1. Tomas Jungwirth, Jan Mašek, Alexander Shick Karel Výborný, Jan Zemen, Vít Novák, et al. Bryan Gallagher, Tom Foxon, Richard Campion, Kevin Edmonds, Andrew Rushforth, et al. Joerg Wunderlich, Andrew Irvine, Elisa Ranieri, et al. Cambridge Nottingham Prague Jairo Sinova (TAMU), Allan MacDonald (UT), et al (except Nov. 26 th ) Making semiconductors magnetic: A physics tango approach to engineering quantum materials Future Directions Workshop October 11 th 2007, Austin TX

2. Technologically motivated and scientifically fueled Incorporate magnetic properties with semiconductor tunability (MRAM, etc) Understanding complex phenomena: Spherical cow of ferromagnetic systems (still very complicated) Engineered control of collective phenomena Benchmark for our understanding of strongly correlated systems Generates new physics: Tunneling AMR Coulomb blockade AMR Nanostructure magnetic anisotropy engineering ENGINEERING OF QUANUTM MATERIALS More knobs than usual in semiconductors: density, strain, chemistry/pressure, SO coupling engineering. Same as in oxides but better control of its consequences.

3. Mn Ga As Mn Ferromagnetic semiconductors GaAs - standard III-V semiconductor Group-II Mn - dilute magnetic moments & holes & holes (Ga,Mn)As - ferromagnetic semiconductor semiconductor More tricky than just hammering an iron nail in a silicon wafer

4. Mn Ga As Mn Mn–hole spin-spin interaction hybridization Hybridization  like-spin level repulsion  J pd S Mn  s hole interaction Mn-d As-p

5. H eff = J pd || -x Mn As Ga h eff = J pd || x Hole Fermi surfaces Ferromagnetic Mn-Mn coupling mediated by holes

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

7. Dilute moment nature of ferromagnetic semiconductors Ga As Mn 10-100x smaller M s One Current induced switching replacing external field Tsoi et al. PRL 98, Mayers Sci 99 Key problems with increasing MRAM capacity (bit density): - Unintentional dipolar cross-links - External field addressing neighboring bits 10-100x weaker dipolar fields 10-100x smaller currents for switching Sinova et al., PRB 04, Yamanouchi et al. Nature 04

8. Integrated read-out, storage, and transistor Low current driven magnetization reversal Parkin, US Patent (2004) Magnetic race track memory Yamanouchi et al., Nature (2004) 2 orders of magnitude lower critical currents in dilute moment (Ga,Mn)As than in conventional metal FMs Sinova, Jungwirth et al., PRB (2004) Wunderlich, et al., PRL (2006) Ideal to study spintronics fundamentals Family of extraordinary MR effects, R(M), in ohmic, tunneling, Coulomb-blockade regimes

9. Low M s (1-10% Mn moment doping): 100-10 x weaker mag. dipolar interactions  can allow for 100-10 x denser integration without unintentional dipolar cross-links Strong SO-coupling: magnetocrystalline anisotropy ~ 10mT & doping and strain dependent  can replace demagnetizing shape anisotropy fields & local control of magnetic configurations... and more yet to be fully exploited

10. GaMnAs Au AlOx Au Tunneling anisotropic magnetoresistance (TAMR) Bistable memory device with a single magnetic layer only Gould, Ruster, Jungwirth, et al., PRL '04 Giant magneto-resistance (Tanaka and Higo, PRL '01) [100] [010] [100] [010] [100] [010] Recently discovered in metals as well!

11. One Dipolar-field-free current induced switching nanostructures Micromagnetics (magnetic anisotropy) without dipolar fields (shape anisotropy) ~100 nm Domain wall Strain controlled magnetocrystalline (SO-induced) anisotropy Can be moved by ~100x smaller currents than in metals Humpfner et al. 06, Wunderlich et al. 06

12. Huge & tunable magnetoresistance in (Ga,Mn)As side-gated nano-constriction FET Wunderlich, Jungwirth, Kaestner, Shick, et al., preprint Single-electron transistor

13. Coulomb blockade anisotropic magnetoresistance Band structure (group velocities, chemical potential scattering rates, chemical potential) depend on Spin-orbit coupling If lead and dot different (different carrier concentrations in our (Ga,Mn)As SET) & electric & magnetic control of Coulomb blockade oscillations

14. Mn-d-like local moments As-p-like holes Mn Ga As Mn EFEF DOS Energy spin  spin  GaAs:Mn – extrinsic p-type semiconductor with 5 d-electron local moment on the Mn impurity valence band As-p-like holes As-p-like holes localized on Mn acceptors << 1% Mn onset of ferromagnetism near MIT Jungwirth et al. RMP ‘06 ~1% Mn >2% Mn STILL LARGELY UNEXPLORED SYSTEMATICALLY: MIT

15. Impurity band to disordered-valence-band cross over in high-doped GaAs:Mn: red-shift of the IR peak in GaMnAs At low doping near the MI transition Non-momentum conserved transitions to localized states at the valence edge take away spectral weight from the low frequency As metallicity/doping increases the localized states near the band edge narrow and the peak red-shifts as the inter-band part adds weight to the low-frequency part

16.  Curie temperature limited to ~110K.  Only metallic for ~3% to 6% Mn  High degree of compensation  Unusual magnetization (temperature dep.)  Significant magnetization deficit But are these intrinsic properties of GaMnAs ?? “110K could be a fundamental limit on T C ” As Ga Mn Problems for GaMnAs (late 2002)

17. Can a dilute moment ferromagnet have a high Curie temperature ? The questions that we need to answer are: 1.Is there an intrinsic limit in the theory models (from the physics of the phase diagram) ? 2.Is there an extrinsic limit from the ability to create the material and its growth (prevents one to reach the optimal spot in the phase diagram)? EXAMPLE OF THE PHYSICS TANGO

18. As Ga Mn T c linear in Mn Ga local moment concentration; falls rapidly with decreasing hole density in more than 50% compensated samples; nearly independent of hole density for compensation < 50%. Jungwirth, Wang, et al. Phys. Rev. B 72, 165204 (2005) Intrinsic properties of (Ga,Mn)As

19. Extrinsic effects: Interstitial Mn - a magnetism killer Yu et al., PRB ’02: ~10-20% of total Mn concentration is incorporated as interstitials Increased T C on annealing corresponds to removal of these defects. Mn As Interstitial Mn is detrimental to magnetic order:  compensating double-donor – reduces carrier density  couples antiferromagnetically to substitutional Mn even in low compensation samples Blinowski PRB ‘03, Mašek, Máca PRB '03

20. Theoretical linear dependence of Mn sub on total Mn confirmed experimentally Mn sub Mn Int Obtain Mn sub & Mn Int assuming change in hole density due to Mn out diffusion Jungwirth, Wang, et al. Phys. Rev. B 72, 165204 (2005) SIMS: measures total Mn concentration. Interstitials only compensation assumed Experimental partial concentrations of Mn Ga and Mn I in as grown samples

21. 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 INTRINSIC LIMIT NO EXTRINSIC LIMIT There is no observable limit to the amount of substitutional Mn we can put in

22. 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

23. - Effective concentration of uncompensated Mn Ga moments has to increase beyond 6% of the current record T c =173K sample. A factor of 2 needed  12% Mn would still be a DMS - Low solubility of group-II Mn in III-V-host GaAs makes growth difficult Low-temperature MBE Strategy A: stick to (Ga,Mn)As - alternative growth modes (i.e. with proper substrate/interface material) allowing for larger and still uniform incorporation of Mn in zincblende GaAs More Mn - problem with solubility Getting to higher Tc: Strategy A

24. Find DMS system as closely related to (Ga,Mn)As as possible with larger hole-Mn spin-spin interaction lower tendency to self-compensation by interstitial Mn larger Mn solubility independent control of local-moment and carrier doping (p- & n-type) Getting to higher Tc: Strategy B

