Fundamental concepts of spintronics Jaroslav Fabian Institute for Theoretical Physics University of Regensburg Stara Lesna, 24. 8. 2008 SFB 689
:outline: what is spintronics? spin injection spin-orbit coupling in solids (next lecture) spin devices conclusions: challenges I. Zutic, J. Fabian, and S. Das Sarma, Spintronics: Fundamentals and applications, Rev. Mod. Phys. 76, 323 (2004) J. Fabian, A. Matos-Abiague, C. Ertler, P. Stano, and I. Zutic, Semiconductor spintronics, Acta Phys. Slov, 57, 566 (2007)
what is spintronics? narrow (device): electronics with spin broad: umbrella for electron spin phenomena in solids
spintronics drive technology fundamental discoveries
The Nobel Prize in Physics 2007 The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2007 jointly to Albert Fert Unité Mixte de Physique CNRS/THALES, Université Paris-Sud, Orsay, France Peter Grünberg Forschungszentrum Jülich, Germany, "for the discovery of Giant Magnetoresistance".
Giant MagnetoResistance P. Grunberg et al. (1988), A. Fert et al. (1988) small resistance large resistance multilayers 30-40% at RT
GMR hard disk read heads From: IBM web site
SPINTRONICS GOALS spin control of electrical properties (I-V characteristics) electrical control of spin (magnetization)
SPINTRONICS 3 REQUIREMENTS EFFICIENT SPIN INJECTION F N SLOW SPIN RELAXATION @ SPIN CONTROL RELIABLE SPIN DETECTION Silsbee-Johnson spin-charge coupling
:(electrical) spin injection:
Johnson-Silsbee spin injection experiment Silsbee: emf appears in the proximity of a ferromagnetic metal and spinpolarized nonmagnetic metal (inverse of spin injection) R. Silsbee, Bull. Mag. Reson. 2, 284 (1980) M. Johnson and R. H. Silsbee, Phys. Rev. Lett. 55, 1790 (1985). E E δm μ 0 N (E) N (E) N (E) N (E) spin injection spin detection
visualizing spin injection S. A. Crooker et al., JAP, 101,081716 (2007) S. A. Crooker at al., Science 309, 2191 (2005)
spin injection into silicon I. Appelbaum et al, Nature 447, 295 (2007) I. Zutic and J. Fabian, Nature (NW) 447, 269 (2007)
spin injection into graphene single-layer on a SiO 2 substrate, room temperature N. Tombros, C. Jozsa, M. Popinciuc, H. T. Jonkman, and B. J. van Wees Electronic spin transport and spin precession in single graphene layers at room temperature, Nature 448, 571 (2007) N. Tombros, S. Tanabe, A. Veligura, C. Jozsa, M. Popinciuc, H. T. Jonkman, and B. J. van Wees Anisotropic spin relaxation in graphene, arxiv:0802.2892
Zincblende band structure (GaAs) optical orientation transitions (a) S 1/2 E CB (b) P 3/2 P 1/2 Γ 6 Γ 8 Γ 7 E g Δ so HH LH SO 0 k m j σ + 1/2 1/2 3 1 1 3 σ + CB σ σ 3/2 1/2 1/2 3/2 2 2 1/2 1/2 SO HH,LH From: I. Zutic, J. Fabian, S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004)
:spin relaxation:
:key concepts: spin relaxation and dephasing B Fe t=0, spin imbalance t=t 1, spin balance impurity phonon spin-orbit coupling
:key concepts: spin relaxation and dephasing Bloch eqs
Time-resolved Faraday rotation Source: web site of Awschalom s group ZnCdSe QW
mechanisms of spin relaxation Elliott-Yafet mechanism elemental metals and semiconductors Dyakonov-Perel mechanism Semiconductors without center of inversion symmetry Bir-Aronov-Pikus mechanism Heavily p-doped semiconductors Hyperfine interaction Electrons bound on impurity sites or confined In quantum dots J. Fabian, A. Matos-Abiague, C. Ertler, P. Stano, and I. Zutic, Semiconductor spintronics, Acta Physica Slovaca, 57, 565 (2007)
spin relaxation in bulk n-gaas relaxation tim e(ns) τ τ τ τ τ τ R. I. Dzhioev et al., Phys. Rev. B 66, 245204 (2002)
spin relaxation in bulk n-si 100 spin relaxation time T 1 [ns] 80 60 40 20 0 7.4 10 14 3.7 10 15 4.5 10 15 7.8 10 15 2.7 10 16 8.0 10 16 0 50 100 150 200 250 300 Temperature [K] D. Lepine, Phys. Rev. B 6, 436 (1972) J. Fabian, A. Matos-Abiague, C. Ertler, P. Stano, and I. Zutic, Acta Physica Slovaca, 57, 565 (2007)
:spin devices: (spin detection)
:semiconductor spintronics devices: spin resonant diodes spin field-effect transistors magnetic semiconductor tunnel junction devices magnetic bipolar junction diodes and transistors spin optoelectronic devices spin galvanics devices spin Hall polarizeds spin-polarized semiconductor lasers spin pumping batteries spin-torque devices spin quantum computers...
