Jairo Sinova (TAMU) NRI e-Workshop Making semiconductors magnetic: A new approach to engineering quantum materials Tomas Jungwirth (TAMU, Institute of Physics, Czech Republic, U. of Nottingham) NERC SWAN

OUTLINE Motivation Ferromagnetic semiconductor materials: –(Ga,Mn)As - general picture –Growth and physical limits on T c –Related FS materials Ferromagnetic semiconductors & spintronics –Tunneling anisotropic magnetoresistive device –Transistors

Ferromagnetic semiconductor research for spintronics: Motivations and strategies 1.Find new effects in this new material and utilize in conventional metal-based spintronics 2. Develop a three-terminal gatable spintronic device to progress from sensors & memories to transistors & logic In the 2 nd part of the talk we show examples of 1. & 2. and a combination of both principles

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

Ga As Mn What happens when a Mn is placed in Ga sites: Mn–hole spin-spin interaction hybridization Hybridization  like-spin level repulsion  J pd S Mn  s hole interaction Mn-d As-p In addition to the Kinetic-exchange coupling, for a single Mn ion, the coulomb interaction gives a trapped hole (polaron) which resides just above the valence band 5 d-electrons with L=0  S=5/2 local moment intermediate acceptor (110 meV)  hole

Mn Ga As Mn EFEF DOS Energy spin  spin  Transition to a ferromagnet when Mn concentration increases 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 Mn Ga As 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- double-donors compensating holes and moments  need to anneal out but without loosing Mn Ga 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

OUTLINE Motivation Ferromagnetic semiconductor materials: –(Ga,Mn)As - general picture –Growth and physical limits on T c –Related FS materials Ferromagnetic semiconductors & spintronics –Tunneling anisotropic magnetoresistive device –Transistors

“... Ohno’s ‘98 T c =110 K is the fundamental upper limit..” Yu et al. ‘03 “…T c = K independent of x Mn >10% contradicting Zener kinetic exchange...” Mack et al. ‘08 “Combinatorial” approach to growth with fixed growth and annealing T’s Tc limit in (Ga,Mn)As remains open 2008 Olejnik et al 185K!!

Can we have high Tc in Diluted Magnetic Semicondcutors? T c linear in Mn Ga local (uncompensated) moment concentration; falls rapidly with decreasing hole density in heavily compensated samples. Define Mn eff = Mn sub -Mn Int NO IDENTIFICATION OF AN INTRINSIC LIMIT NO EXTRINSIC LIMIT (lines – theory, Masek et al 05) Relative Mn concentrations obtained through hole density measurements and saturation moment densities measurements. Qualitative consistent picture within LDA, TB, and k.p

8% Mn Open symbols as grown. Closed symbols annealed High compensation Linear increase of Tc with Mn eff = Mn sub -Mn Int Tc as grown and annealed samples ● Concentration of uncompensated Mn Ga 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

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 GaAs and LiZnAs are twin SC Masek, et al. PRB (2006) LDA+U says that Mn-doped are also twin DMSs n and p type doping through Li/Zn stoichiometry No solubility limit for group-II Mn substituting for group-II Zn !!!!

OUTLINE Motivation Ferromagnetic semiconductor materials: –(Ga,Mn)As - general picture –Growth and physical limits on T c –Related FS materials Ferromagnetic semiconductors & spintronics –Tunneling anisotropic magnetoresistive device –Transistors

AMR ~ 1% MR effect TMR ~ 100% MR effect TAMR Exchange split & SO-coupled bands: Exchange split bands: Au discovered in (Ga,Mn)As Gold et al. PRL’04

As-p-like holes Strong exchange splitting & SO coupling in (Ga,Mn)As Standard MBE techniques for high-quality tunneling structures Mn Ga As Mn

ab intio theory Shick, et al, PRB '06, Park, et al, PRL '08 TAMR in metal structures experiment Park, et al, PRL '08 Also studied by Parkin et al., Weiss et al., etc.

Gating of highly doped (Ga,Mn)As: p-n junction FET p-n junction depletion estimates Olejnik et al., ‘08 ~25% depletion feasible at low voltages (Ga,Mn)As/AlOx FET with large gate voltages, Chiba et al. ‘06

AMR Increasing  and decreasing AMR and T c with depletion Tc

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 via strain

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 metal FMs FSs FSs and metal FS with strong SO

Allan MacDonald U of Texas Tomas Jungwirth Inst. of Phys. ASCR U. of Nottingham Joerg Wunderlich Cambridge-Hitachi Laurens Molenkamp Wuerzburg Mario Borunda Texas A&M U. Other collaborators: Bernd Kästner, Satofumi Souma, Liviu Zarbo, Dimitri Culcer, Qian Niu, S-Q Shen, Brian Gallagher, Tom Fox, Richard Campton Alexey Kovalev Texas A&M U. Liviu Zarbo Texas A&M U. Matching TAMU funds Xin Liu Texas A&M U. Bryan Gallagher U. Of Nottingham Sankar Das Sarma U. of Maryland 30

Conclusion (checks of theory) In the metallic optimally doped regime GaMnAs is well described by a disordered-valence band picture: both dc-data and ac-data are consistent with this scenario. The effective Hamiltonian (MF) and weak scattering theory (no free parameters) describe (III,Mn)V metallic DMSs very well in the optimally annealed regime: Ferromagnetic transition temperatures   Magneto-crystalline anisotropy and coercively   Domain structure   Anisotropic magneto-resistance   Anomalous Hall effect   MO in the visible range   Non-Drude peak in longitudinal ac-conductivity  Ferromagnetic resonance  Domain wall resistance  TAMR  TB+CPA and LDA+U/SIC-LSDA calculations describe well chemical trends, impurity formation energies, lattice constant variations upon doping