CURRENT-INDUCED MAGNETIC DYNAMICS IN NANOSYSTEMS

Transcription

1 CURRENT-INDUCED MAGNETIC DYNAMICS IN NANOSYSTEMS J. Barna Department of Physics Adam Mickiewicz University & Institute of Molecular Physics, Pozna, Poland In collaboration: M Misiorny, I Weymann, AM University, Poznan, Poland P Balaz, M Gmitra, Safarik University, Kosice, Slovakia A Fert, V. Cross, CNRS, Orsay, France V Dugaev, Rzeszow University of Technology, Poland R Swirkowicz, M Wilczynski, Warsaw Univ Technology, Poland

2 Outline: Introduction: GMR effect Normal and inverse switching in nanopillars Anomalous behavior of spin torque in asymmetric systems: zero-field current-induced precessions Switching in tunnel junctions and nanoparticles Molecular magnets

3 CIP geometry Giant Magnetoresistance (GMR): resistance variation with magnetic configuration CIP geometry NOBEL PRIZE 2007!

4 Physical mechanism of GMR Interface roughness P AP DOS E F Two well-defined spin channels: Mott s model (strong exchange field, weak spin-flip scattering) strong spin dependence of transport parameters E 1. No interlayer exchange coupling needed 2. Exponential decrease with interlayer thickness 3. Increase with the number of layers 4. Increase with decreasing temperature 5. Generally both bulk and interface scatterings contribute Interface roughness & scattering on impurities

5 Spin current associated with charge current Charge current is accompanied by spin current Spin current density ih + + ( y) = Ψ ( y) σ Ψ( y) Ψ ( y) Ψ( y) j s µ µ σ µ 2m y Spin current in linear response j = ( j + j ) = h / 2 e )( j j ) j s z ( y j 2 3 ee0 jh dε d k ) = Tr [ vi, σ ] + Gk ( ε + ω) v jg ( ε ) 4ω 2π (2π ) s; i µ ( ω 3 µ k

6 Magnetic switching in nanopillars due to spin-polarized current Can spin polarized current change magnetic state? J Slonczewski, JMMM 159,1,1996 L Berger, PRB54, 9353, 1996 I Positive bias Katine et al., PRL 2000

7 In-plane and out-of-plane torque components exerted on the thin (sensing) layer h s = ( j L j 2 s R ) CPP geometry In-plane component = ais ˆ (ˆ s ˆ) S Out-of-plane component = bisˆ Sˆ

8 Unified description of GMR and CIMS Valet & Fert, PRB 1993 Diffusion equation for the distribution function Diffusive current

9 Spin torque components: diffusive regime Co/Cu/Co = ajs ˆ (ˆ s ˆ) = bjsˆ S S Off-plane torque two orders of magnitude smaller than the in-plane one! ˆ

10 In-plane torque component: normal and inverse switching M Gmitra & JB, positive spin asymmetry of the thick magnetic layer dv/di (Ohm) H (koe) P A P I (ma) negative spin asymmetry of the thick magnetic layer FeCr/Cr/FeCr Inverse CIMS Co/Cu/Co Normal CIMS M. A. Darwish, et al. PRL93, 2004 Spin asymmetry of the thick (reference) layer determines whether the CIMS is normal or inverse!

11 Current-induced magnetic dynamics: macrospin model Landau-Lifshitz- Gilbert equation dsˆ dt H eff = γ µ sˆ H = H ext α sˆ dsˆ dt γ M d 0 eff ϕ θ s 3 e z H a z z + ( sˆ e ) e + qm (ˆ s e s ( x ) e x + )

12 Dynamics of switching Magnetic field favoring spin up (H<0) s z I>0 Weak magnetic field (H<0) normal switching behavior Strong magnetic field (H<0) s z I>0 J H H J M.Gmitra & JB, 2006

13 Dynamics of magnetic switching Co/Cu/Co Switching time: ns Precessional states M.Gmitra & JB, 2006

14 H=700Oe H=0 Microwave voltage osillations For a dc current

15 Unconventional angular variation of spin torque Co/Cu/Py e - e -

16 Asymmetric systems (Co/Cu/Py) Initial P state Initial AP state Zero-field precessional states M.Gmitra & Barna, PRL,APL 2006;PRL 2007

17 Py Co/Cu/Py Microwave nanooscillators: amplitude and frequency controlled by voltage (current)

18 Curren-induced dynamics in magnetic tunnel junctions normal torque (ev/µm 2 ) in-plane torque (ev/µm 2 ) 10 (b) (a) l = r =0.98 ev l = r =1.96 ev l = r =2.40 ev Both in-plane and out-of-plane Components of spin torque are Comparable in magnitude Critical currents smaller by two orders of magnitude Current-induced dynamics observed in some tunnel juntions θ/π Variation with the angle M. Wilczynski, J B, R Swirkowicz, PRB 2008

19 Switching magnetic nanoparticles Gate τ d τ s τ ε E τ ch d > h E ch kt 2 E ch = e / 2C R b R q = h / 2e 2 J B, A Fert, PRL 1998

20 Current induced switching of molecular spin valves Gate Examples of molecular magnets

21 Current induced switching of molecular spin valves Gate Examples of molecular magnets

22 Current-induced switching: transport throgh the LUMO level H = H + H + SMM q H T molecule Electrodes: q=l,r Tunneling

23 Spin of the molecule with one or two extra electrons in the LUMO level Two excess electrons,s t = S One excess electron, S t = S ± 1/2 No excess elect- Rons, S t = S M.Misiorny &JB, PRB 2007; Timm & Elste PRB 2007

24 Current induced switching Gate Intrinsic relaxation rate

25 Switching of SMMs by spin current Nonmagnetic leads M. Misiorny & JB, PRB2007; PRB2008

26 Interplay Method: of ofsequential The real-time and diagrammatic cotunneling technique processes: Real-time diagrammatic technique A( t) H t0 t ~ = T exp i dt ' H T ( t' ) I A( t) I T exp i t t0 dt ' H T ( t' ) I Time evolution of the reduced density matrix d 0 d d R, L, R, L, L, R, L, L, d 0 0 d

27 Conductance TMR and noise in SMM based spin valves F=S/2eI M. Misiorny, I Weymann, & JB, PRB2009

28 Current induced switching of molecular spin valves M. Misiorny, I Weymann, & JB, PRB2009

29 Current induced switching due to cotunneling processes M. Misiorny & JB, phys stat sol (b) 2008

30 Conclusions GMR and CIMS are correlated Stationary precessional states can be induced without external magnetic field Dynamics of MTJs is more complex due to large out-of-plane torque Molecular magnets can be switched by a current pulse