Current-induced switching in a magnetic insulator · 2016. 12. 9. · articlesdoi:10.1038/nmat4812 a) (((a!, )=(.!! // / / ˛

ARTICLESPUBLISHED ONLINE: 21 NOVEMBER 2016 | DOI: 10.1038/NMAT4812

Current-induced switching in a magnetic insulatorCan Onur Avci, Andy Quindeau, Chi-Feng Pai†, Maxwell Mann, Lucas Caretta, Astera S. Tang,Mehmet C. Onbasli, Caroline A. Ross* and Geo�rey S. D. Beach*

The spin Hall e�ect in heavy metals converts charge current into pure spin current, which can be injected into an adjacentferromagnet to exert a torque. This spin–orbit torque (SOT) has been widely used to manipulate the magnetization in metallicferromagnets. In the case of magnetic insulators (MIs), although charge currents cannot flow, spin currents can propagate, butcurrent-induced control of the magnetization in a MI has so far remained elusive. Here we demonstrate spin-current-inducedswitching of a perpendicularly magnetized thulium iron garnet film driven by charge current in a Pt overlayer. We estimatea relatively large spin-mixing conductance and damping-like SOT through spin Hall magnetoresistance and harmonic Hallmeasurements, respectively, indicating considerable spin transparency at the Pt/MI interface. We show that spin currentsinjected across this interface lead to deterministic magnetization reversal at low current densities, paving the road towardsultralow-dissipation spintronic devices based on MIs.

Ferromagnetic thin-film heterostructures provide the basisfor hard disk data storage, magnetic random access memory,magnetic sensor technology based on giant- and tunnelmagnetoresistance, and, more recently, prototypes of logicand memory using domain walls. The ability to manipulatethe magnetic state of thin-film heterostructures is an essentialattribute of these devices, and it is especially attractive if it canbe accomplished electrically, rather than by the applicationof a magnetic field. Compared to conventional spin transfertorque effects demonstrated in spin-valves and magnetic tunneljunctions, the current-induced spin–orbit torque (SOT) in non-magnetic heavy metal/metallic ferromagnet heterostructures1,2and magnetically doped topological insulator heterostructuresat low temperatures3,4 provides a highly efficient way to controlthe magnetization of the ferromagnetic material. With the SOTmechanism it is possible to realize magnetization switching5,6,magnetic oscillations7,8, and ultrafast chiral domain wall motion9,10.Spin currents generated by the spin Hall effect (SHE)11 in heavymetals (HM) have been shown to be a dominant source of thedamping-like SOT responsible for the observed switching anddomain wall motion in such heterostructures.

In the case of magnetic insulators (MIs) such as yttrium irongarnet (YIG), charge currents cannot flow, but spin currentsgenerated by the SHE in an adjacent HM layer can be transmittedacross the HM/MI interface. This has been shown to giverise to novel spin transport phenomena such as spin Hallmagnetoresistance (SMR) in HM/MI structures12–15, and used toexcite high-frequency magnetization dynamics in YIG16,17 drivenby current-induced spin torques. The low damping and long spintransmission length (∼1mm) in magnetic insulators such as YIGmake thempromising candidates for spin-wave communication andultralow-power-dissipation applications16,18, as well as insulatingcomponents of magnetic logic or memory devices. However, SHE-induced control of the magnetization switching in a magneticinsulator has yet to be reported.

In this work, we demonstrate deterministic current-inducedswitching in a perpendicularly magnetized MI using spin torquefrom the SHE in an adjacent Pt layer. The MI used here consists

of an 8-nm-thick thulium iron garnet (Tm3Fe5O12, TmIG)film with perpendicular magnetic anisotropy. We organize thisarticle as follows: first, we report on the structural and magneticcharacterization of TmIG continuous films with X-ray diffraction(XRD), vibrating sample magnetometry (VSM), magneto-opticalKerr effect (MOKE) and magnetic force microscopy (MFM).Next, we characterize SMR by means of transverse (Hall effect)measurements in TmIG/Pt bilayers, fromwhich we extract the spin-mixing conductance G↑↓ that characterizes spin transport acrossthe interface. Then, we perform harmonic Hall effect measurementsto accurately quantify the damping-like SOT driven by the spin Halleffect in Pt. Finally, we show that the perpendicularmagnetization ofTmIG can be reversed by the damping-like SOT stemming from theSHE in an adjacent Pt layer with current densities comparable to orlower than that used to switch all-metallic perpendicular magneticanisotropy (PMA) heterostructures5,6, even though the magneticfilm thickness is one order of magnitude larger. These results are animportant step towards novelmemory or logic devices based onMIs.

