Spintronics in metals and semiconductors Tomas Jungwirth University of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, Kevin Edmonds, Andrew Rushforth, Chris King et al. Hitachi Cambridge Jorg Wunderlich, Andrew Irvine, David Williams, Elisa de Ranieri, Sam Owen, et al. Institute of Physics ASCR Alexander Shick, Karel Výborný, Jan Zemen, Jan Masek, Vít Novák, Kamil Olejník, et al.

Outline 1. Tunneling anisotropic magnetoresistance in transition metals 2. Ferromagnetism in (Ga,Mn)As and related semiconductors 3. Spintronic transistors

Spintronics: Spin-orbit & exchange interactions nucleus rest frame electron rest frame Thomas precession Coulomb repulsion & Pauli exclusion principle  exchange interaction  ferromagnetism  spin-orbit interaction DOS

AMR ~ 1% MR effect TMR ~ 100% MR effect TAMR Exchange int.: Spin-orbit int.: magnetic anisotropy Exchange int.: AFM-FM exchange bias Au

ab intio theory Shick, et al, PRB '06, Park, et al, PRL '08 experiment Park, et al, PRL '08 TAMR in CoPt structures

spontaneous moment magnetic susceptibility Consider uncommon TM combinations Mn/W  ~100% TAMR Consider both Mn-TM FMs & AFMs exchange-spring rotation of the AFM Scholl et al. PRL ‘04 Proposal for AFM-TAMR: first microelectronic device with active AFM component spin-orbit coupling TAMR in TM structures Shick, et al, unpublished Shick, et al, unpublished

Outline 1. Tunneling anisotropic magnetoresistance in transition metals 2. Ferromagnetism in (Ga,Mn)As and related semiconductors 3. Spintronic transistors

Magnetic materials Ferroelectrics/piezoelectrics Semiconductors spintronic magneto-sensors, memories electro-mechanical transducors, large & persistent el. fields transistors, logic, sensitive to doping and electrical gating TM-based  semiconducting multiferroic spintronics sensors & memories  transistors & logic

Ferromagnetic semiconductors GaAs - standard III-V semiconductor Group-II Mn - dilute magnetic moments & holes & holes (Ga,Mn)As - ferromagnetic semiconductor semiconductor Need true FSs not FM inclusions in SCs Mn Ga As Mn

Mn-d-like local moments As-p-like holes Mn Ga As Mn EFEF DOS Energy spin  spin  GaAs:Mn – extrinsic p-type semiconductor FM due to p-d hybridization (Zener local-itinerant kinetic-exchange) valence band As-p-like holes As-p-like holes localized on Mn acceptors << 1% Mn ~1% Mn >2% Mn onset of ferromagnetism near MIT

As-p-like holes Strong spin-orbit coupling Strong SO due to the As p-shell (L=1) character of the top of the valence band VV B eff p s B ex + B eff Note: TAMR discovered in (Ga,Mn)As Gold et al. PRL’04 Mn Ga As Mn

(Ga,Mn)As synthesis Low-T MBE to avoid precipitation High enough T to maintain 2D growth  need to optimize T & stoichiometry for each Mn-doping Inevitable formation of interstitial Mn-donors compensating holes and moments  need to anneal out high-T growth optimal-T growth

Interstitial Mn out-diffusion limited by surface-oxide GaMnAs GaMnAs-oxide Polyscrystalline 20% shorter bonds Mn I ++ O Optimizing annealing-T another key factor Rushforth et al, ‘08 x-ray photoemission Olejnik et al, ‘08 10x shorther annealing with etch

Indiana & California (‘03): “.. Ohno’s ‘98 T c =110 K is the fundamental upper limit..” Yu et al. ‘03 California (‘08): “…T c = K independent of x Mn >10% contradicting Zener kinetic exchange...” Nottingham & Prague (’08): T c up to 185K so far “Combinatorial” approach to growth with fixed growth and annealing T’s ? Mack et al. ‘08 Tc limit in (Ga,Mn)As remains open

Weak hybrid. Delocalized holes long-range coupl. Strong hybrid. Impurity-band holes short-range coupl. InSb GaP d5d5 (Al,Ga,In)(As,P) good candidates, GaAs seems close to the optimal III-V host Other (III,Mn)V’s DMSs Mean-field but low T c MF Large T c MF but low stiffness Kudrnovsky et al. PRB 07

III = I + II  Ga = Li + Zn Other DMS candidates Masek et al. PRL 07 But Mn isovalent in Li(Zn,Mn)As  no Mn concentration limit and self-compensation  possibly both p-type and n-type ferromagnetic SC (Li / Zn stoichiometry) GaAs and LiZnAs are twin SC (Ga,Mn)As and Li(Zn,Mn)As should be twin ferromagnetic SC

Towards spintronics in (Ga,Mn)As: FM & transport Dense-moment MS F << d  -  Eu  - chalcogenides Dilute-moment MS F ~ d  -  Critical contribution to resistivity at T c ~ magnetic susceptibility Broad peak near T c disappeares with annealing (higher uniformity)??? 

