Spintronics in ferromagnetic semiconductor (Ga,Mn)As 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, Byonguk Park, Sam Owen, et al. Institute of Physics ASCR Alexander Shick, Karel Výborný, Jan Zemen, Jan Masek, Vít Novák, Kamil Olejník, et al. University of Texas Allan MaDonald, et al. Texas A&M Jairo Sinova, et al.

Outline 1. Curie temperature and critical transport anomaly 2. Low-voltage control of ferromagnetism in a p-n junction 3. Coulomb-blockade AMR single electron transistor

Electric field controlled spintronics HDD, MRAM controlled by Magnetic field Magnetic Transistor control by Electric field J. Wunderlich, et al. 06 Low-voltage controlled magnetization and MR effects STT MRAM, spin-polarized charge current From storage to logic FS spintronic transitor

Mn-d-like local moments As-p-like holes Mn Ga As Mn EFEF DOS Energy spin  spin  Ferromagnetic semiconductor GaAs:Mn 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

(Ga,Mn)As 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 high-T growth optimal-T growth Optimizing annealing time & temperature (removing int. Mn & keeping Mn Ga in place) again for each Mn- doping is essential

Mack et al. ’08 : “…T c = K independent of x Mn >10% contradicting Zener kinetic exchange...” Nottingham & Prague (’08): T c up to 188 K so far “Combinatorial” approach to growth with fixed growth and annealing conditions ? Mack et al. ‘08 Tc limit in (Ga,Mn)As remains open Olejnik et al., ‘08

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)??? 

TcTc TcTc EuCdSe Ni

d  /dT singularity at T c – consistent with k F ~d  -  Novak, et al.‘08

Optimized materials with x=4-12.5% and Tc=80-185K Remarkably universal both below and above Tc Annealing sequence

As-p-like holes Ferromagnetism & 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 Mn Ga As Mn

Rushforth et al., ‘08 Strain & SO  Electric field control of ferromagnetism k.p kinetic exchange model predicst sensitivity to strains ~10 -4 and hole-density variations of ~ cm -3 slow and requires ~100V

Gating of the highly doped (Ga,Mn)As: p-n junction p-n junction depletion estimates Olejnik et al., ‘08 ~25% depletion feasible at low voltages 2x cm -3

Basic charcteristics of the device can deplete charge at low Vg can “deplete” magnetization at low Vg 30% AMR tuneable by low Vg low Vg dependent competition of uniaxial and cubic anisotropies

Magnetization switching by 10ms low-Vg pulses Due to the Vg-dependent Stoner-Wolfarth “diamond” (tuneable uniaxial and cubic anisotropy terms) dR c /dH normalized dR c /dH -1V 3V -1V3V

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