Structural and magnetic characterizationFerrimagnetic TmIG was chosen because its negativemagnetostriction and negative magnetocrystalline anisotropycoefficient K1 leads to an out-of-plane easy axis19 when grownepitaxially under tensile strain on a (111)-oriented gadoliniumgallium garnet (Gd3Ga5O12, GGG) substrate. Films with layerthicknesses of 8 nm and 30 nm TmIG were grown by pulsed laserdeposition (PLD) onGGG (111). The 8 nm filmwas used in the SOTswitching experiments, whereas the 30 nm film yields a strongersignal for XRD analysis that gives insight into the structure andstrain state of TmIG. The crystalline structure and film thicknesswere investigated by high-resolution XRD and X-ray reflectivity(XRR), respectively. For the 30-nm-thick film the symmetric XRDscans around the (444) GGG-substrate and TmIG peaks (Fig. 1a)demonstrate fully strained TmIG films with the anticipated Lauediffraction peak shape and Laue fringes. Reciprocal space mappingof the 30 nm TmIG film provides evidence that the in-plane latticeparameter of TmIG was coherently strained to match that ofGGG (111) (see Supplementary Information 1). High-resolution

Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. †Present address:Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan. *e-mail: [email protected]; [email protected]

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Figure 1 | Structural and magnetic properties of PMA TmIG. a, X-ray di�raction patterns (2θ−ω scan) of a 30-nm-thick (orange) and a 8-nm-thick(black) TmIG film grown on (111)-oriented GGG substrates. b, Magnetization of both films as a function of out-of-plane magnetic field Hz obtained by VSM.A paramagnetic contribution from the GGG substrate has been subtracted. c, Polar MOKE signal of films. d, MFM image of the as-grown 30-nm-thickTmIG film showing up- and down-oriented domains as regions of light and dark contrast.

transmission electron microscopy of a similar film (not shownhere) revealed a fully coherent interface with no dislocations overat least a 1.5 µm distance. Since the thicker TmIG film showed nostrain relaxation, it is reasonable to assume that the thinner (8 nm)TmIG film is also fully coherent.

Figure 1b,c shows the out-of-plane hysteresis loop for the 8 nmand 30 nm TmIG films measured using VSM and polar MOKE,respectively. Both films exhibit high out-of-plane remanence,indicating strong PMA. The coercivity of the 30 nm film islower than that of the 8 nm film. We tentatively attribute thisbehaviour to the difference in the domain size, which is expectedto be proportional to the square root of thickness, leading to thethickness dependence of the field needed to nucleate and propagatedomains. The saturation magnetization of both films was about100 e.m.u. cm−3, approaching the room-temperature bulk value20 ofMs ∼ 110 e.m.u. cm−3. The magnetic domain configuration wasvisualized in both the as-grown (demagnetized) and saturatedstates via magnetic force microscopy (MFM). Figure 1d shows anMFM image for the demagnetized state in the 30 nm film, whichexhibits a labyrinthine domain structure typical of PMA thin films.After out-of-plane saturation, the domain structure vanishes (notshown), consistent with the high remanence of the correspondinghysteresis loops.

Spin Hall magnetoresistanceSMRmeasurements have been widely used to extract spin transportparameters in transition HM/MI bilayers12–15, from which theinterfacial spin-mixing conductance G↑↓ can be estimated. In suchmeasurements, an electrical current is applied to a HM layer withsizeable SHE deposited on a MI. The charge current j in the HMgenerates a transverse spin current js=θSH(j×σ ) that will either betransmitted or reflected at the HM/MI interface depending on the

relative orientations between the SHE spin polarization σ and themagnetization direction m in the MI. Here θSH represents the spinHall angle (SHA) of theHM. Interfacial spin transmission/reflectionis characterized by the spin-mixing conductanceG↑↓=G↑↓r + iG

↑↓

i .The real part G↑↓r is related to the damping-like torque acting uponm, which is proportional tom× (σ×m). The imaginary partG↑↓i isassociated with a field-like torque, which is proportional to σ×m.The dependence of spin transmission/reflection on m and σ willfurther modulate j in the HM layer due to the inverse spin Halleffect21. This leads to magnetoresistance in the HM, the so-calledSMR,which possesses a symmetry distinct from that of, for example,anisotropic magnetoresistance.

