotropy (PMA) vdW magnets at room temperatures, essential for building compact and thermally stable spintronic devices, is still missing. Here, we report a solution for the field-free, deterministic, and nonvolatile switching of a PMA vdW ferromagnet, Fe3GaTe2, above room temperature (up to 320 K). We use the unconventional out-of-plane anti-damping torque from an adjacent WTe2 layer to enable such switching with a low current density of 2.23 × 106 A cm−2. This study exemplifies the efficacy of low-symmetry vdW materials for spin-orbit torque control of vdW ferromagnets and provides an all-vdW solution for the next generation of scalable and energy-efficient spintronic devices.


Introduc.on
The discovery of emergent magne?sm in two-dimensional van der Waals (vdW) materials [1][2][3] has broadened the material space for developing spintronic devices for energy-efficient, non-vola?lememory and compu?ng applica?ons [4][5][6][7][8] .These applica?ons are par?cularlywell-served by perpendicular magne?c anisotropy (PMA) ferromagnets, which allow fabrica?on of nanometer scale, high-density and thermally stable spintronic devices.vdW materials provide strong PMA alterna?ves [9][10][11][12] to the few bulk op?mal material systems, like CoFeB/MgO [13][14][15] , while providing key advantages like scalability down to monolayer thicknesses, and s?ll maintaining an atomically smooth interface and minimal intermixing with the tunnel barrier of a magne?c tunnel junc?on (MTJ).Ability to switch the vdW PMA ferromagnets above room temperature is necessary for viable applica?ons to harness these capabili?es.Hence, recent reports on achieving current controlled switching of vdW PMA ferromagnets at room temperature are promising 16,17 .However, exis?ng schemes for room temperature current control of vdW ferromagnets u?lize spin-orbit torque (SOT) from heavy-metals or topological insulators and require applica?on of an in-plane magne?c field to allow determinis?c switching.This poses challenges to the development of highdensity, thermally stable SOT-switching devices using vdW ferromagnets.A very recent work (unpublished) has a>empted to use asymmetric growth of Pt on Fe3GaTe2 (single edge-coverage) to ar?ficially break lateral symmetry for showing field-free switching of the vdW ferromagnet up to 300 K 18 .However, such a mechanism is inherently unscalable precluding wafer scale processing and in addi?on, robust non-vola?leswitching remains to be achieved.Thus, the cri?cal challenge of field-free, determinis?c, and non-vola?lecontrol of PMA magne?sm in vdW materials above room temperature has remained unsolved.
Here, we report the first demonstra?on of determinis?c and non-vola?leswitching of a PMA vdW ferromagnet above room temperature without any external magne?c fields.We achieved this by building a bilayer SOT system of room-temperature PMA vdW ferromagnet, Fe3GaTe2 (FGaT) with the low symmetry vdW material WTe2 to harness the unconven?onalout-of-plane an?-damping torque for SOT switching (Fig. 1A).While several approaches to enabling field-free SOT switching of PMA magne?za?on are possible, including STT-assisted SOT switching 19 , anisotropy ?l?ng in the ferromagnet 20,21 , ar?ficially breaking lateral symmetry 22 , and u?lizing intrinsically low symmetry spin-orbit coupling layers [23][24][25][26][27] , we have employed WTe2 because it is par?cularly interes?ng for control of vdW magnets allowing crea?on of vdW heterostructures and ensuring pris?ne interfaces and no laace strain.Charge current injec?on along the low symmetry a-axis of WTe2 generates an unconven?onal,out-of-plane an?-damping SOT,  !" ##$ , of the form  $ × ̂×  $ ( $ , ̂ are unit vectors along ferromagnet magne?za?on and WTe2/ferromagnet interface) 28,29 and this torque can be u?lized for field-free switching of PMA ferromagnets [24][25][26][27] .However, this mechanism has not been previously demonstrated for room-temperature field-free switching of vdW materials.Employing our FGaT/WTe2 heterostructure devices, we demonstrate determinis?c switching using a low current density of 2.23 × 10 6 A/cm 2 up to 320 K.We also show that such field-free determinis?c switching is seen exclusively when the charge current is injected parallel to the low-symmetry axis of WTe2, asser?ng the role of crystal symmetry in enabling such field-free switching of PMA magne?sm.

