Magneto-optic Studies Of Spin Dynamics And Spin Torque In .
Topological Spintronics Workshop, May 13, 2016Magneto-optic Studies of Spin Dynamics andSpin Torque in High Spin-Orbit MaterialsRoland KawakamiDepartment of PhysicsThe Ohio State University
AcknowledgementsStudents & PostdocsBeth BushongYunqiu Kelly LuoDante O’HaraMichael NewburgerSimranjeet SinghAdam AhmedIgor PinchukCollaboratorsKathleen McCreary (NRL)Berend Jonker (NRL)
Outline Overview Spin Dynamics in Transition Metal Dichalcogenides Spin Torque Dynamics in FM/HM bilayers Summary
Overview: Spin-Orbit Coupling in 2D MaterialsLow spin-orbit coupling is good for spin transportPicture ofGraphene exhibits spin transport at room temperaturewith spin diffusion lengths up to tens of micronsW. Han, RKK, M. Gmitra, J. Fabian,Nature Nano. 9, 794–807 (2014)
Overview: Spin-Orbit Coupling in 2D MaterialsHeavy GrapheneGraphene (C)Transition MetalDichalcogenides (TMD)Silicene (Si) Germanene (Ge) Stanene (Sn)WeakSPIN ORBIT COUPLINGMoS2 (TMD)Strong Long spin lifetimes Spin Hall effect Spin Transport at RT Quantum spin Hall effectA wide range of spin-dependent phenomena can be arealized in 2D materials by tuning spin-orbit coupling
Overview: Spin-Orbit Coupling in 2D Materials2D Spin Transport Channels(Low SOC)Graphene2D Insulators/Barriershex. Boron NitridePhosphorene2D Ferromagnets2D Spin-Optical MaterialsTMDs(?) Mn:WSe2(?) GeCrTe3(?) Doped Graphene2D Spin Hall Materials,(High SOC)2D Topological Materials(?) StaneneTMDs(?) TMDs(?) Heavy graphene(?) Layered ZintlUnprecedented ability to combine properties throughvertical stacking and proximity effects
Outline Overview Spin Dynamics in Transition Metal Dichalcogenides Spin Torque Dynamics in FM/HM bilayers Summary
Monolayer Transition Metal DichalcogenideMonolayer TMD, such as WS2, with hexagonal structure and inversion symmetry breakingWS2Spin-valley coupling due to large spin-orbit interaction
Monolayer Transition Metal Dichalcogenide(a)-KK-KIntensity (a.u.)(c)-Kσ-KΓKKσ -K(d)100006KBerry curvatureValley Hall Effect5000 spin-valley couplingTheory: D. Xiao et al, PRL108, 196802 (2012)Spin Hall Effect0600700Linear ProExperiment: K. F. Mak et al,344, 10001489 (2014)800Science900
Ultrafast Optical Microscopy of Spin Dynamics inTransition Metal Dichalcogenides(a)-KK-KWS2Intensity (a.u.)(c)-KKΓKKσ-σ -K(d)100006K What is the spin lifetime of WS2?5000 Strong Berry curvature for spin/valley Hall effect. How are the spin and valley0 degrees of freedom600coupled?(e)700 800 900 1000Wavelength (nm)1.06KLinear Pro
Chemical Vapor Deposition Grown WS2 High quality, large area,single layer flakes n-type WS2 From collaborators atNaval ResearchLaboratory (NRL),Kathleen McCreary andBerry Jonker20 mm
Monolayer WS2 Photoluminescence6.2 K 532 nm excitation Monolayer TMDs showstrong PL, with no PL atlower energies Lower energy peaksno indirecttransitionindicate an indirect gaptransition, characteristic ofmulti-layer WS2 PL peak is at 630 nm (Aexciton)
Time Resolved Kerr Rotation Microscopy Layout 625 nm wavelength 76 MHz rep rate 150 fs pulse widthDelay line to adjustpump-probe time delay
Time Resolved Kerr Rotation Microscopy Layout
Recent Developments in TRKR on TMDZhu, et al. Phys. Rev. B 90, 161302(R) (2014).WSe2: 6 ps at 4 K, 1.5 ps at 125 KPlechinger, G., Nagler, P., C., S. & Korn, T. ArXiv:1404.7674 (2014).MoS2: 10 ps at 4 KDal Conte, S. et al. ArXiv: 1502.06817 (2015).MoS2: 5 ps at 77 KYan, T., et al. arXiv:1507.04599v1 (2015).WSe2: 120 ps at 10 KYang et. al (Crooker), Nature Phys. 11, 830 (2015).MoS2: 5 ns at 10 K, signals up to 40 KIntervalley scattering model for spinrelaxationHsu, W.-T., et al., Nat. Commun. 6:8963 doi:10.1038/ncomms9963 (2015).WSe2: 1 ns at 10 K, signals up to RTThis work: Bushong et. al., arxiv: 1602.03568 (2016) WS2: Imaging TRKR
Time Resolved Kerr Rotation of WS2T 6.2 Kt 3 psBi-exponentialdecayt 5.6 nsMonolayer WS2 exhibits long spin lifetimes
Spatial Mapping of the Kerr RotationΔt 300 psSpatial variation of spin polarization in WS2
Time Resolved Kerr Rotation MappingSpatial variation of spin density in WS2
High Resolution Imaging of Spin Dynamics5 mmKerrRotation(a.u.)1.5Dt 80 psDt 250 psDt 600 ps1.00.50.0Dt 2000 psDt 4000 psDt 11000 psImages appear to be more symmetrical withincreasing time delay