In the SMR scenario, the longitudinal (R) and transverse Hall(RH) resistance in a HM/MI bilayer can be expressed as13,15,22

R=R0+1RSMR sin2 θ sin2 ϕ (1)

RH=RSMRH sin2θ sin2ϕ+RAHE,SMRH cosθ+R

OHEH Hz (2)

where R0 and 1RSMR represent the m-independent longitudinalresistance and the modulation due to the SMR, respectively. RSMRH ,RAHE,SMRH and R

OHEH represent the transverse manifestation of SMR,

the SMR-induced anomalous Hall effect (AHE) resistance, and theordinary Hall resistance of the HM, respectively. The magnetizationangles θ , ϕ are defined in Fig. 2a, as well as the coordinate systemused throughout the article. We note that the transverse SMR andthe SMR-induced AHE are identical to the planar Hall resistanceand anomalous Hall resistance in conducting ferromagnets bysymmetry.

Figure 2b–d summarizes measurements of transverse resistanceRH on TmIG(8 nm)/Pt(5 nm) bilayers that reveal SMR in our films

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Figure 2 | SMRmeasurements on a TmIG/Pt bilayer device. a, Hall device schematics, coordinate systems and the electrical measurement set-up. b, Hallresistance RH is measured with an out-of-plane-field Hz and an a.c. current of amplitude 1.5 mA (r.m.s.), corresponding to j=4.3× 1010 A m−2 in the Ptlayer. RH tracks the magnetization vector, due to the SMR-induced AHE, which switches between up and down states at the coercive field ofHc= 110±5 Oe. The linear negative slope arises from the ordinary Hall e�ect, scaling linearly with Hz and independent of the magnetization direction.c, RH measured while sweeping an in-plane field (HIP) at ϕH=0◦, 45◦, 135◦ in the presence of a constant Hz=+200 Oe to ensure that the magnetizationremains single domain during measurement. The curves at ϕH=45◦(135◦) are representative of SMR behaviour where a large positive (negative) signalappears when m is saturated in-plane. Orange curves are simulations based on experimental parameters and an estimated perpendicular magneticanisotropy field HK=2,700 Oe. d, Summary of RH measurements performed at di�erent ϕH as shown in c. A fit to the data according to equation (2)reveals RSMRH =6.5 m. Note that a constant sample-dependent o�set is subtracted from the raw data shown in b and c.

and allow the associated transport parameters to be quantified(see Supplementary Information 2 for the longitudinal SMRmeasurements). The bilayer was patterned intoHall cross devices, asshown schematically in Fig. 2a, and all electricalmeasurements wereperformed using a standard lock-in technique (see Methods). RHversusHz (Fig. 2b) shows 100% remanence and sharpmagnetizationreversal at a coercive field Hc = 110± 5 Oe. We attribute thereduction in Hc as compared to the continuous film (Fig. 1)to geometrical effects due to patterning. The linear background(green dotted line) arises from the ordinary Hall effect in Pt,consistent with equation (2). We find RAHE,SMRH =−0.46m andROHEH =−5.8mT

−1, which yield ρAHE,SMRxy =−2.3× 10−4 µ cm

and ρOHExy =−2.9× 10−3 µ cmT−1, respectively, comparable to

recently reported values in Pt/YIG22. Next, we measure RH as afunction of in-plane field (HIP) at different field angles ϕH. Figure 2cshows measurements recorded at ϕH= 0◦, 45◦ and 135◦, where theSMR contribution to RH is either zero at ϕH = 0◦ or maximum(minimum) at ϕH=45◦(135◦) according to equation (2). Note thatwe have applied a fixed out-of-plane field HOOP= 200Oe duringthe HIP sweep to avoid magnetic domain formation and to ensurecoherent rotation of the magnetization. By simulating the SMRwith a macrospin model using experimental parameters (Fig. 2c,orange curves) we obtain a perpendicular magnetic anisotropy field