Results
Our heterostructure devices use exfoliated sheets of FGaT and WTe2, with pa>erned electrical contacts, and hexagonal boron nitride (hBN) encapsula?onfor air-stability, as illustrated schema?cally through Fig. 1A.The heterostructures were assembled using dry viscoelas?ctransfer process 30 and electrodes were pa>erned using a combina?on of e-beam lithography and e-beam evapora?on of Ti/Au (more details in Methods).The Td-phase of WTe2 used here belongs to the Pmn21 space group.As shown in Fig. 1B, the crystal structure of WTe2 is such that it preserves mirror symmetry about the bc-plane ( %& ), while it breaks the mirror symmetry along the ac-plane ( '& ), where c is the out-of-plane crystallographic axis.As a result, spin-orbit coupling induced spin-accumula?on, and consequently the spin-orbit torque, in response to a current flowing along the a-axis and the b-axis varies significantly.These two cases are treated in detail in the following discussion, using two devices, D1 with FGaT (25.8 nm)/WTe2 (21.6 nm) and D2 with FGaT (17.9 nm)/WTe2 (23.8 nm).An op?cal image of the device D1 is shown in Fig. 1C, with the FGaT, WTe2 and hBN flakes indicated.The crystallographic a and b-axes of the WTe2 flakes were iden?fied using polarized Raman spectroscopy in the backsca>ering geometry ( ,  , ) ̅ , where  , is a unit vector in the sample plane, along the azimuthal angle  as defined in Fig. 1C.Fig. 1D shows a color plot of the polarized Raman spectra of the WTe2 flake in D1 (see Supplementary Fig. S2 for D2).WTe2 exhibits two types of prominent Ag peaks with two-fold symmetries, which can be used to iden?fy its a and b-axes 31,32 .The minima in the type-I peaks (81 cm -1 and 212 cm -1 ), which coincides with the maxima in the type-II peak (165 cm -1 ) corresponds to the a-axis of the WTe2 crystal.
Magneto-transport characteriza?on of the FGaT/WTe2 devices using anomalous Hall effect helps to establish that the inherent ferromagne?c characteris?cs of FGaT are well preserved in the heterostructure device and can be effec?velyprobed through transverse voltage monitoring for current-induced magne?za?on switching experiments.Fig. 2A, B show the anomalous Hall effect curves for the device D1, for field swept along sample normal ( ∥ ) and temperatures in the range 10 K to 340 K.The device exhibits a large coercivity (up to 8.25 kOe at 10 K) at low temperatures, which diminishes with temperature (Fig. 2C) such that  & = 210 Oe at 300 K and near-zero star?ng 330 K.The anomalous Hall resistance,  () !*+ goes to zero above 320 K too, marking a ferromagnet to paramagnet transi?on between 320 K to 330 K.The anomalous Hall effect curve corresponding to field swept close to sample plane ( ⊥ ) is shown in Fig. 2D.It exhibits the characteris?cs of a PMA magnet, going to near-zero resistance values only at high inplane magne?c fields, with an anisotropy field of about 35 kOe, corrobora?ngthat the strong perpendicular magne?c anisotropy of FGaT is preserved in the heterostructure device.
Fig. 3A provides a schema?crepresenta?on of the spin-orbit torque mechanism at play when the applied current is parallel to the high-symmetry, b-axis.In this case, the applied current has no effect on the crystal's bc-mirror plane symmetry ( %& ).In accordance with Curie's principle 33 , since the causali?es (crystal structure and applied current) preserve  %& , the resultant spin-current (and accumula?on)must also preserve  %& .This forbids a ver?cal spin-polariza?on ( , ) component in the ver?cally flowing spin-current, since the  , pseudovector transforms an?