Spatially Resolved PhotoluminescencePhotoluminescenceKerr Rotation5 mmDt 80 psDt 250 psDt 600
10TRKR vs. Photoluminescence5Dt 80 ps20(a)5 mm1510y 15 mmy 15 mmy 11 mmy 11 mmy 7 mmy 7 mmy 3 mmy 3 mm5010Regions of brighty 11 mmPL haveshort spinlifetimes(d)15y 15 mm105y 11 mm0600640680720Wavelength (nm)360064068072020(e)1510503.02.52.01.51.001Kerr rotation (a.u.)0.5y 7 mm0.0y 7 mmy position (mm)y 15 mm32-320(c)1minescence Intensity (a.u.)0b)2Time Delay (ns)x10Dt 80 ps0PL intensity (a.u.)1y position (mm)Kerr Rotation (a.u.)y position (mm)(c)Photoluminescence Intensity (a.u.)(b)0
Possible Explanation(a)-KKKσ -K(d)Short10000Spin Lifetime,Intensity (a.u.)σ-Strong PhotoluminescenceLong Spin Lifetime,6KWeak PhotoluminescenceθK5000Linear ProbeOb0600(e)700 800 900 1000Wavelength (nm)CircularPump.)1.06K2.0)(c)5 μm-KSelectively excite spins-K bandΓinto the conductionK(b)K
-K(b)K0K 01-Kσ1020BSO (T)σ 30Spin-OrbitStabilizedConventional0-1BSO 2 T(d)BSO 25 T0 0.5 1 61.5K 2 0 1 23 4 5700 mT02010500 mT0BSO (T)1Conventional0-1300 mT5 μmBSO 2 T8(e)6423000200Spin-OrbitMagnetiStabilized(f)BSO 254 T0 0.5 1 1.5 2 0 1 2KerrRotation (a.u.)5-KΓalculated SZ (a.u.)(c)Kerr Rotation(a.u.)CalculatedSZ (a.u.)CalculatedLifetime (ns)(b)Spin Lifetime (ns)(d)(a)5Spin Lifetime (ns)CalculatedLifetime (ns)(b)Kerr Rotation (a.u.)Role of spin-orbit splitting33 4 5100TimemT Delay (ns)Time Delay (ns)20stabilized4. Spin-orbitFigure scatteringIntervalleymodelfor spin spins in W1 B (bluemagne?c fieldand transverse0 mTextrelaxationlife?me as a func?on of spin-orbit field, cYang et. al (Crooker), Nature Phys.0 11,At low spin-orbit coupling, the spins exh830 (2015).0 couplin50 1024At much6 larger8 spin-orbitcurve).θ
In-Plane Magnetic Field DependenceNon-precessing spin in the spin-orbit stabilized regime
In-Plane Magnetic Field DependenceZoom in with finer scans:Zoomed in and offsetOscillations are 3% of total signal
In-Plane Magnetic Field Dependence1.5nL (GHz)1/T2* (GHz)201.01080.5000.0200 400 600 8000Magnetic Field (mT)200 400 600 800Magnetic Field (mT)Small population of precessing spins
40TemperatureDependence0200 400600 800300 mTSpin Lifetime (ns)(f)100 mT0 mT43KerrRotation (a.u.)Magnetic Field (mT)030 K60 K110 K180 K2Time Delay (ns)42106elay (ns)8050 100150 200Temperature (K)m of the ini?al spin direc?on S (red arrow)in the plane of the WS island. b, Spin
OutlookNext stepsImage Dynamics of Spin Hall EffectTune Fermi level-K(b)KEF-KKΓσ-5 μmσ -K(d)6KθKLinear Probe100x
Outline Overview Spin Dynamics in Transition Metal Dichalcogenides Spin Torque Dynamics in FM/HM bilayers Summary
Spin Torque Dynamics in FM/HM BilayersUse TRKR microscopy to image magnetization switchingdynamics Spin-orbit torque switching Magneto-electric switching .FMMJCHeavy Metal (HM)JS Sub ps temporal resolution explore faster switching mechanisms Submicron spatial resolution
Quantifying spin-orbit torquesSpin-orbit torqueschange the equilibriumdirection of MQuantify the Anti-Damping Torque and Field-Like Torque
Quantifying spin-orbit torquesOur data: Pt(6nm)/Fe(4nm)John Xiao’s data:Pt(5nm)/CoFeB(0.85nm)90V (mV)80Regular LongitudinalMOKE hysteresis loop70605040-20-1001020Magnetic Field (mT)9.2SOT polar MOKESOT polarMOKEHeff MeffhSOT tT (m x s)V (mV)Dmpolar hSOT hoersted9.19.08.912 mA10 mA8.8-20-10010Magnetic Field (mT)20
MBE growth of magnetic multilayersMBE growth: Fe, Pd, Cu, Bi, AgFe on MgO(001)Cu on Pd(001)Pd on Fe(001)Fe on Cu(001)Bi0.03Pd0.97 on Fe(001)Cu on Fe(001)
Summary Observed complex spatial depedence of spin density in WS2 Anticorrelation between PL and TRKR in WS2 Spin in WS2 are stabilized by spin-orbit against external fields andthermal fluctuations Progress on spin-orbit torques in FM/HM bilayers
Low spin-orbit coupling is good for spin transport Graphene exhibits spin transport at room temperature with spin diffusion lengths up to tens of microns Picture of W. Han, RKK, M. Gmitra, J. Fabian, Nature Nano. 9, 794–807 (2014) Overview: Spin-Orbit Coupling in 2D Materials
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Spin current operator is normally defined as: Issues with this definition: 1. Spin is not conserved so the spin current does not satisfy the continuity equation 2. Spin current can occur in equilibrium 3. Spin currents also can exist in insulators 4. Spin current cannot
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