HK≈2,700Oe. Finally, by repeating the measurements at variousϕH we find that RH ∝ sin 2ϕ, as shown in Fig. 2d, as expectedfrom the SMR. A fit to the data yields RSMRH = 6.5m, which leadsto ρSMRxy =3.25×10

−3 µ cm. This value is approximately 14 timeslarger than the SMR-induced AHE, implying that the real partof the spin-mixing conductance is significantly larger than theimaginary part.

Following the SMR model given in ref. 23 and by usingρSMRxy , ρ

AHE,SMRxy obtained above, we can calculate G

↑↓. By takingρPtxx=40.4 µ cm as measured on the same device, assumingθSH=0.08 for Pt24 and taking the spin diffusion length to beλPt = 1.4 nm (ref. 23), we estimate G↑↓r = 1.0 × 10

14 −1 m−2and G↑↓i =7.1×10

12−1 m−2, which compare well with commonlyaccepted values for YIG/Pt13,14.

Determination of damping-like spin–orbit torqueThe large G↑↓r suggests the possibility of achieving significantspin injection across the TmIG/Pt interface, and hence exerting adamping-like torque on the magnetization. To quantify the SHE-induced SOT, we performed harmonic Hall effect measurementssimilar to those commonly used in all-metallic HM/ferromagnetbilayers1,25. In such measurements, an a.c. current inducesoscillations in the magnetization due to alternating SOT, giving rise



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Figure 3 | Determination of the damping-like SOT in TmIG/Pt by harmonic measurements. a, Measurement geometry for the harmonic Hall voltagemeasurements. Blue and red arrows show the e�ective fields associated with damping-like (HDL) and field-like (HFL) SOT, respectively, for current flowalong the x-axis and magnetization tilted along the y-axis. b, First-harmonic Hall voltage (Vω) mainly driven by the SMR-induced AHE, acquired during anin-plane field sweep of Hy=±200 Oe with an applied current of jr.m.s.=2.1× 1011 A m−2. The curvature is di�erent for mup and mdown due to misalignmentof the external field with respect to the y-axis, causing a small SMR contribution which is even with respect to magnetization reversal whereas SMR-inducedAHE is odd. c, Second-harmonic voltage (V2ω) recorded under the same conditions as in b. Di�erent slopes are a consequence of a current-induced spinSeebeck e�ect voltage (see text and Supplementary Information 4 and 5 for more details). d, V2ω plotted versus (2VSMRH sin

2 θ)/(HsinθH) to quantify HDLby evaluating the slope given by the linear fit (black curves). A constant sample-dependent o�set has been subtracted from the data presented in b and c.

to a second-harmonic Hall voltage, V2ω. Usually, the damping-likeand field-like torques are probed by measuring V2ω while sweepingan in-plane magnetic field that tilts the magnetization in the zxand zy planes, respectively. Under these conditions the a.c. currentgenerates out-of-plane magnetization oscillations in the respectiveplanes, and the resulting AHE oscillations dominate the V2ω signal.In the present system, however, the SMR is much larger than theSMR-induced AHE, and hence V2ω mainly reflects the in-planecomponent of magnetization oscillations. In this case, the damping-like SOT is expected to give the dominant contribution to V2ωwhen the magnetization is tilted in the zy plane, since the field-liketorque drives out-of-plane oscillations, whose contribution to V2ωscales with the much smaller SMR-induced AHE. At the same time,thermal contributions to V2ω from the spin Seebeck effect (SSE)arising from the Joule-heating-induced perpendicular temperaturegradient scale as VSSE ∝∇Tz ×m, and are hence minimized inthis configuration26. Therefore, harmonic Hall measurementsunder a transverse field are well-suited to quantitatively extract thedamping-like SOT in this system.

