Perpendicular magnetic anisotropy induced by Rashba spin- orbit interaction

Transcription

1 New Perspectives in Spintronics and Mesoscopic Physics ISSP Kashiwa, U Tokyo 2015 June 16 Perpendicular magnetic anisotropy induced by Rashba spin- orbit interaction 家田淳一 Jun ichi Ieda Advanced Science Research Center (ASRC), Japan Atomic Energy Agency (JAEA), Tokai, Japan In collaboration with S. E. Barnes (U. Miami), S. Maekawa (ASRC, JAEA)

2 Introduction Outline Rashba effects in Spintronics Model Rashba mechanism of magnetic anisotropy Electric- Field Effect Experiments and predictions Summary

3 Rashba Effect H R = α R p ( σ ẑ ) Inversion symmetry breaking - > E(k, ) E(- k, ) Time reversal symmetry - > E(k, ) E(k, ) E. I. Rashba: Fiz. Teverd. Tela (Leningrad) 2 (1960) 1224; Y. A. Bychkov and E. I. Rashba: J. Phys., C 17 (1984) physics of spin- orbit coupling penetrated into numerous branches of condensed matter physics. I guess that our paper became successful because it was one of first, and timely, steps of this journey. by E. I. Rashba from editor blog of JETP Letters

4 Surface Rashba Splitting Au (111) ε = 2 2 k k,σ 2m σα k R = 2 ( 2m k σ k ) 2 0 E R E R = 2 2m k 2 0 = mα R = m eη SO 2 E 2 ~3.5meV ~ 35T! k 2 = k x 2 + k y 2

5 Huge Magnetic Field Bi on Ag (111) ~200meV ~ 2000T! Ast et al. PRL (2007).

6 Rashba Splitting Rashba splitting band: ε! = "2 ( k,σ 2m k σ k ) 2 E # 0 R Spins are in- plane µ B B R = α R k y,k x,0 ( ) Rashba energy: E R = mα 2 R 2! 2 Energy gain ~ 3.5meV for Au(111) ~ 35T!

7 Rashba in Spintronics Domain wall motion [Miron: Nat. Mat. (2011)] Spin filtering [Kohda: Nat. Com. (2012)] Spin- charge conversion [J. Sanchez: Nat. Com. (2013)] Non- equilibrium state: current, magnetization dynamics

8 Aim Perpendicular Magnetic Anisotropy (PMA) è Indispensable properties for MRAM Simple bit structure for efficient magnetization flips High rmal stability (vs. superparamagnetism) High density memory bit Gate control of PMA è Electric control of magnetism We propose Rashba- induced PMA.

9 Question How does in- plane Rashba field lead to PMA? Answer Cooperation with exchange interaction. Ultrathin ferromagnetic film Rashba Field: B R BBR R ky, kx,0

10 Reinforcement exchange Rashba field

11 Model Ferromagnet with Rashba SOI: H = 2 2m k 2 2 x + k y ( ) J 0 S ˆm σ + α ( R k y σ x k x σ ) y Magnetization (Order parameter) ˆ sin cos,sin sin, cos m Rashba Field: B R BBR R ky, kx,0 Ultrathin ferromagnetic film Rashba parameter: α R R SOEz

12 α R & α R2 terms Total field (J 0 S in y- z plane). µ B B tot = J 0 S + µ B B y R sinθ z B Ry sin x B R x ( ) 2 + ( µ B B y R cosθ ) 2 + ( x µ B B ) 2 R! J 0 S + α R k x sinθ J0 S B R y y B Ry cos α R 2 ( J 0 S k 2 cos 2 2 θ + k x y ). 1/2 Fermi sea shift & exchange reinforcement (partial) Rashba splitting

13 Out- of- plane configuration quantization axis JS Rashba Field B R J 0 S Energy gain due to reinforced exchange splitting (no energy gain from Rashba splitting)

14 In- plane configuration J 0 S y BR JS x BR Rashba fields go into exchange (B Rx ) and Rashba (B Ry ) splittings.

15 Rashba- PMA ε k,σ = 2 2m ( k x σ k 0 sinθ ) k y E R sin 2 θ σ ( J 0 S) α R ( k 2 x cos 2 2 θ + k ) y 1/2 Averaging over occupied states Magnetic Anisotropy Energy: E MA E R E R = mα 2 R 2 2 T = 2 2m Rashba splitting gain in- plane MA exchange splitting gain PMA 2T 1 cos JS 0 ~ 35T (Au) k 2 k 2 ( x x ) 2

16 Two interfaces interaction I/F/I tri- layer with gating Magnetic Anisotropy Energy: E MA = E R 1 2T J 0 S E R = m ( α top bottom + α ) 2 R R 2! 2 = me2 η 2! 2 SO top ~ 2me2 2 η SO ( E ) 2! 2 bias cos2 θ ( E top 0 + E ) bias + η bottom SO ( E bottom 0 + E ) bias 2

17 MgO/FeB/MgO (a) Applied Physics Express 6 (2013) Voltage-Induced Magnetic Anisotropy Changes in an Ultrathin FeB Layer Sandwiched between Two MgO Layers Ru cap MgO (1.5 nm) Fe 80 B 20 (1.5 nm) MgO (2.5 nm) Takayuki Nozaki 1;2, Kay Yakushiji 1;2, Shingo Tamaru 1, Masaki Sekine 1, Rie Matsumoto 1;2, Makoto Konoto 1;2, Hitoshi Kubota 1;2, Akio Fukushima 1;2, and Shinji Yuasa 1;2 Appl. Phys. Express 6 (2013) Fe (50 nm) T. Nozaki e Magnetic AnisotropyEnergy E perp t (µj/m 2 ) FeB M in-plane / M S (a.u.) H (Oe) Bias voltage (mv) E Seed/Buffer layers A highly effective anisotropy change was induced positive bias voltages with a slope MgO of (001) 108substrate fj/vm. In addit an even function voltage dependence of perpendicu anisotropy was observed, in contrast with odd funct voltage dependence observed Fig. 1. previously (a) Schematic for illustration a single Mo barrier structure. TheseFe/MgO/Fe results indicate 80 B 20 /MgO that structure. M( sandwich structure 2 2has plane potential magnetic tofields. offerinset a newdrawing route controlling 2me voltage effect. of bottom Fe and top Fe 80 B 20 l SO 2 2T Acknowledgments We thank H. Imamura, H. Maehara, and K. Naka MA E 2 bias 1 for fruitful discussion and E. Usuda for assistance with experiments. h J0S 1) M. Weisheit, S. Fähler, A. Marty, Y. Souche, C. Poinsignon, and D. Givord: Science 315 (2007) ) T. Maruyama, Y. Shiota, T. Nozaki, K. Ohta, N. Toda, M. Mizuguch A. A. Tulapurkar, T. Shinjo, M. Shiraishi, S. Mizukami, Y. Ando, an Suzuki: Nat. Nanotechnol. 4 (2009) 158.