J. Fabian, A. Matos-Abiague, C. Ertler, and P. Stano, Semiconductor spintronics, Acta Phys. Slov, 57, 566 (2007)
International Technology Roadmap 2004 for Semiconductors: Emerging Research Logic Devices RSFQ 1-D structures resonant tunneling SET molecular QCA spin transistor risk 2005, 2006
International Technology Roadmap 2004 for Semiconductors: Emerging Research Logic Devices RSFQ 1-D structures resonant tunneling SET molecular QCA spin transistor risk 2007
detour: material case study: GaMnAs 5-15 % Mn p-doped (Mn replaces Ga) degenerate: p = 10 20-10 21 /cm 3 Tc = 170 K ferromagnetism and carrier density coupled kλ about 3 (localization?) impurity or valence band? quantum coherence effects observed GaMnAs, from Jungwirth et al, Rev. Mod. Phys. 78, 809 (2006)
Where does GaMnAs fit? No good answer yet
magnetic Resonant Tunnel Diodes A. Slobodskyy et al, Phys. Rev. Lett. 90, 246601 (2003) C. Ertler and J. Fabian, Appl. Phys. Lett. 89, 193507 (2006) C. Ertler and J. Fabian, Phys. Rev. B 75 195323 (2007) ZnSe ZnSe BeZnSe BeZnSe ZnMnSe ZnMnSe b) ZnSe ZnSe Current (0-150 μa) 8% Mn T=1.3K 0T 3T 6T B 1.3 K a) Voltage (0-0.2 V) efficient spin filtering spin detection fast switching times coherence issues RT operation? Current Density (A/cm 2 ) 2.5 2 1.5 1 0.5 3 x 105 T = 4.2 K Energy (mev) 100 out ΔV 0 2 0 10 20 30 z (nm) 0 out 0 ΔV 0.05 0.1 0.15 0.2 0.25 1 Voltage (V) 50 Δ E = 0 Δ E = 5 mev Δ E = 10 mev Δ E = 15 mev Δ E = 20 mev Δ E = 25 mev Δ E = 40 mev ΔV 3 out
:selfsustained magneto-electric oscillations in MRTDs: C. Ertler and J. Fabian, Phys. Rev. Lett. 101, 077202 (2008) Intrinsic bistability leads to temporal oscillations in the current, magnetizaion, and particle density (a) x 10 15 j max (b) 20 j (a.u.) (c) j (a.u.) 10 5 I j min 0 0 10 20 30 Voltage (mv) 10 5 0 x 10 15 II j tot j j 50 100 150 200 Time (t*) Δ (mev) (d) n (1/cm 2 ) 15 10 5 I Δ max Δ min 0 0 10 20 30 Voltage (mv) 14 x 1011 12 10 8 6 4 2 50 100 150 200 Time (t*) II n tot n n
:nanospintronics: spin-based quantum information processing D. Loss and D. P. DiVincenzo, PRA 57, 120 (1998) single and few spins manipulation and detection spin relaxation and decoherence entanglement control (EDAP: Fabian and Hohenester, PRB 72, 201304 (R) 2005)
closing: challenges in spintronics room-temperature ferromagnetic semiconductors, n and p type, identification of mechanisms for ferromagnetic long-range order magnetic heterostructures: ferromagnetic quantum wells and quantum dots spin-polarized transport through magnetic interfaces and inhomogeneities, accurate determination of spin polarization of ferromagnets development of silicon (Si, Si:Ge) spintronics: spin injection, spin relaxation, magnetism (?), quantum dots demonstration of semiconductor spin transistors--power gain and magnetologic: spin FETs, bipolar spin transistors niche devices for GaMnAs or other dilute magnetic semiconductors, specific functionalities
closing: challenges in spintronics control of ferromagnetism by gating or current injection, spin-transfer torque spin dynamics and spin pumping phenomena in spin transport control of spin-orbit coupling by gate and doping, interface properties single channel devices Spin transport in carbon nanotubes, graphene spin quantum information processing: single and few spin manipulation, relaxation and decoherence, spin entanglement control