Figure 3b and c shows the first- and second-harmonic signalswhenm is tilted along±y with a swept field Hy and an a.c. currentdensity jr.m.s.∼= 2.1× 1011 Am−2. Ideally, the first-harmonic signalVω arises solely from the SMR-induced AHE in this geometry(see equation (2)), which decreases with increasing Hy as m tiltstowards the plane (Vω = V AHE,SMRH cos θ). However, unintentionalmisalignment of H with respect to the y axis creates non-negligible

SMR (∝ sin2 θ) and OHE (∝ Hz ) contributions that distort theexpected quadratic AHE signal, as depicted in Fig. 3b.

We now focus on V2ω which, as shown in Fig. 3c, varies linearlywith Hy , having opposite slopes for up (mup) and down (mdown)magnetization states. This symmetry is typical for a SOT signal andcan be driven by both HFL and HDL according to ref. 1:

V ϕ=90◦

2ω =(V AHE,SMRH −2V

SMRH cosθ sin2ϕ

) dcosθdH

HFLsin (θH−θ)

+2V SMRH sin2θ cos2ϕ

HDLH sinθH

(3)

where θH is the out-of-plane angle of the external field. SinceV SMRH � V

AHE,SMRH , and assuming HDL≈HFL, the signal in Fig. 3c

must be dominated by the second term on the right-hand side.Therefore, to estimate the damping-like SOT we plot V2ω asa function of (2V SMRH sin

2 θ)/(H sinθH) in Fig. 3d (note thatcos2ϕ∼= sinθH∼=1 and sin 2ϕ∼=0 when Hy tilts the samplemagnetization in the zy plane) and take the slope, which directlyyields HDL in units of Oe. We obtain −9.4Oe and +15.1Oe formup and mdown, respectively, giving on average HDL=12.3±2.8 Oeper jr.m.s. ∼= 2.1× 1011 Am−2, with the sign compatible with theSHE in Pt. The difference in slopes between mup and mdown is dueto a small x-component creating a non-negligible VSSE, adding apositive slope for both up and down orientation (see Supplementary

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NATUREMATERIALS DOI: 10.1038/NMAT4812 ARTICLES

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Figure 4 | Current-induced magnetization switching of TmIG/Pt. a, Hall resistance measurements serving as reference for mup and mdown states(a constant o�set and planar Hall e�ect (PHE) contribution are subtracted). b, Magnetization switching with 5-ms-long current pulses ofj=±2.34× 1011 A m−2 amplitude in the presence of an external field of Hx=±500 Oe. The switching polarity changes on reversing either the currentpolarity or field direction. c, Switching behaviour of the device as a function of the injected current density for a given field of Hx=+500 Oe. Above thecritical current of jc=±1.7× 1011 A m−2 the magnetization switches systematically and reproducibly. d, Magnetization switching phase diagram showingthe switching behaviour as a function of applied current and field. It is constructed with the help of measurements shown in c performed withHx=0→ 1,200 Oe.

Information 4). By assuming that HDL is entirely driven by theSHE, we can estimate the SHA via27 θSH= (2e/})(MstPtHDL/j) usingappropriate material parameters, where e is the elementary charge,} is the reduced Planck constant,Ms is the saturationmagnetizationof TmIG and tPt is the Pt layer thickness. We find θSH≈ 1%, whichconstitutes the lower bound due to current spread to theHall voltagebranches, effectively reducing the current density of the central areaof the Hall cross1,28. When this correction is taken into account weestimate θSH∼1.5–2%, close to the reported values for Pt in contactwith YIG13.

Current-induced magnetization switchingNext, we show that the large damping-like SOT revealed byharmonic measurements can be used to switch the magnetizationof TmIG, as summarized in Fig. 4. First, we recorded the Hallresistance during a Hz sweep, as shown in Fig. 4a, from whichwe subtracted a sample-dependent constant offset and the OHEcontribution. This serves as reference for mup and mdown, to whichwe compare the action of the current pulses. We then successivelyapplied ±5-ms-long pulses of amplitude j= 2.34× 1011 Am−2 inthe presence of an in-plane field of Hx = ±500 Oe along thecurrent injection axis. We note that for deterministic SOT-inducedswitching in PMA materials it is necessary to apply an in-planeexternal field to break the rotational symmetry of HDL (ref. 5,6).In Fig. 4b we show the measured Hall resistance after each currentpulse in the presence of a constant Hx . We observe that RH changesbetween 80–100%with respect to its full amplitude—that is, 0.7m,after every pulse. The switching polarity reverses when reversing

the applied field direction +H x→−H x , as expected from theSOT-induced switching. Reference measurements performed withthe identical configuration showed that the switching polarity iscompatible with that of Pt/Co when the structural inversion factoris taken into account (Supplementary Information 6).

