Spintronics in metals and semiconductors

Spintronics in metals and semiconductors Tomas Jungwirth University of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, Kevin Edmonds, Andrew Rushforth, Chris King et al. Institute of Physics ASCR Alexander Shick, Karel Výborný, Jan Zemen, Jan Masek, Vít Novák,Kamil Olejník, et al. Hitachi Cambridge Jorg Wunderlich, Andrew Irvine, David Williams, Elisa de Ranieri, Byonguk Park, Sam Owen, etal. Texas A&M Jairo Sinova, et al. University of Texas Allan MaDonald, 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  spin-orbit interaction DOS Coulomb repulsion & Pauli exclusion principle  exchange interaction  ferromagnetism

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

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

TAMR in TM structures Consider uncommon TM combinations Mn/W  ~100% TAMR Consider both Mn-TM FMs & AFMs Shick, et al, unpublished spontaneous moment magnetic susceptibility spin-orbit coupling exchange-spring rotation of the AFM Scholl et al. PRL ‘04 Proposal for AFM-TAMR: first microelectronic device with active AFM component Shick, et al, unpublished

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

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

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

(Ga,Mn)As synthesis high-T growth • 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 optimal-T growth

Interstitial Mn out-diffusion limited by surface-oxide Polyscrystalline 20% shorter bonds O GaMnAs-oxide x-ray photoemission MnI++ GaMnAs Olejnik et al, ‘08 10x shorther annealing with etch Optimizing annealing time & temperature (removing int. Mn & keeping MnGa in place) is essential Rushforth et al, unpublished

Tc limit in (Ga,Mn)As remains open Indiana & California (‘03): “ .. Ohno’s ‘98 Tc=110 K is the fundamental upper limit ..” Yu et al. ‘03 Nottingham & Prague (’08): Tc up to 188 K so far California (‘08): “…Tc =150-165 K independent of xMn>10% contradicting Zener kinetic exchange ...” ? Mack et al. ‘08 “Combinatorial” approach to growth with fixed growth and annealing cond.

Other (III,Mn)V’s DMSs Kudrnovsky et al. PRB 07 Delocalized holes long-range coupl. Weak hybrid. Mean-field but low TcMF InSb d5 Impurity-band holes short-range coupl. Strong hybrid. Large TcMF but low stiffness GaP (Al,Ga,In)(As,P) good candidates, GaAs seems close to the optimal III-V host

Other DMS candidates III = I + II  Ga = Li + Zn GaAs and LiZnAs are twin SC (Ga,Mn)As and Li(Zn,Mn)As should be twin ferromagnetic SC • 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) Masek et al. PRL 07

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

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

Tc Ni EuCdSe Tc

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

SO couped carries scattering coherently off Coulomb & polarized-magnetic potential of Mn AMR in DMSs MnGa ~ magnetic. only > > sign and magnitude (numerical) consistent with experiment max AMR

Remark: Extraordinary MRs & quantum coherent transport phenomena UCF dirty metal

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

AMR Increasing  and decreasing AMR, Tc, coercivity with depletion

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

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

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

AMR nature of the effect Coulomb blockade AMR normal AMR

Single-electron charging energy controlled by Vg and M M [010] [110] F Q VD [100] Source Drain [110] [010] Gate VG Q0 Q0 e2/2C magnetic electric & SO-coupling  (M) control of Coulomb blockade oscillations

Theory confirms chemical potential anisotropies in (Ga,Mn)As & predicts CBAMR in SO-coupled room-Tc metal FMs • CBAMR if change of |(M)| ~ e2/2C • In our (Ga,Mn)As ~ meV (~ 10 Kelvin) • In room-T ferromagnet change of |(M)|~100K • Room-T conventional SET • (e2/2C>300K) possible

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

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

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

Spintronics in metals and semiconductors