Spin transfer & Current-induced magnetization reversal Andr - PowerPoint PPT Presentation

Spin transfer & Current-induced magnetization reversal André Thiaville Laboratoire de physique des solides Université Paris-sud, Orsay France European School of Magnetism, 1 Constanta, 2005: André THIAVILLE

Electronic structure of magnetic 3d metals s The simple s-d model d electrons : localized, carry magnetism d s electrons : delocalized, carry current E F 2 ne τ σ = m π 2 1 2 τ = V k T N ( E ) diff B F h Minority σ ↑ > σ Majority usually ↓ electrons electrons European School of Magnetism, 2 Constanta, 2005: André THIAVILLE

Spin transfer : principle I (large) F2 F1 electrons After reorientation of their p spin to m , an angular F1 momentum has been given m F2 European School of Magnetism, 3 Constanta, 2005: André THIAVILLE

Magnitude of the spin transfer effect J : current density [C/(m 2 s)] m 1 m r ( ) r r J h − = per unit surface dt s s P d L e 2 s 1 2 M r s 1 r = − Ultrathin layer of thickness D s L D m γ r µ Jg P ( ) ( ) r r r r r  d m 1 = − = × ×  B m m m m m  τ dt 2 eM D 1 ⊥ 1 − spin transfer s European School of Magnetism, 4 Constanta, 2005: André THIAVILLE

LLG + spin transfer term r r r ( ) r r r r r d m d m 1 = γ × + α × + × × H m m m m m τ dt dt 0 eff 1 m 1 m r  d m   dt spin transfer destabilizing stabilizing European School of Magnetism, 5 Constanta, 2005: André THIAVILLE

Sign of the spin transfer effect (mnemonics) given Favors a parallel in alignment electrons in out out of F to F1 m 1 m given Favors an anti-parallel out alignment out in electrons in of F to F1 m 1 m European School of Magnetism, 6 Constanta, 2005: André THIAVILLE

Electron motion through a multilayer θ > = θ ↑> + θ ↓> cos( / 2 ) sin( / 2 ) out in electrons m 1 m Spin-dependent transmission and reflection European School of Magnetism, 7 Constanta, 2005: André THIAVILLE

Order of magnitude of the current needed r r r ( ) r r r r r d m d m 1 = γ × + α × + × × H m m m m m τ dt dt 0 eff 1 1 Stability calculation = α γ M τ 0 s critical 2 αµ M e D = 0 s J P critical h α = = = = 0 . 01 , M 0 . 8 MA / m , D 3 nm , P 30 % s 11 2 ⇒ = J 1 . 3 10 A / m c European School of Magnetism, 8 Constanta, 2005: André THIAVILLE

Samples have to be small Oersted field associated with the current 2 R I R = = H Oersted J π 2 R 2 1 γ = H τ − transfer 0 spin 2 = π I J R P h > ⇔ < H H R µ − e M D spin transfer Oersted 0 s = µ = = → < P 1 , M 1 T , D 3 nm R 200 nm 0 s European School of Magnetism, 9 Constanta, 2005: André THIAVILLE

First demonstration of current-induced magnetization reversal diameter ≈ 150 nm 5 10 11 A/m 2 J.A. Katine et al. Phys. Rev. Lett. 84 , 3149 (2000)

size ≈ 60x100 nm 2 5 10 11 A/m 2 F.J. Albert et al. Appl. Phys. Lett. 77 (2000)

The second experimental demonstration of current-induced magnetization reversal Pillar cross-section : 200 x 600 nm 2 J. Grollier et al., Appl. Phys. Lett. 78, 3663 (2001) European School of Magnetism, 12 Constanta, 2005: André THIAVILLE

Main sample architectures Pillars Point Contacts z e - e - H European School of Magnetism, 13 Constanta, 2005: André THIAVILLE J. Miltat, Nano Spin School , Cargese 05-2005

