Spin-electronics: a new challenge in science and technology ``Teaching electrons new tricks

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Claude Skinner
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1 Spin-electronics: a new challenge in science and technology ``Teaching electrons new tricks P. Bruno Max-Planck Institute of Microstructure Physics, Halle, Germany Summary: Introduction: what is spin-electronics? fundamental spin-electronics effects: - giant magneto-resistance - tunneling magneto-resistance and their applications: - heads for magnetic disks - magnetic random-access memories further spin-electronic effects: - spin-injection in semi-conductors - spin-transistors perspectives further documentation: what is an electron? particle with negative electric charge q = - e and spin 1/2 (magnetic moment m = µ B ) =

2 electron as seen by an electronician: electronics = manipulation of electrons by using their charge for storage and processing of information the spin is (almost) completely neglected principal electronic device: MOSFET source metallic gate oxide drain (SiO 2 ) application: logic gates random access memory inconvenients: volatility of the information energy consumption limited density of information semi-conductor (Si) + - metallic gate conducting channel (2D electron gas) transistor blocked

electron as seen by a magnetician: - - - - - - - purpose of magnetism: develop materials in which the electron spins tend to align parallel to each (magnets) the charge of the electrons plays a

3 electron as seen by a magnetician: purpose of magnetism: develop materials in which the electron spins tend to align parallel to each (magnets) the charge of the electrons plays a secondary role application: mass storage of information (magnetic disks and tapes) advantages: non-volatility high storage density no energy consumption inconvenients: mechanical access to information

4 Purpose of spin-electronics: ``Teaching electrons new tricks combine electronics and magnetism in order to make new devices in which both the charge and the spin of the electron play an active role new fundamental physical questions: interface ferromagnet / semi-conductor injection of spin-polaried electrons into a semi-conductor spin-polaried electronic transport in a semi-conductor (problem of spin-relaxation)... new phenomena: giant magneto-resistance tunneling magneto-resistance... new devices and applications: read-out heads for magnetic disks increased storage capacity magnetic random access memories (M-RAM) (non volatile)... Giant magneto-resistance (GMR) Baibich et al., PRL 61, 2472 (1988) Binasch et al., PRB 39, 4828 (1989) ferromagnetic metal (Fe, Co,...) current in-plane (CIP) non-magnetic metal (Cu, Ru,...) current perpendicular to the plane (CPP)

5 mechanism of GMR: spin-dependent scattering two-current model ferromagnetic (F) configuration R F < R AF antiferromagnetic (AF) configuration RAF R A F can be larger than 50% RAF + RF

6 Applications of GMR: reading head for magnetic disks 1 GByte drive

7 Tunneling magneto-resistance (TMR) Jullière, Phys. Lett. 54A, 225 (1975) Moodera et al., PRL 74, 3273 (1995) insulating barrier θ ferromagnetic electrodes Mechanism of tunneling magneto-resistance parallel (P) configuration G P > G AP antiparallel (AP) configuration

8 Applications of TMR: magnetic random access memories (M-RAM) "bit" lines tunnel barrier FM electrodes "word" lines hot-electron spin-transistor Monsma et al. PRL 74, 5260 (1995) Science 281, 407 (1998)

9 Electric field control of ferromagnetism in semi-conductors H. Ohno et al.,nature 408, 944 (2000) In 1-x Mn x As Mn 2+ (S=5/2), p-dopant

10 H. Ohno et al.,nature 408, 944 (2000) Spin injection into a semi-conductor Ohno et al., Nature 402, 790 (1999)

11 transport of spin-polaried electrons in a semi-conductors spin-injection by optical orientation hν E g SO

12 Kikkawa et al., Nature 397, 139 (1999) spin-injection into a superconductor

13 Soulen et al., Science 282, 85 (1998) Magnetic switching due to spin-injection

14 AF differential resistance F current density Myers et al., Science 285, 867 (1999) Katine et al., PRL 84, 3149 (2000) coherent spin-polaried transport through a carbon nanotube Tsukagoshi et al., Nature 401, 572 (1999)

15 Perspectives and conclusion: spin-electronics = new field in science and technology combining electronics and magnetism new fundamental problems strong potential for technological application: - GMR magnetic sensors (automobile and mechanical industry) - GMR reading heads for magnetic disks and tapes (increased capacity) - magnetic RAM (non-volatile, high density) long-term perspectives: use of the spin-coherence to perform quantum computation in solid state devices

16 Paradigm: Datta-Das transistor wide gap III-V SC Schottky gate FM source FM drain x narrow gap III-V SC conduction channel Datta and Das, APL 56, 665 (1990) What is the Rashba effect? H = h 2 2 k 2m + V + α ( k ˆ ) σ B eff 2DEG y x

