1. Making semiconductors magnetic: new materials properties, devices, and future NRI SWAN JAIRO SINOVA Texas A&M University Institute of Physics ASCR Hitachi Cambridge Jorg Wunderlich, A. Irvine, et al Institute of Physics ASCR Tomas Jungwirth, Vít Novák, et al Texas A&M L. Zarbo University of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, et al. 215th ECS Meeting - San Francisco, CA May 27 th 2009

2. OUTLINE Motivation Ferromagnetic semiconductor materials: –(Ga,Mn)As - general picture –Growth, physical limits on T c –Related FS materials (searching for room temperature) –Understanding critical behavior in transport Ferromagnetic semiconductors & spintronics –Tunneling anisotropic magnetoresistive device –Transistors (4 types)

3. 1.Create a material that marriages the tunability of semiconductors and the collective behavior of ferromagnets; once created search for room temperature systems 2.Study new effects in this new material and utilize in metal-based spintronics 3.Develop a three-terminal gated spintronic device to progress from sensors & memories to transistors & logic Ferromagnetic semiconductor research : Motivations and strategies

4. (Ga,Mn)As GENERAL PICTURE

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

6. Ga As 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

7. 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

8. 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 (Ga,Mn)As GROWTH

9. 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

10. T c LIMITS AND STRATEGIES

11. “... Ohno’s ‘98 T c =110 K is the fundamental upper limit..” Yu et al. ‘03 “…T c =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 T’s Tc limit in (Ga,Mn)As remains open 2008 Olejnik et al 188K!!

12. 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

13. 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

14. How well do we understand (Ga,Mn)As? 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  Transport critical behaviour  TB+CPA and LDA+U/SIC-LSDA calculations describe well chemical trends, impurity formation energies, lattice constant variations upon doping

15. 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 !!!!

16. UNDERSTANDING CRITICAL BEHAVIOUR IN TRANSPORT

17. Solving a puzzle 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)??? 

18. TcTc EuCdSe When density of carriers is smaller than density of local moments what matters is the long range behavior of Γ (which goes as susceptibility) When density of carriers is similar to density of local moments what matters is the short range behavior of Γ (which goes as the energy) Ni TcTc

19. Optimized materials with x=4-12.5% and Tc=80-185K Remarkably universal both below and above Tc Annealing sequence d  /dT singularity at T c – consistent with k F ~d  -  V. Novak, et al “Singularity in temperature derivative of resistivity in (Ga,Mn)As at the Curie point”, Phys. Rev. Lett. 101, 077201 (2008).

20. OUTLINE Motivation Ferromagnetic semiconductor materials: –(Ga,Mn)As - general picture –Growth, physical limits on T c –Related FS materials (searching for room temperature) –Understanding critical behavior in transport Ferromagnetic semiconductors & spintronics –Tunneling anisotropic magnetoresistive device –Transistors (4 types)

21. 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

22. 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.

23. DMS DEVICES

24. 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

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

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

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

28. 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

29. & 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 Theory confirms chemical potential anisotropies in (Ga,Mn)As & predicts CBAMR in SO-coupled room-T c metal FMs

30. 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 0 0 0 OFF ON OFF 0 0 1 1 ON OFF AB Vout 00 0 10 1 01 1 11 1 0 0 1 ON OFF 0 0 1 ON 1 1 1 1 OFF ON 1 1 OFF 1 “OR” Nonvolatile programmable logic

31. 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 00 0 10 1 01 1 11 1 “OR” Nonvolatile programmable logic

32. 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

33. 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 33

34. EXTRAS

35. AMR nature of the effect normal AMR Coulomb blockade AMR

36. 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

37. 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