-symmetrically upon reflec?on in the bc-plane.As a result, the spin-accumula?on at the FGaT/WTe2 interface only has an in-plane spin-polariza?on, similar to the case of heavy metal/ferromagnet and topological insulator/ferromagnet systems.Such an in-plane spin accumula?oncan only produce determinis?c switching in the presence of an externally applied field along the current direc?on.Fig. 3B shows the response of device D1 to current pulses applied along the b-axis of WTe2 in the absence of any external field, at 300 K.As expected, the in-plane an?-damping torque from spinaccumula?on at the FGaT/WTe2 interface drives the FGaT magne?za?on in-plane ( , = 0) resul?ng in a near-zero anomalous Hall resistance, for a current magnitude of about ±4.5 mA (9.51 × 10 5 A/cm 2 ).Upon lowering the current drive to zero, the FGaT remains effec?vely demagne?zed as its various domains orient randomly due to lack of a symmetry breaking field.The four curves in Fig. 3B verify this for all combina?ons of current drive (posi?ve or nega?ve) and ini?al magne?za?on direc?on ( , = ±1 ≡  () = ±1.2Ω).The ini?al magne?za?on state is set by applying a field of ±2 kOe along the sample normal before star?ng current sweeps.Contrary to the above case, driving a current of the same magnitude in the presence of a nonzero external field ( = ±500 Oe) parallel to current axis,  ∥  ∥ , results in determinis?c, par?al switching of the FGaT magne?za?on.As shown in Fig. 3C, reversing the direc?on of applied field reverses the chirality of the current-induced switching loops, as is expected for such a system.Field-assisted determinis?c and non-vola?leswitching of out-of-plane magne?za?on of FGaT could also be achieved in D1 for the case of  ∥  ∥  (Fig. 3D) at 300 K.
In contrast to the above discussed case, when current is applied along the low-symmetry a-axis of WTe2, the applied current breaks the bc-mirror plane symmetry ( %& ).Thus, the causali?es break both the mirror plane symmetries ( '& broken by crystal structure,  %& broken by applied current), and a ver?cal spin-polariza?on component in the ver?cal spin-current is now permissible.This scenario is depicted schema?cally in Fig. 4B.The ver?cal component of spinaccumula?on at the FGaT/WTe2 interface can now apply a symmetry breaking, unconven?onal,out-of-plane an?-damping spin orbit torque,  !" ##$ , on the FGaT magne?za?on. !" ##$ is an?symmetric in current and hence, the FGaT magne?za?on can be toggled determinis?cally between  , = ±1 by applying posi?ve and nega?ve current pulses.Device D2, with current applied along the a-axis of its WTe2 flake is used to study this scenario.Details on the device, its Raman spectra and magneto-transport data are included in Supplementary Fig. S2 and S3.Fig. 4A shows the field-free current induced switching loops of D2 for temperatures ranging from 300 K to 325 K.At 300 K, complete switching could be induced using ±8 mA (see Supplementary Fig. S4), equivalent to a current density of 2.23 × 10 6 A/cm 2 .Increasing the temperature from 300 K to 325 K resulted in shrinking of the anomalous Hall resistance spliang, un?l no clear looping behavior could be observed at 330 K and beyond (Fig. 4C).This aligns with the fact that magne?za?on of FGaT would decrease with increasing temperature, resul?ng in a decreasing  () !*+ un?l it eventually vanishes beyond its Curie temperature (320 K -330 K).Fig. 4D shows the field-free determinis?c and non-vola?leswitching of PMA magne?za?on of FGaT by a train of current pulses, 1 ms long and ±8 mA in amplitude, applied along the low-symmetry axis of WTe2,  ∥ , at 300 K. We could observe such determinis?cswitching right up to 320 K as reported in Supplementary Fig. S6, providing the first demonstra?on of field-free, determinis?c switching of out-of-plane magne?za?on in a vdW ferromagnet above room temperature.