Experimental results : point contact geometry W. H. Rippard et al., PRL'2004 Point Contact Geometry : ≈ 40 nm diameter ( ) ∆ R ≈ ∆ R Max 1 − m 1 ⋅ m 2

Experimental Results : Pillar Geometry A Rather Complex Set of Experimental Results S. I. Kiselev et al., NATURE'2003

Experimental Results : Pillar Geometry with Exchange Biasing at a Skewed Angle I. Krivorotov et al., Science'2005 European School of Magnetism, 16 Constanta, 2005: André THIAVILLE

Energy and spin transfer effect r µ Jg P ( ) ( )  r r r r r d m 1 = − = × ×  B m m m m m τ  dt 2 eM D ⊥ 1 1 − spin transfer s Effective field for the spin transfer term r × r 1 = H m m γ τ eff, spintransf er 1 0 m 1 r θ r r r r d 1 = − µ = × = dE M H . d m ( m m ). d m γ τ γτ 0 s eff 1 No energy term to be associated with the spin transfer torque term ! θ m European School of Magnetism, 17 Constanta, 2005: André THIAVILLE

Elliptical Elements M x H Eff European School of Magnetism, 18 Constanta, 2005: André THIAVILLE

Micromagnetic Regime: Precessional States (T=300 °K) Eigenmode Driven Mode B. Montigny & J. Miltat, J. Appl. Phys. 97 10C708 (2005) European School of Magnetism, 19 Constanta, 2005: André THIAVILLE

Micromagnetics vs experiments � Switching, generation of microwaves, are qualitatively reproduced perfectly, but � Computed Power Spectral Density line widths too large when compared to experimental data even in the MS approximation � Comparison between experiments and micromagnetic simulations strongly suggest that micromagnetics leads to excessive spatial incoherence � At the same time, extremely narrow line widths, even in the pillar geometry, call for markedly weakly damped systems � Such features seem hardly compatible within the framework of existing theories European School of Magnetism, 20 Constanta, 2005: André THIAVILLE J. Miltat, Nano Spin School , Cargese 05-2005

Giant magnetoresistance (GMR) I (CIP) I (CPP) R AP = 2R P R P CIP : needs layers thinner CPP : needs layers only than the mean free path thinner than the spin diffusion length European School of Magnetism, 21 Constanta, 2005: André THIAVILLE

Current polarization variation l sf Co Cu j ↑ Majority spins j ↓ σ = σ j / j / δ = = m j j j / 2 ↑ ↑ ↓ ↓ ↑ ↓ = σ σ = α j / j / Spin ↑ ↓ ↑ ↓ = P 0 accumulation − j j α − M.D. Stiles, A. Zangwill, 1 = ↑ ↓ = = β P + α + J. Appl. Phys. 91 , 6812 (2002) j j 1 ↑ ↓ European School of Magnetism, 22 Constanta, 2005: André THIAVILLE

A non-collinear spin enters Ferromagnetic metal E E F θ > = θ ↑> + θ ↓> ( 0 ) cos( / 2 ) sin( / 2 ) ∆ θ > = θ ↑> ( x ) exp( ik x ) cos( / 2 ) n ↑ ( E ) n ↓ ( E ) ↑ + θ ↓> exp( ik x ) sin( / 2 ) ↓ > k ( E ) k ( E ) ↑ ↓ E π π 2 4 = ≈ F 1 nm − ∆ k k k ↑ ↓ F Disparition of the transverse component European School of Magnetism, 23 Constanta, 2005: André THIAVILLE

Wall displacement by current Pulses : 0.5 µ s 1.2 10 12 A/m 2 A. Yamaguchi et al. Phys. Rev. Lett. 92 077205 (2004) European School of Magnetism, 24 Constanta, 2005: André THIAVILLE

Conclusions A fascinating field Experiments are far away before theory Just putting the simple Slonczewski term into LLG does not suffice to explain quantitatively everything European School of Magnetism, 25 Constanta, 2005: André THIAVILLE