Spin pumping and spin transport in magne0c metal and insulator ...

Spin pumping and spin transport in magne0c metal and insulator heterostructures Eric Montoya Surface Science Laboratory Simon Fraser University

Why use spin currents? We can eliminate circumvent these problems: • Joule Hea0ng • Circuit Capacitance • Electron migra0on Outline: • Introduce spin pumping • Spin transport in Au • Spin pumping from magne0c insulator (if 0me) Au Fe Au Fe Au Fe Au YIG Magne0c metal Magne0c insulator

Ferromagnetic Resonance(FMR)Anritsu signal generator: 1-70 GHzelectromagnet: 2.8 T∂ M∂tSpin Dynamics (LLG)Landau Lifshitz Gilbert M = −γ × Heff + α M × ∂ n∂tn =MM sBulk damping is noise in spin orbit (s-o) interactionInterface damping is spin pumpingH eff36GHz !MFMR linewidth:ωΔH = αγGα =γM s

FM1NMSpin Pumping FM2I mm NMm = −gµ B Sx0 LSpin current arises from the 0me retarded response to interlayer exchange coupling The spin current can be expressed as an accumulated magne0c moment (/area) I m = −gµ BM ∂ˆn∂M Re [g ↑↓ ] × ⊗ ˆx= 1 ∂m4πM s ∂t∂t d FM ∂tSpin mixing conductance for metals Re [g ↑↓ ]= 1 [|r ↑,n − r ↓,n | 2 + |t ↑,n − t ↓,n | 2 ]2n|r ↑(↓),n | 2 + |t ↑(↓),n | 2 ∗=1 r ↑,n r ↓,n + t ↑,n t ↓,n 0g ↑↓ = n∗∗[1 − Re[r ↑,n r ↓,n + t ↑,n t ↓,n∗≈ k F 24π ∼ N 2/3Density of electrons per spin direc0on in NM

FM1NMSpin Pumping I mFM2δ sd = v Fτsf τ m3m NM0 LThe accumulated magne0c moment then diffuses away from then interface I m = −gµ BM ∂ˆn∂m Re [g ↑↓ ] × ⊗ ˆxNM= D ∂2 m NM4πM s ∂t∂t ∂x 2 − 1 mNMτ sfxD = v F 2 τ m3Boundary FM1/NM FM1/NM/FM2x =0I m − 1 2 v Fm NM = −D ∂ m NM∂xx = L∂m NM∂x=0 −D ∂ m NM∂x= 1 2 v Fm NM

Spin Pumping Theory 9Α 10 3 s6300 100 200 300 400 500 600dAu ALF b =F(d FM , 4"M S , g , g, # B ) F sd =F(! sf , ! m , d NM , v F )×F b ! sf is only unknown parameter

Sample Growth by MBE • GaAs(001) Template – Atomic H etching – 650eV Ar + spu^ering (con0nuous rota0on) – 4x6 reconstruc0on RHEED monitored • 16Fe and 12Fe have different FMR fields • At 16Fe FMR, 16Fe acts as spin pump and 12Fe acts as spin sink 12Fe (1.7nm)!16Fe (2.3nm)!GaAs(001)!20Au (4.1nm)!300Au (61.2nm)!or!20Au (4.08nm)!Single Layers Double Layers

Charge Transport !!295R 88= 2.5R 10295= 6.1• Van der Pauw measurements – 10K-­‐300K τ m = σm ene 2• Contribu0on due to bulk phonon and interface sca^ering • Using Mathiassen’s Rule: 1= 1 + 1 τ m τ p τ i• Interface sca^ering contribu0on independent of temperature

Ferromagne0c Resonance • FMR followed Gilbert damping phenomenology: ∆H(ω) =α ω γ + ∆H(0)• Enhanced Gilbert damping due to spin pumping is an interface effect • Spin momentum accumulates at the Fe/Au interface

H Oe15010050Gilbert Damping 00 10 20 30 40f GHz• $ sp greatest in ballis0c limit for double layer • $ sp increases with decreasing temperature for double layers • $ sp decreases with decreasing temperature for single layers Α 10 3 963300Au SL 300Au DL 20Au SL 20Au DL 0100 150 200 250 300Temperature K