Conclusion
We u?lize the unconven?onal,out-of-plane an?-damping spin orbit torque,  !" ##$ , generated from WTe2 upon charge current injec?on along its low-symmetry a-axis to switch the magne?za?on of underlying FGaT, in the FGaT/WTe2 heterostructure devices.We clearly show that the  !" ##$ induced field-free switching occurs exclusively for charge current injec?on along WTe2 a-axis, while charge injec?on along the b-axis results in demagne?za?on of underlying FGaT.Thus, we have reported the first demonstra?on of field-free magne?za?on switching of a perpendicular magne?c anisotropy vdW ferromagnet above room temperature (up to 320 K) using a low current density of 2.23 × 10 6 A/cm 2 .The proposed all-vdW architecture can also provide unique advantages like improved interface quality needed for efficient spin-orbit torques, possibili?esfor gate-voltage tuning to assist SOT switching, and prospects for flexible and transparent spintronic technologies.This work asserts the role of crystal symmetry in spin-orbit coupling layers of an SOT switching device using a low-symmetry vdW material, and provides a new, scalable all-vdW approach to developing energy-efficient spintronic devices.

Device fabrica+on
The Fe3GaTe2/WTe2 devices reported here were fabricated using heterostructure assembly of exfoliated vdW flakes.Bulk FGaT was grown using a previously reported process 17 .Bulk WTe2 and hBN were commercially sourced from HQ Graphene and Ossila, respec?vely.FGaT flakes were exfoliated on Si/SiO2 (280 nm) substrates using mechanical exfolia?on.WTe2 flakes, exfoliated on PDMS stamps were transferred on to selected FGaT flakes using the dry viscoelas?ctransfer process.Electrodes were then pa>erned on the FGaT/WTe2 heterostructure using a combina?on of e-beam lithography with the posi?ve e-beam resist PMMA 950, and e-beam evapora?on of Ti/Au (5 nm/60 nm).The devices were then encapsulated with thick exfoliated flakes of hBN, using dry viscoelas?ctransfer.All exfolia?on and vdW transfer processes were performed inside the inert environment of a N2-filled glovebox (O2, H2O < 0.01 ppm).Thicknesses of the cons?tuent flakes were characterized amer encapsula?onusing a Cypher VRS AFM.Polarized Raman spectra of WTe2 flakes was acquired using a 532 nm laser with a WITec Alpha300 Apyron Confocal Raman microscope, by rota?ng the polarizer and analyzer while the sample was sta?c.

Transport measurements
All transport measurements were performed in a 9 T PPMS DynaCool system.Measurements were performed by sourcing current using a Keithley 6221 current source and measuring the transverse voltage across the devices, using a Keithley 2182A nanovoltmeter.Anomalous Hall effect measurements with field sweeps were performed using a drive current of 50 -200 µA.For the current-induced switching measurements, a 1 ms pulse of write-current was followed by 999 ms of read pulses (±200 µA).Field could be applied in and out of the sample plane using the PPMS' horizontal rotator module.Fig. 3: Field-assisted (only) switching for  ∥ .(A) Schema?c illustra?on of the scenario where current is sourced along the high-symmetry axis,  ∥ .Symmetry constraints allow only an inplane component of spin-accumula?on along the FGaT/WTe2 interface, resul?ng in a non-zero inplane an?-damping torque ( !" -$ ≠ 0) but a zero out-of-plane an?-damping torque ( !" ##$ = 0).(B) Response of the device to current pulses applied along the b-axis for zero external field at 300 K.The blue and green (yellow and red) curves correspond to current pulses swept from 0 → −4.5 mA → 0 (0 → +4.5 mA → 0 mA), for the device ini?alized at  , = 1 and  , = −1, respec?vely.The device undergoes complete demagne?za?on by 4.5 mA in all the four cases.(C) Current sweeps up to 4.5 mA result in par?al magne?za?on switching in the presence of an externally applied field,  ∥  ∥  of ±500 Oe, with changing the direc?on of field resul?ng in chirality reversal of the current-induced switching curves.Black dashed lines in (B) and (C) correspond to  , = ±1.(D) Field-assisted determinis?c, non-vola?leswitching of FGaT magne?za?on using a train of 1 ms long current pulses, ±4.5 mA in magnitude, under +500 Oe in-plane magne?c field,  ∥  ∥ .The curve at each temperature is an average of four consecu?ve current pulse sweeps acquired for that temperature, with error bars indica?ng standard devia?on of each data point across the four sweeps (Individual sweeps reported in Supplementary Fig. S5).Data offset along y-axis for clarity.(B) Schema?c illustra?on of this scenario where current is sourced along the low-symmetry axis,  ∥ .Broken mirror plane symmetries allow an out-of-plane component of spin-accumula?on along the FGaT/WTe2 interface, resul?ng in a non-zero out-of-plane an?-damping torque ( !" -$ ≠ 0), asymmetric in current direc?on, enabling field-free determinis?c switching of the underlying FGaT's magne?za?on.(C) Temperature dependence of the anomalous Hall resistance spliang in the current-induced switching loops.Clear switching can be observed up to 325 K (green region), with decreasing  () !*+ denoted with solid square point, while no clear switching loops could be observed star?ng 330 K (red region), and hence the  () !*+ is set to zero (hollow square points).(D) Demonstra?on of field-free, determinis?c, non-vola?leswitching of out-of-plane FGaT magne?za?on in the FGaT/WTe2 device using 1 ms long pulses of current, ±8 mA in amplitude, applied along the a-axis.The data is acquired at 300 K in two sets of 50 s long pulsing sequences, with periodic and randomized current pulses, respec?vely.