Figure 4c shows the switching behaviour of the same sample asa function of j for Hx =+500Oe. Each data point represents theoutcome of averaging ten measurement sequences where, duringeach sequence, we measured RH after a given pulse was appliedsynchronously with Hx , and compared with the reference RHrecorded after positive and negative out-of-plane saturation fieldpulses (for details of the measurement protocol see SupplementaryInformation 6). We see that the switching events are systematicabove the critical current density of jc= 1.8× 1011 Am−2 for thisspecific Hx . We note that the switching events above jc saturatearound 90%,meaning that the probed area of theHall cross does notfully reverse. This behaviour was reproducible for several devicesand tentatively explained by partial switching of areas in the vicinityof the Hall cross where j is relatively lower but still contributes to RH.

To examine the switching behaviour in more detail, we repeatedthe measurements shown in Fig. 4c for fields between 0–1,200Oe,and constructed the switching probability phase diagram shown inFig. 4d. Red (blue) colour shows wherem can (cannot) be switchedbetween up and down states with the combined action of j andHx , as determined by averaging ten switching events. We observeseveral interesting features. First, the minimum Hx required forswitching is as low as several tens of Oe. Second, the switchingphase boundary exhibits two regimes above and belowHx=900Oe.



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ARTICLES NATUREMATERIALS DOI: 10.1038/NMAT4812We speculate that for Hx > 900Oe it becomes comparable to aneffective decrease of the anisotropy field due to Joule heating,and thus a significant decrease of the energy necessary to switchm. On the other hand, for Hx < 900Oe the behaviour is quasi-linear, similar to the data reported for, for example, Ta/CoFeB29.Finally, we find that switching is possible for current densities aslow as j=0.5×1011Am−2, much lower than in all-metallic systems,despite the relatively small θSH found with harmonic measurementsin the single-domain state. Since magnetization reversal is mediatedby a domain nucleation and propagation30, this behaviour suggeststhat the damping-like SOT acting on domain walls is far moreefficient than in metallic systems. We also note that Joule heating(see Supplementary Information 7) probably reduces the coercivity,which may play a role in reducing the switching current thresholdat higher current densities.

In summary, we show highly efficient magnetization switchingof an insulating ferrimagnet with PMA utilizing the spin Halleffect of an adjacent Pt layer. Spin Hall magnetoresistanceand harmonic measurements show considerable spin-mixingconductance and damping-like torque, explaining the observedbehaviour. These results open up a range of possibilities from bothfundamental and applied viewpoints. Detection of perpendicularmagnetization in an insulator via the anomalous Hall effectwith unprecedented precision makes characterization of magneticdomain wall dynamics electrically accessible. Furthermore, theharmonic measurement scheme introduced here makes the SHEcharacterization of metal/insulating systems feasible and fast.Finally, the deterministic magnetization switching and its detectionrepresent a significant advance towards the development of room-temperature spintronic devices based on magnetic insulators.

MethodsMethods and any associated references are available in the onlineversion of the paper.

Received 12 April 2016; accepted 12 September 2016;published online 21 November 2016

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AcknowledgementsThe authors would like to acknowledge support from C-SPIN, one of the six SRCSTARnet Centers, sponsored by MARCO and DARPA. A.Q. acknowledges funding fromthe Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) and fromthe Max-Planck-Institute of Microstructure Physics. C.O.A. and C.-F.P. thank K. Uedaand A. J. Tan for fruitful discussions.

Author contributionsG.S.D.B. and C.A.R. proposed and supervised the study. C.O.A., M.M., C.-F.P. andG.S.D.B. designed the transport experiments. A.Q., A.S.T. and M.C.O. fabricated theTmIG samples. A.Q. performed structural and magnetic analysis. M.M. carried outphotolithography processing. C.O.A., M.M. and A.Q. carried out transportmeasurements. M.M. and L.C. designed and established the electrical measurementequipment.