25. conc. of wide gap component 0 1 lattice constant (A) 5.4 5.7 (Al,Ga)As Ga(As,P) (Al,Ga)As & Ga(As,P) hosts d5d5 d5d5 local moment - hole spin-spin coupling J pd S. s Mn d - As(P) p overlapMn d level - valence band splitting GaAs & (Al,Ga)As (Al,Ga)As & Ga(As,P)GaAs Ga(As,P) Mn As Ga

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

27. Using DEEP mathematics to find a new material 3=1+2 Steps so far in strategy B: larger hole-Mn spin-spin interaction : DONE BUT DANGER IN PHASE DIAGRAM lower tendency to self-compensation by interstitial Mn: DONE larger Mn solubility ? independent control of local-moment and carrier doping (p- & n-type)?

28. 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 Masek, et al. PRB (2006) LDA+U says that Mn-doped are also twin DMSs No solubility limit for group-II Mn substituting for group-II Zn !!!!

29. Electron mediated Mn-Mn coupling n-type Li(Zn,Mn)As - similar to hole mediated coupling in p-type (Ga,Mn)As L As p-orb. Ga s-orb. As p-orb. EFEF Comparable T c '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 Li(Mn,Zn)As just one candidate of the whole I(Mn,II)V family

30. ● Apply same strategic approach to Oxides, other strongly correlated materials (new different DMSs, etc) ● Exploit further new properties and physics: are the same physics present in DMSs in Oxides and other strongly correlated materials? ● Fill in the phase diagram SO WHAT NEXT?

31. BEFORE 2000 CONCLUSION: directors cut BUT it takes MANY to do the physics tango!! Texas A&M U., U. Texas, Nottingham, U. Wuerzburg, Cambirdge Hitachi, …. It IS true that it takes two to tango 2000-2004 2006 NEW DMSs, More heterostructures NEXT

32. EXTRA

33. MAGNETIC ANISOTROPY M. Abolfath, T. Jungwirth, J. Brum, A.H. MacDonald, Phys. Rev. B 63, 035305 (2001) Condensation energy depends on magnetization orientation compressive strain   tensile strain experiment:

34. Potashnik et al 2001 Lopez-Sanchez and Bery 2003 Hwang and Das Sarma 2005 Resistivity temperature dependence of metallic GaMnAs

35.  theory experiment Ferromagnetic resonance: Gilbert damping A a,k (  )

36. Anisotropic Magnetoresistance exp. T. Jungwirth, M. Abolfath, J. Sinova, J. Kucera, A.H. MacDonald, Appl. Phys. Lett. 2002

37. ANOMALOUS HALL EFFECT T. Jungwirth, Q. Niu, A.H. MacDonald, Phys. Rev. Lett. 88, 207208 (2002) anomalous velocity: M0M0 M=0 J pd N pd (meV) Berry curvature: AHE without disorder

38. ANOMALOUS HALL EFFECT IN GaMnAs Experiments Clean limit theory Minimal disorder theory

39. CBAMR  new device concepts

40. (Ga,Mn)As material 5 d-electrons with L=0  S=5/2 local moment intermediate 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 Mn Ga As Mn Ohno, Dietl et al. (1998,2000); Jungwirth, Sinova, Mašek, Kučera, MacDonald, Rev. Mod. Phys. (2006), http://unix12.fzu.cz/ms

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

42. d4d4 d Weak hybrid. Delocalized holes long-range coupl. Strong hybrid. Impurity-band holes short-range coupl. d 5  d 4 no holes InSb, InAs, GaAs T c : 7  173 K (GaN ?) GaP T c : 65 K Similar hole localization tendencies in (Al,Ga)As and Ga(As,P) Scarpulla, et al. PRL (2005) d5d5 Limits to carrier-mediated ferromagnetism in (Mn,III)V