Fig. S6:
Determinis?c switching up to 320 K. Field free determinis?c, non-vola?leswitching of the OOP magne?za?on of FGaT in device D2, using the train of current pulses, 1 ms long and ±8 mA in magnitude (top panel), with  ∥ , at 310 K (middle panel) and 320 K (lower panel).The data is acquired in two sets of 50 s long pulsing sequences, with periodic and randomized current pulses, respec?vely.

Fig. 1 :Fig. 2 :
Fig. 1: Fe3GaTe2/WTe2 heterostructure device.(A) Schema?c diagram of the Fe3GaTe2/WTe2 heterostructure devices used in this study.(B) Schema?c model of WTe2 crystal's ab-plane, with the a and b-axes labelled.The crystal preserves mirror-plane symmetry in the bc-plane while breaks it in the ac-plane.(C) Op?cal image of device D1, with the WTe2 (21.6 nm), FGaT (25.8 nm) and hBN flakes labelled.Crystallographic axes of the WTe2 flake (determined through polar Raman spectra) and the defini?on of azimuthal angle  in the Raman spectra are also indicated.Scale bar: 10 µm (D) Polarized Raman spectra of the WTe2-flake in (C).The minima (maxima) in type-I Ag modes at 81 cm -1 and 212 cm -1 (type-II Ag mode at 165 cm -1 ) around  = 90 o corresponds to the a-axis of the WTe2 flake.

Fig. 4 :
Fig.4: Field-free switching for  ∥ .(A) Response of the device D2 to current pulse sweeps, along a-axis, for varying temperatures without any external field.The curve at each temperature is an average of four consecu?ve current pulse sweeps acquired for that temperature, with error bars indica?ng standard devia?on of each data point across the four sweeps (Individual sweeps reported in Supplementary Fig.S5).Data offset along y-axis for clarity.(B) Schema?c illustra?on of this scenario where current is sourced along the low-symmetry axis,  ∥ .Broken mirror plane symmetries allow an out-of-plane component of spin-accumula?on along the FGaT/WTe2 interface, resul?ng in a non-zero out-of-plane an?-damping torque ( !" -$ ≠ 0), asymmetric in current direc?on, enabling field-free determinis?c switching of the underlying FGaT's magne?za?on.(C) Temperature dependence of the anomalous Hall resistance spliang in the current-induced switching loops.Clear switching can be observed up to 325 K (green region), with decreasing  () !*+ denoted with solid square point, while no clear switching loops could be Fig. S1: Topographical data for device D1.

Fig. S1 :
Fig. S1: Topographical data for device D1. (A) Op?cal image of the device, with red box indica?ng the region used for AFM measurements.(B) AFM topography micrograph of the region in red box.(C) Height profile along the red line in (B).

Fig. S2 :
Fig. S2: Device D2, its Raman spectra and topography.(A) Op?cal image of the FGaT/WTe2 heterostructure before pa>erning electrodes.crystallographic axes of WTe2 (determined using polar Raman spectra) and the defini?on of azimuthal angle  in the polar Raman measurements is indicated.(B) Polarized Raman spectra of the WTe2 flake in D2. (C) Op?cal image of the device D2, amer pa>erning electrodes and encapsula?onwith hBN.Red boxes correspond to area scanned in AFM for determining the thicknesses of the cons?tuent WTe2 (box D) and FGaT (box F) flakes.(D) AFM topography micrograph of red box D and (E) the height profile along the red line.(F) AFM topography micrograph of the red box F and (G) the height profile along the red line.

Fig. S5 :
Fig. S5: Current-pulsing loops across varying temperatures.(A) Four consecu?ve current pulsing loops acquired for D2, with  ∥ , without any external field field-assisted ini?aliza?on between consecu?ve loops either) at 300 K. Black dashed lines are a visual aid deno?ng the same loop spliang.(B-F) Similar data for temperatures 305 K -325 K in steps of 5 K.