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Ferromagnetic ordering in (Ga,Mn)As related zincblende semiconductors

Ferromagnetic ordering in (Ga,Mn)As related zincblende semiconductors. Tom áš Jungwirth. Universit y of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, Kevin Edmonds, Andrew Rushforth, Devin Giddings et al. Institute of Physics ASCR

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Ferromagnetic ordering in (Ga,Mn)As related zincblende semiconductors

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  1. Ferromagnetic ordering in (Ga,Mn)As related zincblende semiconductors Tomáš Jungwirth University of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, Kevin Edmonds, Andrew Rushforth, Devin Giddings et al. Institute of Physics ASCR František Máca, Jan Mašek, Jan Kučera Josef Kudrnovský, Alexander Shick Karel Výborný, Jan Zemen, Vít Novák, Miroslav Cukr, Kamil Olejník, et al. in collaboration with Hitachi Cambridge Polish Acad. of Sci. UT & Texas A&M Univ. Wuerzburg Jorg Wunderlich Tomasz Dietl Allan MacDonald Laurenc Molenkamp David Williams, et al. Mike Sawicki Jairo Sinova Charles Gould, et al.

  2. Spintronics in (Ga,Mn)As dilute moment ferromagnetic semiconductor Spintronic transistor Huge hysteretic low-field MR Sign & magnitude tunable by small gate valtages Wunderlich, et al., PRL (2006) Current driven magnetization reversal 2 orders of magnitude lower critical currents in dilute moment (Ga,Mn)As than in conventional metal FMs Sinova, Jungwirth et al., PRB (2004) Magnetic race track memory Parkin, US Patent (2004) Yamanouchi et al., Nature (2004)

  3. OUTLINE 1.Mn-doped GaAs material - only a factor of 2 short room-T otherwise outstanding properties 2.Mn-doped (Al,Ga)As and Ga(As,P) - useful materials to compare with and Ga(As,P) could lead to higher Tc 3. Mn-doped LiZnAs - still very closely related to (Ga,Mn)As and might heal its key problems

  4. Mn As Ga (Ga,Mn)As material 5 d-electrons with L=0 S=5/2 local moment moderately shallow acceptor (110 meV)  hole -Mn local moments too dilute (near-neghbors cople AF) - Holes do not polarize in pure GaAs - Hole mediated Mn-Mn FM coupling Ohno, Dietl et al. (1998,2000);Jungwirth, Sinova, Mašek, Kučera, MacDonald, Rev. Mod. Phys. (2006), http://unix12.fzu.cz/ms

  5. Universal scaling of (Tc / Mn-moment) vs. (hole / Mn-moment) theory expectations Robust mean-field-like ferromagnet experiment hole density / Mn-moment density Jungwirth, Wang, et al. PRB (2005)

  6. coupling strength / Fermi energy band-electron density / local-moment density Magnetism in systems with coupled dilute moments and delocalized band electrons (Ga,Mn)As

  7. Mnsub MnInt Mnsub - As + MnInt Ga More Mn - interstitial incorporation Covalent SCs do not like doping self-compensation by interstitial Mn Interstitial MnInt is detrimental to magnetic order charge and moment compensation defect Yu et al., PRB ’02;Blinowski PRB ‘03; Mašek, Máca PRB '03 Can be annealed out Tc 95K in as-grown (9% Mn) to 173 in annealed (6% Mnsub) but MnGa < nominal Mn theory & exp. Jungwirth, Wang, et al. PRB (2005)

  8. More Mn - problem with solubility - Effective concentration of uncompensated MnGa moments has to increase beyond 6% of the current record Tc=173K sample. A factor of 2 need (12% Mn is still a DMS). - Low solubility of group-II Mn in III-V-host GaAs makes growth difficult Low-temperature MBE

  9. A startegy: • Find DMS system as closely related to (Ga,Mn)As as possible to with • larger hole-Mn spin-spin interaction • lower tendency to self-compensation by Mnint • larger Mn solubility • independent control of local-moment and carrier doping (p- & n-type)

  10. d5 d5 Mn As Ga (Al,Ga)As & Ga(As,P) hosts 5.7 (Al,Ga)As lattice constant (A) Ga(As,P) 5.4 0 1 conc. of wide gap component local moment - hole spin-spin coupling Jpd S . s Mn d - As(P) poverlap Mn d level - valence band splitting GaAs & (Al,Ga)As Ga(As,P) GaAs (Al,Ga)As & Ga(As,P)

