USING SPIN IN (FUTURE) ELECTRONIC DEVICES Tomas Jungwirth University of Nottingham Bryan Gallagher, Kevin Edmonds Tom Foxon, Richard Campion, et al. IP ASCR, Prague Jan Mašek,Alexander Shick Jan Kučera, František Máca Texas A&M Jairo Sinova, et al. University of Wuerzburg Laurens Molenkamp, Charles Gould et al. University of Texas Allan MaDonald, Qian Niu, et al. Hitachi Cambridge Jorg Wunderlich, Bernd Kaestner et al.

OUTLINE - Current and future (???) spintronic devices Electron has a charge (electronics) and spin (spintronics) Electrons do not actually “spin”, they produce a magnetic moment that is equivalent to an electron spinning clockwise or anti-clockwise OUTLINE - Current and future (???) spintronic devices - Challenges for spintronics  research topics - Electrical manipulation of spin in normal semiconductors (Spin Hall effect) - Ferromagnetic semiconductors - materials and devices

CURRENT SPINTRONIC DEVICES

HARD DISKS

HARD DISK DRIVE READ HEADS spintronic read heads horse-shoe read/write heads

Anisotropic magnetoresistance (AMR) read head 1992 - dawn of spintronics Ferromagnetism  large response (many spins) to small magnetic fields Spin-orbit coupling  spin response detected electrically

Giant magnetoresistance (GMR) read head 1997 GMR

Operation through electron charge manipulation MEMORY CHIPS .DRAM (capacitor) - high density, cheep x slow, high power, volatile .SRAM (transistors) - low power, fast x low density, expensive, volatile .Flash (floating gate) - non-volatile x slow, limited life, expensive Operation through electron charge manipulation

MRAM – universal memory (fast, small, non-volatile) Tunneling magneto-resistance effect RAM chip that won't forget ↓ instant on-and-off computers

MRAM – universal memory (fast, small, non-volatile) Tunneling magneto-resistance effect RAM chip that won't forget ↓ instant on-and-off computers

FUTURE (? or ???) SPINTRONIC DEVICES

PROCESSORS Low-dissipation microelectronics                                                         The power we use at home and outside of work accounts for only about a fifth of the total energy consumed in the United States every year, according to the Department of Energy. (ABCNEWS.com) Where Does All the Power Go? United States Energy Consumption: An Overview                          April 24 — We have electronic gizmos for just about every part of our daily lives, from brushing our teeth to staying in touch no matter where we are. Our swollen houses are stuffed with TVs, computers, and ever-larger and more complicated appliances. PROCESSORS Low-dissipation microelectronics Long spin-coherence times → information carried by spin-currents Instead of electrical currents. Functionality based on spin-dynamics, e.g., domain wall motion NOT gate Allwood et al., Science ’02

QUANTUM COMPUTERS 1 a + b Classical bit massive quantum Q-bit Classical bit massive quantum parallelism Q-bit a + b

CHALLENGES FOR SPINTRONICS

EXANGE-BIAS FM AFM fails when scaled down to ~10 nm dimensions Look for other MR concepts

EXTERNAL MAGNETIC FIELD problems with integration - extra wires, addressing neighboring bits

Current (insted of magnetic field) induced switching Buhrman & Ralph, NNUN ABSTRACTS '02 Slonczewski, JMMM '96; Berger, PRB '96 Angular momentum conservation  spin-torque

local, reliable, but fairly large currents needed magnetic field current Myers et al., Science '99; PRL '02 local, reliable, but fairly large currents needed Likely the future of MRAMs

INTEGRATION WITH SEMICONDUCTOR ELECTRONICS  Spin-valve transistor  Metal ferromagnet to semiconductor spin-injector  All-semiconductor spintronics - electrical manipulation of spins (no external magnetic field) - making semiconductors ferromagnetic

ELECTRICAL MANIPULATION OF SPINS IN NORMAL SEMICONDUCTORS - SPIN HALL EFFECT

Ordinary Hall effect B I V Lorentz force deflect charged-particles towards the edge B V I _ + + + + + + + + + + + + + _ _ _ _ _ _ _ _ _ _ FL Detected by measuring transverse voltage

Spin Hall effect Spin-orbit coupling “force” deflects like-spin particles I _ FSO V=0 non-magnetic Spin-current generation in non-magnetic systems without applying external magnetic fields Spin accumulation without charge accumulation excludes simple electrical detection Kato, Myars, Gossard, Awschalom, Science Wunderlich, Kaestner, Sinova, Jungwirth, PRL '04

Spin-orbit coupling (relativistic effect) Produces an electric field Ingredients: - potential V(r) - motion of an electron E In the rest frame of an electron the electric field generates and effective magnetic field - gives an effective interaction with the electron’s magnetic moment Since I am the first talk and the topic of spin-orbit coupling will come up often let me introduce its basic notion. The spin orbit coupling interactions is nothing but an effective magnetic field interaction felt by a moving charge due to a changing electric field due to its relative motion in a potential generating such an electric field. Maxwell’s equations show us that this relative motion of the charge and the electric field induces an effective magnetic field proportional to the orbital momentum of the quasiparticle. As illustrated in the equation shown the spin quantization axis for such a E Beff k

Skew scattering off impurity potential

SO-coupling from host atoms in a perfect crystal l=0 for electrons  weak SO l=1 for holes  strong SO Enhanced in asymmetric QW

 z-component of spin due to precession in effective "Zeeman" field Classical dynamics in k-dependent (Rashba) field: LLG equations for small drift  adiabatic solution: 

