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

Transcription

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

2 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

3 Ferromagnetic Resonance (FMR) Anritsu signal generator: 1-70 GHz electromagnet: 2.8 T M t Spin Dynamics (LLG) Landau Lifshitz Gilbert M = γ Heff + α M n t n = M M s Bulk damping is noise in spin orbit (s-o) interaction Interface damping is spin pumping H eff 36GHz! M FMR linewidth: ω ΔH = α γ G α = γm s

4 FM1 NM Spin Pumping FM2 I m m NM m = gµ B S x 0 L Spin 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 B M ˆn M Re [g ] ˆx = 1 m 4πM s t t d FM t Spin mixing conductance for metals Re [g ]= 1 [ r,n r,n 2 + t,n t,n 2 ] 2 n r ( ),n 2 + t ( ),n 2 =1 r,n r,n + t,n t,n 0 g = n [1 Re[r,n r,n + t,n t,n k F 2 4π N 2/3 Density of electrons per spin direc0on in NM

5 FM1 NM Spin Pumping I m FM2 δ sd = v F τsf τ m 3 m NM 0 L The accumulated magne0c moment then diffuses away from then interface I m = gµ B M ˆn m Re [g ] ˆx NM = D 2 m NM 4πM s t t x 2 1 mnm τ sf x D = v F 2 τ m 3 Boundary FM1/NM FM1/NM/FM2 x =0 I m 1 2 v F m NM = D m NM x x = L m NM x =0 D m NM x = 1 2 v F m NM

6 Spin Pumping Theory 9 Α 10 3 s dau AL F 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

7 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

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

9 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

10 H Oe Gilbert Damping f 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 Au SL 20Au SL 300Au DL 20Au DL Temperature K

11 Relaxa0on parameters Τ s τ sf (90K) τ p (90K) = 22.1 τ sf (290K) τ p (290K) =8.4! sf increases faster than! p as temperature decreases! i very weakly dependent on temperature Temperature K Spin 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?

12 Previous Studies T. Monchesky et. al. Phys.Rev.B. 71 (2005) B. Kardasz et. al. J.Appl.Phys. 103 (2008) From 80-5nm thickness of Au! i increases by a factor of 12! sf only increases by a factor of 1.5! sf can only weakly be dependent on interface sca^ering Temperature dependence of! sf governed by mul?- phonon scaaering

13 Spin pumping at YIG/Au interface recently new ideas and systems being developed for generation of pure spin currents for driving Spin Transfer Torque (STT) devices John Slonczewski has shown higher spin efficiency can be achieved by thermal gradients using Magnetic Insulator (MI)/NM heterostructures J. Slonczewski, PRB 82, (2010) new emerging field spincoloritronics Arne Brataas and Gerrit Bauer have shown that the spin pumping generation is determined at MI/NM interfaces by spin mixing conductance?????what is g "# at the YIG/Au interface????

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

15 YIG surface chemistry YIG: Y 3 Fe 2 (FeO 4 ) 3 Fe XPS 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) O C Y As prepared YIG has surface deficiency of Fe Common for even thick PLD prepared YIG

16 Spin pumping from YIG 9nmYIG a 9nm YIG/6.1nm Au/4.3nm Fe/6.1nm Au b Amplitude a.u. H = 15.9Oe Amplitude a.u. H = 21.2Oe Transfer of angular momentum to Fe layer is seen as loss of angular momentum in YIG layer H koe H koe 60 Damping! 50 g!" #1.3$10 14 cm %2 12% efficiency compared to Fe H Oe frequency GHz

17 Evalua0ng g!" in Ar+ etched YIG H Oe Low angle Ar+ etched 10minutes 0.8kV, 400 o C f GHz 9YIG(etched)/6.1Au/4.3Fe/6.1Au! = YIG(etched)/6.1Au! = g!" = 5.1#10 14 cm $2 70% of predicted by first principles calculations X. Jia et al. Europhysics Letters 96, (2011) 50% of Fe/Au

18 XPS on YIG 678&./ 01. &2(& 6:8&./ 01. &2(& 698& 34&5&& 6$8& 34&5& Prolonged etching lead to metallic Fe - > Suppresses pumping!"#$"#%&'#()%*&+(,-& required change for increased g!" E b [ev] metallic state of Fe decreases rapidly g!" # 3.7$10 14 cm %2

19 spin pump/sink effect can be used to investigate the spin transport parameters in magnetic nanostructures Conclusions: spin pumping at YIG/Au is efficient 70% of theory calc. 50% of Fe/Au evidence that a time retarded interlayer exchange coupling creates spin pumping! 4! "g!"sin 2 # $! 4! g!"! 2k T m B YIG & % V coh M s Efficiency of spin pumping comparison ( #T Au ) J. Xiao et al. PRB 81, (2010) Microwave driven: for f=10ghz and Θ=90 o 2x10 10 Thermal excitation: for ΔT=10 K, V coh =2.7x10 3 nm 3 ω eff =2x10 2 MHz 1.0x10 8 STT (60% polarization): for 2x10 6 Acm -2 : 2x10 10 ' ) ( (!T Au ) "! eff = " 2k B T m YIG $ # V coh M s! % ' & nm -2

20 Simon Fraser University Physics, Magnetism Dr. Bret Heinrich Prof. Emeritus Eric Montoya PhD Candidate Dr. Bartek Kardasz research associate Dr. Capucine Burrowes PDF Dr. Erol Girt Professor Charles Eyrich MSc Candidate Dr. Monika Aurora PhD Candidate Dr. Wendell Huttema PDF artist: Mr. Clarence Mills Ken Myrtle Research Assistant Prof. Mingzhong Wu, Dr.Young-Yeal Song, and Dr. Yiyan Sun Physics Department Colorado State University Fort Collins, USA Acknowledgements: NSERC, CIfAR, NSF, NIST

21 Conclusions Temperature dependence of! sf from 290K to 90K! sf increases by factor of 10! p increases by factor of 4! i negligible dependence Thickness dependence of! sf 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

22 deposition of Fe on bare YIG results in metallic state of Fe 1 MLFe deposited at 500 o C no spin pumping Binding Energy [ev] metallic state of Fe YIG state of Fe YIG surface H atom etching showed in XPS a strong presenceof metallic state of Fe no spin pumping Binding Energy [ev] spin current blockade by metallic Fe