  11. (Al,Ga)As p-d coupling and Tc in mixed (Al,Ga)As and Ga(As,P) theory 10% Mn Ga(As,P) Smaller lattice const. more important for enhancing p-d coupling than larger gap  Mixing P in GaAs more favorable for increasing mean-field Tc than Al Up to a factor of ~1.5 Tc enhancement 10% Mn Ga(As,P) 5% Mn theory Mašek, et al. preprint (2006) Microscopic TBA/CPA or LDA+U/CPA

  12. Mnint formation in mixed (Al,Ga)As and Ga(As,P) higher in (Al,Ga)As and Ga(As,P) than in GaAs smaller interstitial space only in Ga(As,P) theory No reduction of Mnint in (Al,Ga)As Mixing P in GaAs more favorable for suppressing Mnint formation

  13. Limits to carrier-mediated ferromagnetism in (Mn,III)V Delocalized holes long-range coupl. Weak hybrid. InSb, InAs, GaAsTc: 7  173 K d5 Similar hole localization tendencies in (Al,Ga)As and Ga(As,P) Impurity-band holes short-range coupl. Strong hybrid. GaP Tc: 65 K Scarpulla, et al. PRL (2005) d 5 d 4 no holes d (GaN ?) d4

  14. III = I + II  Ga = Li + Zn GaAs and LiZnAs are twin SC Wei, Zunger '86; Bacewicz, Ciszek '88; Kuriyama, et al. '87,'94; Wood, Strohmayer '05 LDA+U sais that Mn-doped are also twin DMSs Masek, et al. preprint (2006)

  15. No solubility limit for group-II Mn substituting for group-II Zn theory Additional interstitial Li in Ga tetrahedral position - donors n-type Li(Zn,Mn)As

  16. L As p-orb. Ga s-orb. As p-orb. Electron mediated Mn-Mn coupling n-type Li(Zn,Mn)As - similar to hole mediated coupling in p-type (Ga,Mn)As EF Comparable Tc's at comparable Mn and carrier doping and Li(Mn,Zn)As lifts all the limitations of Mn solubility, correlated local-moment and carrier densities, and p-type only in (Ga,Mn)As

  17. Conclusions 1.Relatevily small technological investment in Mn-doped (Al,Ga)As - useful info about trends in III-V's and for Ga(As,P) 2. More tech. difficult Mn-doped Ga(As,P) - could lead to higher Tc 3. Adventureous Mn-doped LiZnAs - might heal key problems in (Ga,Mn)As & n-type FS

  18. super-exchange (anti-ferro) kinetic exchange (RKKY) Mn Mn Mn As Ga Intrinsic properties of Ga1-xMnxAs room-Tc for x=10% hole density (nm-3) hole density / Mn density Effective kinetic-exchange Hamiltonian, microscopic TBA or LDA+U • Tc linear in MnGa local moment concentration • Falls rapidly with decreasing hole density in more than • 50% compensated samples • Nearly independent of hole density for compensation < 50%.

  19. Extrinsic effects - covalent SC do not like doping self-compensation by interstitial Mn - MnGa As + MnI Ga Interstitial MnI is detrimental to magnetic order: compensating double-donor – reduces carrier density attracted to substitutional MnGa acceptor and couples antiferromagnetically to MnGa even at low compensation Yu et al., PRB ’02;Blinowski PRB ‘03; Mašek, Máca PRB '03

  20. Tc=173K 8% Mn Tc in as-grown and annealed samples Open symbols as-grown. Closed symbols annealed Total Mn doping (%)

  21. - MnGa As + Closed symbols are annealed samples MnI Ga High compensation Linear increase of Tc with effective Mn moment doping Mneff = MnGa-MnI Effective Mn doping (%) Tc increases with Mneff when compensation is less than ~40%. No saturation of Tc at high Mn concentrations

  22. Mn As L Ga (III,Mn)V materials: Microscopic picture of Mn-hole coupling in (Ga,Mn)As Mn GaAs d5 Ga s-orb. 1 Mn 0.1eV acceptor many Mn  As p-orb.  As 4p - Mn 3d hybridization d5

  23. Mn d  level Ed Ed Mn d  level = Mixed (Al,Ga)As and Ga(As,P) hosts 1/|Ed| + 1/|Ed| ~ const. |Vpd|2 ~ alc-7 Hole - local moment Kondo coupling: Mean-field Curie temperature: 4% in GaP and AlAs 50% in GaP

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