Relaxa0on parameters Τ 10 13 s2.52.01.51.0τ sf (90K)τ p (90K) = 22.1τ sf (290K)τ p (290K) =8.4• ! sf increases faster than ! p as temperature decreases • ! i very weakly dependent on temperature 0.50.00 50 100 150 200 250Temperature KSpin flip sca^ering dominated by phonon processes Combined influence of temperature dependent spin flip sca^ering at interfaces and bulk phonon sca^ering? or Mul0-­‐phonon sca^ering that does not contribute strongly to resis0vity?

ONCHESKY et al.the boundary condi-Previous Studies J. Appl. Phys. 103, 07C509PHYSICAL REVIEW B 71, 214440 5magnetic moment con-6T. Monchesky et. al. Phys.Rev.B. 71 (2005) B. Kardasz et. al. J.Appl.Phys. 103 (2008) FIG. 6. In situ measurement of the conductivity FIG. as 1. aDependence function of of the additional damping by spin pumping, sp ,u thickness deposited on 28 ML Fe/GaAs001. The two curvesreF1/NM/F2 calculated using first-principles the From bound-parameters are equivalent described by line to AEq.! 80-­‐5nm density functional thickness Au cap layercalculations. of thickness Au ! d Au the Au/16Fe/GaAs001 samples. Ti increases by a factor of 12 he solid line P=0, S ↑ =0.55, S ↓ =0.77 is the best fit given the setfe in Fig. 7 below. sf only increases by a factor of 1.5 The dashed lineP=0.41,M/F2S ↑ =0.03,interfaceS ↓ =0.65arehas 5 wave frequency f, Hf. Hf followed well a linear dependencethe highest The 2 among error thebars paramterson line A, which demonstrates the sensitivity of the fit to thewere determined FIG. 7. Fitting fromofsmall the conductivity slope variations of 20 ML in Au/1 theFe/7 ML Au/28 ML Fe/GaAs001 using first-principles catting parameters. ! measurements. The solid line shows fitting using the spin pumpingsf can only weakly be dependent on interface sca^ering with the following parameters.: tions. The filled points correspond to the parameters which givg˜↑↓ =2.410 15 cm −2 , el =1.210 −147correct sheet resistance for a parallel configuration of magnetihree fitting parameters remained: S ↑ ,S ↓ , and sf =1510P vac/Au . Unforunatelya unique fit to the data was not achieved with the the correct sheet resistance for an antiparallel zero applied−14 s.ments resistance at saturation in Fig. 2 and the open pointsata in Fig. Temperature 6 alone. dependence of ! configuration. The triangles, diamonds, and squares are calcuare The valid magnetoresistance for the case data fromparameters: sf governedAu/Fe/Au/Fe/ intr by mul?-­‐phononfor =3.510 P vac/Au =0, 0.25, −3 scaAering, and g˜↑↓ 0.5, =2.410 respectively. The 15 cm dashed −2 lines , el laaAs001 was used to help determine the specularity −14 pa- A and B −14 are interpolations of the points of intersection whichsymbols represent the measured data from the H dependence on m

!Spin pumping at YIG/Au interfacerecently new ideas and systems being developed for generationof pure spin currents for driving Spin Transfer Torque (STT) devicesJohn Slonczewski has shown higher spin efficiency can be achieved by thermal gradientsusing Magnetic Insulator (MI)/NM heterostructuresJ. Slonczewski, PRB 82, 054403 (2010)new emerging fieldspincoloritronicsArne Brataas and Gerrit Bauer have shown that the spin pumping generationis determined at MI/NM interfaces by spin mixing conductanceg "#?????what is at the YIG/Au interface ????

