Spintronics in metals and semiconductors Tomas Jungwirth Institute

Spintronics in metals and semiconductors Tomas Jungwirth 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, et al. University of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, Kevin Edmonds, Andrew Rushforth, Chris King et al. Texas A&M Jairo Sinova, et al. University of Texas Allan Ma. Donald, 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

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

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

Consider uncommon TM combinations Mn/W ~100% TAMR Shick, et al, TAMR in TM structures unpublished Consider both Mn-TM FMs & AFMs spontaneous moment cou plin ility eptib rbit susc n-o spi netic mag g exchange-spring rotation of the AFM Scholl et al. PRL ‘ 04 Proposal for AFM-TAMR: first microelectronic device with active AFM component Shick, et al, unpublished

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

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

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

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

(Ga, Mn)As synthesis high-T growth Low-T MBE to avoid precipitation High enough T to maintain 2 D 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

Polyscrystalline 20% shorter bonds Interstitial Mn out-diffusion limited by surface-oxide O Ga. Mn. As-oxide x-ray photoemission Ga. Mn. As Mn. I++ Olejnik et al, ‘ 08 10 x shorther annealing with etch Optimizing annealing time & temperature (removing int. Mn & keeping Mn. Ga 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 California (‘ 08): “…Tc =150 -165 K independent of x. Mn>10% contradicting Zener kinetic exchange. . . ” Mack et al. ‘ 08 “Combinatorial” approach to growth with fixed growth and annealing cond. Nottingham & Prague (’ 08): Tc up to 188 K so far ?

Other (III, Mn)V’s DMSs Kudrnovsky et al. PRB 07 Weak hybrid. Mean-field but low Tc. MF In. Sb Strong hybrid. Delocalized holes long-range coupl. d 5 Impurity-band holes short-range coupl. Large Tc. MF but low stiffness Ga. P (Al, Ga, In)(As, P) good candidates, Ga. As seems close to the optimal III-V host

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

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

Critical contribution at Tc to d /d. T like TM FMs (Ga, Mn)As (Prague Nottingham) Fe Fisher & Langer ’ 68 Ni Novak et al. , ‘ 08 d /d. T ~ cv F ~ d -

Tc Eu. Cd. Se Ni Tc

Ferromagnetism & strong spin-orbit coupling Ga Mn As-p-like holes V s p Beff 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 Mn. Ga ~ magnetic. only > > max AMR sign and magnitude (numerical) consistent with experiment

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

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 ~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 normal AMR Coulomb blockade AMR

Single-electron charging energy controlled by Vg and M Source Q VD Drain Q 0 Gate VG e 2/2 C [110] [010] M F [100] [110] electric & magnetic control of Coulomb blockade oscillations [010] SO-coupling (M)

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

Nonvolatile programmable logic Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device 1 0 V DD ON OFF ON VB ON OFF 10 Vout 10 ON OFF VA 1 0 0 1 VA ON OFF 1 0 OFF ON OFF “OR” A 0 1 B 0 0 1 1 Vout 0 1 1 1

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

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