Additional informationSupplementary information is available in the online version of the paper. Reprints andpermissions information is available online at www.nature.com/reprints.Correspondence and requests for materials should be addressed to C.A.R. or G.S.D.B.

Competing financial interestsThe authors declare no competing financial interests.

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http://dx.doi.org/10.1038/nmat4812http://dx.doi.org/10.1038/nmat4812http://dx.doi.org/10.1038/nmat4812http://dx.doi.org/10.1038/nmat4812http://www.nature.com/reprintswww.nature.com/naturematerials

NATUREMATERIALS DOI: 10.1038/NMAT4812 ARTICLESMethodsMaterial growth. A stoichiometric TmIG target was fabricated using high-puritypowders of the respective binary oxides, which were sintered and cold-pressed intoa one-inch diameter target. The material was deposited onto 1 cm-by-1 cmGGG(111) substrates via PLD at a laser wavelength of 248 nm, a fluence of∼1.5 J cm−2, and a target-to-substrate distance of 5 cm. During deposition, thesubstrate temperature was 810 ◦C and the oxygen pressure was 200mtorr. Nopost-deposition annealing process was performed, but the cooling step to roomtemperature at 200mtorr oxygen pressure occurred at−5 ◦Cmin−1.

Device fabrication. After the deposition of the TmIG(8 nm) thin film via PLD, a5-nm-thick layer of the spin Hall metal Pt was deposited by sputtering. The basepressure was 9×10−7 torr, and the deposition rate was 1.5 nmmin−1. Hall crosseswith lateral dimensions of 7 µm× 6 µm were patterned via a top-down ion millingprocess after using a standard photolithography method to define the mesastructures. Ta(4 nm)/Au(100 nm) were deposited as contact pads for electricalmeasurements.

Magnetic characterization. In-plane and out-of-plane hysteresis loopswere measured using VSM, and the out-of-plane loop shape was verified using

a polar MOKE magnetometer. A paramagnetic background from thesubstrate was subtracted from the VSM data shown in Fig. 1b. TheMFMmeasurements were conducted within a scan area of 10 µm2 usinga magnetic Co/Cr-coated antimony-doped Si tip at a lift heightof 20 nm.

Electrical measurements.Hall effect measurements reported in Fig. 2 wereperformed by injecting an a.c. voltage of Vr.m.s.=0.5V with frequencyω/2π=3,678Hz, giving rise to a current of amplitude Ir.m.s.=1.5mA, using astandard lock-in amplifier. A typical device resistance was Rdevice≈300, to whicha capacitor from a bias-T (1 µF) and a resistor (50) were connected in series. Themeasured a.c. Hall voltage was averaged over several scans and was converted to aHall resistance using RH= (Vω/I). To generate measurable torques and perform theharmonic measurements shown in Fig. 3, we increased the voltage to Vr.m.s.=3Vand recorded the second-harmonic Hall voltage. Each curve shown in Fig. 3b,cwas averaged over 200 loops with a field sweep rate of 0.4Hz. To perform theswitching measurements reported in Fig. 4 we have used a bias-T (1 µF–1.05 k) todecouple the output from the pulse generator and the lock-in amplifier. Pulses weregenerated by amplifying a digital-to-analog converter output with a Crown DC150audio amplifier.

NATUREMATERIALS | www.nature.com/naturematerials



Current-induced switching in a magnetic insulatorStructural and magnetic characterizationSpin Hall magnetoresistanceDetermination of damping-like spin–orbit torqueCurrent-induced magnetization switchingMethodsFigure 1 Structural and magnetic properties of PMA TmIG.Figure 2 SMR measurements on a TmIG/Pt bilayer device.Figure 3 Determination of the damping-like SOT in TmIG/Pt by harmonic measurements.Figure 4 Current-induced magnetization switching of TmIG/Pt.ReferencesAcknowledgementsAuthor contributionsAdditional informationCompeting financial interestsMethodsMaterial growth.Device fabrication.Magnetic characterization.Electrical measurements.

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