Spin mixing conductance in magne0c insulators g "# = 1 2( | r "n$ r n#| 2 + | t n"$ t n#| 2)%n!t n"#= 0r n"#=1$ e i% n "#!$n( )g !" ! = 1# cos(! ! n#! " n)B. Heinrich et al. PRL, 107, 066604 (2011)C. Burrowes et al. APL , 100, 092403 (2012)

YIG surface chemistry YIG: Y 3 Fe 2 (FeO 4 ) 3 • Grown on (111) Gd 3 Ga 5 O 12 substrate by PLD at 700C and 0.1Torr O 2 • Thickness d=9nm (low angle XRD) • 4"Ms=1.31kG(SQUID), g=2.027 (FMR) • Surface roughness 0.5nm (AFM) FeOCXPSYAs prepared YIG has surface deficiency of Fe Common for even thick PLD prepared YIG

Spin pumping from YIG 9nmYIGa9nm YIG/6.1nm Au/4.3nm Fe/6.1nm AubAmplitude a.u.∆H = 15.9OeAmplitude a.u.∆H = 21.2OeTransfer of angular momentum to Fe layer is seen as loss of angular momentum in YIG layer 7.3 7.6 7.9H kOe7.3 7.6 7.9H kOe60Damping! 50g !"#1.3$10 14 cm %212% efficiency compared to Fe H Oe4030201000 10 20 30 40frequency GHz

Evalua0ng g !"in Ar+ etched YIG H Oe120100806040200Low angle Ar+ etched10minutes0.8kV, 400 o C0 10 20 30 40f GHz9YIG(etched)/6.1Au/4.3Fe/6.1Au ! = 0.00699YIG(etched)/6.1Au! = 0.0014g !"= 5.1#10 14 cm $270% of predictedby first principles calculationsX. Jia et al. Europhysics Letters 96, 17005 (2011)50% of Fe/Au

XPS on YIG678&./ 01. &2(&6:8&./ 01. &2(&698&34&5&&6$8&34&5&Prolonged etching lead to metallic Fe -­‐> Suppresses pumping !"#$"#%&'#()%*&+(,-&required change for increasedg !"725 720 715 710 705 700E b [eV]metallic state of Fedecreases rapidlyg !"# 3.7$10 14 cm %2

spin pump/sink effectcan be used toinvestigate the spin transport parameters in magnetic nanostructuresConclusions:spin pumping at YIG/Au is efficient70% of theory calc. 50% of Fe/Auevidence that a time retarded interlayer exchange couplingcreates spin pumping!4! "g !"sin 2 #$!4! g !"! 2k T mB YIG&%V cohM sEfficiency of spin pumping comparison( #T Au )J. Xiao et al. PRB 81, 214418 (2010)Microwave driven: for f=10GHz and Θ=90 o 2x10 10Thermal excitation: for ΔT=10 K , V coh =2.7x10 3 nm 3 ω eff =2x10 2 MHz 1.0x10 8STT (60% polarization): for 2x10 6 Acm -2 : 2x10 10')(( !T Au )"! eff= " 2k BT mYIG$#V cohM s!%'&nm -2

Simon Fraser UniversityPhysics, MagnetismDr. Bret Heinrich Prof. Emeritus Eric MontoyaPhD CandidateDr. Bartek Kardaszresearch associateDr. Capucine BurrowesPDFDr. Erol GirtProfessorCharles EyrichMSc CandidateDr. Monika AuroraPhD CandidateDr. Wendell HuttemaPDFartist: Mr. Clarence MillsKen MyrtleResearch AssistantProf. Mingzhong Wu, Dr.Young-Yeal Song, and Dr. Yiyan SunPhysics DepartmentColorado State UniversityFort Collins, USAAcknowledgements:NSERC, CIfAR , NSF, NIST

Conclusions Temperature dependence of ! sf Thickness dependence of ! sf from 290K to 90K ! sf increases by factor of 10 ! p increases by factor of 4 ! i negligible dependence from 80nm to 5nm ! sf increases by factor of 1.5 ! p constant ! i increases by factor of 12 Temperature dependence of ! sf governed by mul0-­‐phonon sca^ering

deposition of Fe on bare YIGresults in metallic state of Fe1 MLFe depositedat 500 o Cno spin pumping704 708 712 716Binding Energy [eV]metallic state of FeYIG state of FeYIG surface H atom etching showed in XPSa strong presenceof metallic state of Feno spin pumping704 708 712 716 720Binding Energy [eV]spin current blockade by metallic Fe