Making semiconductors magnetic: new materials properties, devices, and future

1. NRI SWAN Making semiconductors magnetic: new materials properties, devices, and future 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&ML. Zarbo 215th ECS Meeting - San Francisco, CA May 27th 2009 University of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, et al.

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

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

4. (Ga,Mn)As GENERAL PICTURE

5. 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 +

6. Ga As Mn What happens when a Mn is placed in Ga sites: Mn–hole spin-spin interaction As-p 5 d-electrons with L=0 S=5/2 local moment intermediate acceptor (110 meV)  hole Mn-d hybridization Hybridization  like-spin level repulsion  Jpd SMn shole interaction 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

7. Ga Ga Mn As As Mn Mn Transition to a ferromagnet when Mn concentration increases 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 Ga Mn As Mn FM due to p-d hybridization (Zener local-itinerant kinetic-exchange)

8. (Ga,Mn)As GROWTH 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-double-donors compensating holes and moments •  need to anneal out but without loosing MnGa optimal-T growth

9. 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-T another key factor Rushforth et al, ‘08

10. Tc LIMITS AND STRATEGIES

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

12. Can we have high Tc in Diluted Magnetic Semicondcutors? NO IDENTIFICATION OF AN INTRINSIC LIMIT NO EXTRINSIC LIMIT Tc linear in MnGa local (uncompensated) moment concentration; falls rapidly with decreasing hole density in heavily compensated samples. (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 Define Mneff = Mnsub-MnInt

13. Linear increase of Tc with Mneff = Mnsub-MnInt High compensation 8% Mn Tc as grown and annealed samples Open symbols as grown. Closed symbols annealed • Concentration of uncompensated MnGa 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 n and p type doping through Li/Zn stoichiometry LDA+U says that Mn-doped are also twin DMSs No solubility limit for group-II Mn substituting for group-II Zn !!!! Masek, et al. PRB (2006)

16. UNDERSTANDING CRITICAL BEHAVIOUR IN TRANSPORT

17.  Solving a puzzle 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

18. 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) Tc Ni EuCdSe Tc

19. d/dT singularity at Tc – consistent with kF~d- Annealing sequence Optimized materials with x=4-12.5% and Tc=80-185K Remarkably universal both below and above Tc 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 Tc • Related FS materials (searching for room temperature) • Understanding critical behavior in transport • Ferromagnetic semiconductors & spintronics • Tunneling anisotropic magnetoresistive device • Transistors (4 types)

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

22. TAMR in metal structures experiment Park, et al, PRL '08 ab intio theory Shick, et al, PRB '06, 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 (Ga,Mn)As/AlOx FET with large gate voltages, Chiba et al. ‘06 p-n junction depletion estimates ~25% depletion feasible at low voltages Olejnik et al., ‘08

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

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

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

28. (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

29. 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

30. V DD V V A B V B V A Nonvolatile programmable logic 1 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 0 1 1 1 0 0 1 1 Vout A B Vout 0 0 0 1 0 1 0 1 1 1 1 1 OFF ON OFF 1 1 0 0 “OR” OFF ON OFF ON

31. V DD V V A B V B V A Nonvolatile programmable logic 1 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”

32. Device design Physics of SO & exchange Materials FSs and metal FS with strong SO Chemical potential  CBAMR SET FSs Tunneling DOS  TAMR Tunneling device metal FMs Group velocity & lifetime  AMR Resistor

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

34. EXTRAS

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

36. 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 ferromagnetchange of |(M)|~100K • Room-T conventional SET • (e2/2C>300K) possible

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