18 I/F/N tri- layer Depending on N- layer Rashba fields cancel Rashba fields add gating experiments distinguish cases.

19 samples. Figu age dependen of thin C energy are ob face anisotrop uated to be $ APPLIED PHYSICS LETTERS 103, (2013) Ru-MTJ. Inte Opposite signs of voltage-induced perpendicular magnetic anisotropy In Ta-MTJ, FIG. 2 change in CoFeBjMgO junctions with different underlayers CoFeB layer) Shiota et al. Appl. Phys. Lett. 103, 082 ization! de!ric Bonell,1,2 Shinji Miwa,1,2 Norikazu Mizuochi,1 Teruya Shinjo,1 Yoichi Shiota,1,2 Fre or works tunnelw and Yoshishige Suzuki1,2,a) 1 applied or hand, Graduate School of Engineering Science, Osaka University,Appl. 1-3 Machikaneyama, Osaka 560Phys. Lett.Toyonaka, 103, (2013) respectively. We use a value for saturation magn 8531, Japan penden 2 charged thin CREST, Japan Science Technology, Honcho, Kawaguchi,l Saitama , Japan for energy 0MS ¼ 1.78 T for Ta-MTJ and l0ms ¼ 1.98 T has not yet (Received 3 June 2013; accepted 7 August 2013; published 23 August 2013) CoFeB asonline measured by vibrating sample magnetometry inb MTJ We report a voltage-induced perpendicular magnetic anisotropy (PMA) Figures change in sputter-deposited samples. 2(c) and 2(d) presentjunctions. applied Oppos TajCoFeBjMgO and RujCoFeBjMgO junctions. The PMA change is quantitatively evaluated by Clear evi, where t is dependence ofwe Eperp field dependence of tunneling magnetoresistanceage for various bias voltages. findtthat CoFeB CoFeB induce rf voltage-ind both sign and amplitude of voltage effect depend underlayer, Ta or layer. Ru, belowlinear changes of on thin CoFeB in were ao CoFeB layer. The rf voltage-induced ferromagnetic resonance spectra also support underlayervoltage materi is app energy are observed in both material-dependent direction of voltage torque. The present study shows that underlayer is one MTJs, and change o a bias W Publishing LLC. of key parameters for controlling voltage effect. V 2013 faceaipanisotropy energy for an electric field of tee. 1 V/nm [ uated to be $33 fj/(vm) for Ta-MTJ andvoltage þ18 fj/e andhave oppos small Ru-MTJ. Interestingly, se two values FIG. 2. Normalized in-plane magnetoutput d Ru (5 nm)jco16fe64b20 (1.4 nm)jmgo (2.0 nm)jthick CoThis Voltage control of magnetic properties has been investi16 ization curves determined from In Ta-MTJ, a negative voltage (positively char 1 3 4,5 multiferroics, or Fe64B20jcap (hereafter called Ta-MTJ and Ru-MTJ, respecgated in magnetic semiconductors, tee. The tunnel magnetoresistance for various CoFeB increases into PMA. Thisbias tendency ag hybrids with piezoelectric materials and ferromagnetic materitively). The layer) MTJ films were patterned pillar-shaped 2 external mag 11,16,20, ) bybias-voltage electron-beamdelithography and Ar als.6,7 Recently, a voltage-induced perpendicular magnetic ani- applied junctionsvoltages (4! 1 lmand or works with TajCoFeBjMgO junctions. magnetic anisotropy sotropy (PMA) change in ferromagnetic metals has been of pendence ion milling.ofsince films with small saturation field are plane w FIG.control 1. Tunnel magnetoresistance astoper a observe function in-planemagnetic magnetic or hand, avoltage-induced positive voltage in film Ru-MTJ (n great interest for magnetization in spintronics devices, energy easy of anisotropy area times thickness of " axis of pillar. C for 1.0 h because it enables room temperature operation with low-power change, annealing temperature of 300 field for (a) Ta-MTJ and (b) Ru-MTJ. The resistance-area product (RA) is PMA. This, for (a) and (b) TaCoFeB, E perptthin CoFeB charged layer) increases " CoFeB 20.5 h (Ru-MTJ) was employed to Relative consumption work function: C for (Ta-MTJ) or 200 and coherent plotted manipulation of magnetization. The Ru-MTJ. insets show for designed junctionmtj size area 4 ' 1and lm.(d) is chosen to m and of(c) has not yet been observed stacks with CoF make CoFeB layers with small saturationin field. Note that, Even though electric field is screened at metal/insulator which are similar to magnitude of voltage effect estifirst-principles calculations, P3 Ta Fe Ru Fe layer structures of magnetic tunnel junctions. The thicknesses of each Opposite directions for22, voltage-ϕ modification =-0.25 <0 ϕ 11-ϕ =0.21 ev >0 Figures according to Refs. and anneal condition does not interface, ϕ a significant of ev surface magnetic anjunctions. in nanometers are8 22 given ininduced parenses. mated from staticlayer magnetoresistance measurements. hybridization at FejMgO in magnetic anisotropy change MgO/Co16Fe64B20/Ta(Ru) C

20 Summary PMA mechanism due to Rashba effect Two energy gain processes: Reinforced exchange splitting - > perp. MA Residual Rashba splitting - > in- plane MA Magnetic anisotropy energy scales with Rashba energy E R and can be changed by applied field E. Ref.: Scientific Reports 4, 4105 (2014).