Novel co-planar spin-LED Conventional vertical spin-LED Novel co-planar spin-LED Y. Ohno et al.: Nature 402, 790 (1999) R. Fiederling et al.: Nature 402, 787 (1999) B. T. Jonker et al.: PRB 62, 8180 (2000) X. Jiang et al.: PRL 90, 256603 (2003) R. Wang et al.: APL 86, 052901 (2005) … ● No hetero-interface along the LED current ● Spin detection directly in the 2DHG ● Light emission near edge of the 2DHG ● 2DHG with strong and tunable SO 2DHG 2DEG Spin polarization detected through circular polarization of emitted light

EXPERIMENT Spin Hall Effect 2DHG 2DEG VT VD

Spin Hall Effect Device Experiment “A” Experiment “B” This micrograph shows our device used for studying the SHE. A p-channel containing a 2DHG is bordered by two n-regions containing the 2DEG, therefore upper and lower quasi-lateral pn-junctions are defined at both sides of the channel.

Experiment “A” Experiment “B” Opposite perpendicular polarization for opposite Ip currents or opposite edges  SPIN HALL EFFECT

FERROMAGNETIC SEMICONDUCTORS

MnGa As Ga (Ga,Mn)As diluted magnetic semiconductor Low-T MBE - random but uniform Mn distribution up to ~ 10% doping MnGa As Ga 5 d-electrons with L=0, S=5/2 moderately shallow acceptor

Jpd = + 0.6 meV nm3 Theoretical descriptions Microscopic: atomic orbitals & Coulomb correlation of d-electrons & hopping Jpd = + 0.6 meV nm3 Jpd SMn.shole Effective magnetic: Coulomb correlation of d-electrons & hopping AF kinetic-exchange coupling

Intrinsic properties of (Ga,Mn)As: Tc linear in MnGa local moment Jungwirth, Wang, et al. cond-mat/0505215 Intrinsic properties of (Ga,Mn)As: Tc linear in MnGa local moment concentration; falls rapidly with decreasing hole density in more than 50% compensated samples; nearly independent of hole density for compensation < 50%.

Extrinsic effects: Interstitial Mn - a magnetism killer Interstitial Mn is detrimental to magnetic order: compensating double-donor – reduces carrier density couples antiferromagnetically to substitutional Mn even in low compensation samples  smaller effective number of Mn moments Blinowski PRB ‘03, Mašek, Máca PRB '03 Mn As Yu et al., PRB ’02: ~10-20% of total Mn concentration is incorporated as interstitials Increased TC on annealing corresponds to removal of these defects.

Tc as grown and annealed samples Tc=173K 8% Mn Open symbols as grown. Closed symbols annealed Jungwirth, Wang, et al. cond-mat/0505215

Number of holes per Mneff Tc/xeff vs p/Mneff High (>40%) compensation Number of holes per Mneff Jungwirth, Wang, et al. cond-mat/0505215

Generation of Mnint during growth Theoretical linear dependence of Mnsub on total Mn confirmed experimentally Mnsub MnInt Jungwirth, Wang, et al. cond-mat/0505215

Prospects of (Ga,Mn)As based materials with room Tc - Concentration of uncompensated MnGa moments has to reach ~10% only 6.2% in the current record Tc=173K sample - Charge compensation not so important unless > 40% - No indication from theory or experiment that the problem is other than technological - better control of growth-T, stoichiometry; new growth or chemical composition strategies to incorporate more MnGa local moments or enhance p-d coupling

Tunneling anisotropic magnetoresistance no exchange-bias needed Giant magneto-resistance (Ga,Mn)As Au Au no exchange-bias needed [100] [010] Single magnetic layer sensor or memory Gould, Ruster, Jungwirth, et al., PRL '04

Spin-orbit coupling and anisotropies M || <111> M || <100> spin-split bands at M≠0 Dietl et al., Science '00 (Abolfath, Jungwirth et al., PRB '01 Magnetization orientation dependences Hole total energy over Fermi volume → magnetic anisotropy Group velocities at the Fermi surface and density of states for scattering → in plane magneto-resistance anisotropy Density of states at the Fermi energy → anisotropic tunnel magneto-resistance

GaMnAs Nanocontact TAMR 5nm thick 2% Mn GaMnAs Hall bars & nanoconstrictions Current [110] 30nm Constriction 30nm constriction Tunnelling conduction at low temperatures & voltages Giddings, Khalid, Jungwirth, Sinova et al. PRL '05

Landauer-Büttiker tunnelling probabilites Wavevector dependent tunnelling probabilityT (ky, kz) Red high T; blue low T. y x jt z Magnetisation in plane Magnetization perpendicular to plane Magnetization in plane x z y jt constriction: strong z-confinement (ultra-thin film) less strong y –confinement (constriction)

Very large TAMR in single nanocontacts 30nm constriction Very large TAMR in single nanocontacts 1400%

3m bar 30nm constriction B || z B|| y B || y B || x AMR & TAMR AMR in unstructured bar TAMR in constriction MR response of constricted device and bar are very similar in character but largely enhanced in the tunnel constriction

Final remark: spintronics in footsteps of electronics Spintronic nano-transistor field-controlled MR device Spintronic wire AMR device Spintronic diode GMR, TMR, TAMR device