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Jairo Sinova ( TAMU )

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

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Jairo Sinova ( TAMU )

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

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

  3. Ferromagnetic semiconductor research for spintronics: • Motivations and strategies • 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 2nd part of the talk we show examples of 1. & 2. and a combination of both principles

  4. 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

  5. Ga Mn 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

  6. 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)

  7. (Ga,Mn)As synthesis 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

  8. 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

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

  10. 185K!! 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

  11. Can we have high Tc in Diluted Magnetic Semicondcutors? NO EXTRINSIC LIMIT NO IDENTIFICATION OF AN INTRINSIC 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

  12. 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

  13. Other (III,Mn)V’s DMSs Kudrnovsky et al. PRB 07 Delocalized holes long-range coupl. Weak hybrid. Mean-field but low TcMF InSb d5 Impurity-band holes short-range coupl. Strong hybrid. Large TcMF but low stiffness GaP (Al,Ga,In)(As,P) good candidates, GaAs seems close to the optimal III-V host

  14. 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)

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

  16. 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

  17. 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

  18. 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.

  19. 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

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

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

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

  23. (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

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

  25. 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

  26. 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

  27. 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

  28. V DD V V A B V B V A 1 0 Nonvolatile programmable logic 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”

  29. 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

  30. 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 Sankar Das Sarma U. of Maryland 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

  31. 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

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