17 Physical origin of Rashba effect: combination of spin-orbit interaction and structural asymmetry s p ) ( V H SO ε ( ) σ k k + + = ˆ α V m H h B eff = SO 2 ) ( 1 ) ( 1 d d 2 ), ( g E g V E V P ε ε ε α Envelop function approximation de Andrada e Silva et al., PRB 50, 8523 (1994) ( ) 2, ), ( d ε ψ ε α α = ε E g SO E g SO

18 Electronic properties of a 2DEG with Rashba effect ε H = h 2 k 2 2m + V + α ( k ˆ ) σ Beff k y k x k y k x Comparison with the spin-splitting in a ferromagnet ε k y k x k y k x Experimental evidence of Rashba effect: beating features in Shubnikov de Haas oscillations Das et al., PRB 39, 1411 (1989)

19 Weak antilocaliation due to (Rashba) spin-orbit interaction Knap et al., PRB 53, 3912 (1996) B (T) ``Tuning of the Rashba effect via a gate voltage Nitta et al., PRL 78, 1335 (1997)

20 Engels et al., PRB 55, R1958 (1997) 1D motion coherent spin precession B eff x e _ 2D motion incoherent spin precession ( D yakonov-perel mechanism) B eff y x

21 Spin-relaxation (2D) momentum relaxation time: τ P mean free path: L P = v F τ P diffusion constant: D~ v 2 F τ P spin-precession time: τ S spin-precession length: L S = v F τ S precession angle between two collisions: θ = τ P / τ S = L P / L S case of weak spin-orbit coupling (θ << 1) angle of diffusion in spin-space after N collisions: α N ~ θ N 1/2 number N * of collisions necessary for having full spin-depolariation: π ~ θ N* 1/2 spin-relaxation time: τ * = N* τ P ~ τ S2 / τ P (increases with disorder!!!) spin-diffusion length: L * ~ ( D τ * ) 1/2 ~ L S (independent of disorder) case of strong spin-orbit coupling (θ 1 or θ > 1 ) spin-relaxation time: τ * ~ τ P spin-relaxation length: L * ~ L P Model for electronic transport calculations ideal lead central region (disorder, Rashba effect...) ideal lead y W x L H = 2 h 2m 2 + V( r) iα ˆ σ 2 r r discretiation on a square lattice of parameter a tight-binding model Relevant physical parameters: mean-free path: L P lenght for a spin precession of 2π: L S Fermi-wavelength: λ F width W and length L of the channel

22 Conductance within the Landauer-Büttiker formalism C = C + C + C + C spin-conserving spin-flip spin-resolved conductance: 2 σ σ e σσ C = T ( εf ) h transmittance for input spin σ, output spin σ, and energy ε F : 0 1 N N+1 σσ σ σ σσ σ σ T ( εf ) = Tr Γ1 G 1N ΓN G N1 + H + 0,1 G H, N N, N + 1 (0)+ G 0,0 1 (0) + G N+ 1N, + 1 (0) + (0) ( G G ) 0, 1 Γ 1 = i H1,0 0,0 0,0 H recursive calculation of the Green s function + calculation of the surface Green s function: layer addition (or removal) invariance self-consistent (Dyson-like) equation

23 spin-precession lenght = 104 mean free path = 30 width = 80 width = 50 width = 30 width = 20 width = 10 C = (C + C ) + (C + C ) spin-conserving conductance = C + C spin quantiation axis along X mean free path = 30 spin-flip conductance = C + C total conductance spin-conserving conductance spin-flip conductance polariation: P (C + C ) - (C + C ) (C + C ) + (C + C )

24 total conductance spin-conserving conductance spin-flip conductance (mean free path = 30) spin quantiation axis along Y spin quantiation axis along X spin quantiation axis along Z y x total conductance spin-conserving conductance spin-flip conductance (mean free path = 120) spin quantiation axis along Y spin quantiation axis along X spin quantiation axis along Z y x

25 spin-precession lenght = 104 spin quantiation axis along Y mean free path = 30 width = 10 width = 20 width = 30 width = 50 width = 80 y x length = width = 20 LS LP = LP = 120 LP = 30 LP = 10 LP = 4

26 length = width = 20 LS LP = LP = 120 LP = 30 LP = 10 LP = 4 length = width = 80 LS LP = LP = 120 LP = 30 LP = 10 LP = 4

27 length = width = 80 LS LP = LP = 120 LP = 30 LP = 10 LP = 4

The 2007 Nobel Prize in Physics. Albert Fert and Peter Grünberg

The 2007 Nobel Prize in Physics Albert Fert and Peter Grünberg Albert Fert and Peter Grünberg are well-known for having opened a new route in science and technology by their discovery of the Giant MagnetoResistance