Ni (Ga,Mn)As (Prague Nottingham) Fe Critical contribution at Tc to d  /dT like TM FMs d  /dT ~ c v F ~ d  -  Fisher & Langer ’68 Novak et al., ‘08

TcTc d  /dT Scattering off short range correlated spin-fluctuation Fisher&Langer ‘68

Outline 1. Tunneling anisotropic magnetoresistance in transition metals 2. Ferromagnetism in (Ga,Mn)As and related semiconductors 3. Spintronic transistors

Gating of the highly doped (Ga,Mn)As: p-n junction FET p-n junction depletion estimates Olejnik et al., ‘08 ~25% depletion feasible at low voltages

AMR Increasing  and decreasing AMR and T c with depletion

Persistent variations of magnetic properties with ferroelectric gates Stolichnov et al., Nat. Mat.‘08

Electro-mechanical gating with piezo-stressors Rushforth et al., ‘08 Strain & SO  Electrically controlled magnetic anisotropies

Single-electron transistor Two "gates": electric and magnetic (Ga,Mn)As spintronic single-electron transistor Huge, gatable, and hysteretic MR Wunderlich et al. PRL ‘06

AMR nature of the effect normal AMR Coulomb blockade AMR

& electric & magnetic control of Coulomb blockade oscillations Q0Q0 Q0Q0 e 2 /2C  [ 010 ]  M [ 110 ] [ 100 ] [ 110 ] [ 010 ] SO-coupling   (M) SourceDrain Gate VGVG VDVD Q Single-electron charging energy controlled by V g and M

CBAMR if change of |  (M)| ~ e 2 /2C CBAMR if change of |  (M)| ~ e 2 /2C  In our (Ga,Mn)As ~ meV (~ 10 Kelvin)In our (Ga,Mn)As ~ meV (~ 10 Kelvin) In room-T ferromagnet change of |  (M)|~100KIn room-T ferromagnet change of |  (M)|~100K Room-T conventional SET (e 2 /2C  >300K) possible Theory confirms chemical potential anisotropies in (Ga,Mn)As & predicts CBAMR in SO-coupled room-T c metal FMs

Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device 0 ON OFF 1 0 ON OFF 1 V DD V A V B V A V B Vout OFF ON OFF ON OFF AB Vout ON OFF ON OFF ON 1 1 OFF 1 “OR” Nonvolatile programmable logic

V DD V A V B V A V B Vout Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device 0 ON OFF 1 0 ON OFF 1 AB Vout “OR” Nonvolatile programmable logic

Physics of SO & exchange SET Resistor Tunneling device Chemical potential  CBAMR Tunneling DOS  TAMR Group velocity & lifetime  AMR Device designMaterials TM FMs (III,Mn)V, I(II,Mn)V DMSs Mn-based TM FMs&AFMs TM FMs, MnAs, MnSb

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Dawn of spintronics Anisotropic magnetoresistance (AMR) – 1850’s  1990’s Giant magnetoresistance (GMR) – 1988  1997 Inductive read/write element Magnetoresistive read element

MRAM – universal memory fast, small, low-power, durable, and non-volatile First commercial 4Mb MRAM

RAM chip that actually won't forget  instant on-and-off computers Based on Tunneling Magneto-Resistance (similar to GMR but insulating spacer)

Giant Magneto-Resistance      ~ 10% MR effect DOS  AP PP >

Tunneling Magneto-Resistance ~ 100% MR effect DOS   DOS 

Spin Transfer Torque writing

Ga As Mn x smaller M s One Key problems with increasing MRAM capacity (bit density): - Unintentional dipolar cross-links - External field addressing neighboring bits x weaker dipolar fields x smaller currents for switching Dilute moment nature of ferromagnetic semiconductors

Magnetism in systems with coupled dilute moments and delocalized band electrons (Ga,Mn)As coupling strength / Fermi energy band-electron density / local-moment density

Hole transport and ferromagnetism at relatively large dopings conducting p-type GaAs: - shallow acc. (C, Be) ~ cm -3 - Mn ~10 20 cm -3 Non-equilibrium growth - technological difficulties

Electric-field controlled ferromagnetism in FET or piezo/FM hybrid V gate Ferro SC Photogenerated ferromagnetism Ferro SC GaSb B (mT) ħħ Magnetization

Variable controlled strain using a Piezo stressor A.W. Rushforth, J. Zemen, K. Vyborny, et al. arXiv: Strain induced by piezo voltage +/- 150V: ~ (at 50K) Easy axis rotation by 50 deg for V piezo = -150V  +150V M. Overby, et al., arXiv:

Fast Precessional switching via gatevoltage

Spintronics with spin-currents only Magnetic domain “race-track” memory

n n p SHE mikročip, 100  A supercondicting magnet, 100 A Spin Hall effect detected optically in GaAs-based structures Same magnetization achieved by external field generated by a superconducting magnet with 10 6 x larger dimensions & 10 6 x larger currents Cu SHE detected elecrically in metals SHE edge spin accumulation can be extracted and moved further into the circuit

Datta-Das transistor Spintronics in nominally non-magnetic materials

intrinsic skew scattering I _ F SO _ _ _ Spin Hall effect spin-dependent deflection  transverse edge spin polarization

Information reading  Ferro Magnetization  Current Information reading & storage Tunneling magneto-resistance sensor and memory bit Information reading & storage & writing Current induced magnetization switching Information reading & storage & writing & processing : Spintronic transistor: magnetoresistance controlled by gate voltage New materials Ferromagnetic semiconductors, Multiferroics Non-magnetic SO-coupled systems Mn Ga As Mn Spintronics explores new avenues for: