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5. Magnetic properties and disorder; recent developments; questions for the future  Magnetic properties and disorder Anisotropic RKKY interaction Effect.

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Presentation on theme: "5. Magnetic properties and disorder; recent developments; questions for the future  Magnetic properties and disorder Anisotropic RKKY interaction Effect."— Presentation transcript:

1 5. Magnetic properties and disorder; recent developments; questions for the future  Magnetic properties and disorder Anisotropic RKKY interaction Effect of weak disorder on the magnetization Disorder and magnetization: Strong disorder approach Lightly doped, strongly disordered DMS: Percolation picture Stability of collinear magnetic state, magnetic fluctuations Spin-charge coupling: possible stripe order Magnetic order and transport: Resistive anomaly  Recent experiments – new puzzles  Magnetic semiconductors – what next?

2 impurity spins (random positions) carriers spin-spin interaction Coulomb disorder defects carrier scattering carrier-carrier interaction magnetic order magnetic fluctuations Magnetic properties and disorder Complicated system of coupled carriers and spins impurity spins (random positions) carriers spin-spin interaction Coulomb disorder defects carrier scattering magnetic order magnetic fluctuations carrier-carrier interaction

3 Experiments:  more disorder → straight/convex magnetization curves  annealing seems to reduce disorder (curves more mean-field-like)  magnetization for T ! 0 is significantly less than saturation Potashnik et al. (2001) Park et al., PRB 68, (2003) Mn -implanted GaAs:C (Ga,Mn)As

4 Anisotropic RKKY interaction Reminder: local moment polarizes carrier band, other local moments see oscillating magnetization (“integrate out the carriers”) RKKY interaction for local carrier-impurity exchange J pd and parabolic band (Lecture 3) is isotropic in real and spin space  Zaránd & Jankó, PRL 89, (2002): local J pd and spherical (4-band) approximation with spin-orbit interaction → H RKKY isotropic in real space, highly anisotropic in spin space → frustration  Brey & Gómez-Santos: PRB 68, (2003): non-local J pd with Gaussian form, realistic 6-band k ¢ p Hamiltonian → weakly anisotropic in real and spin space isotropic in real space isotropic in spin space

5 Attempt at more realistic description: C.T. & MacDonald, PRB 71, (2005) (a) Start from Slater-Koster tight binding theory for GaAs with spin-orbit coupling [Chadi (1977)] (b) Incorporation of Mn d-orbitals hybridization with GaAs sp-orbitals: photoemission (Okabayashi), ab-initio (Sanvito) interactions: Hubbard U and Hund‘s 1st rule J H to preserve spherical symmetry of d-shell in real and spin space: Parmenter, PRB 8, 1273 (1973) – 5-orbital Anderson model does not 16 bands

6 (c) Canonical perturbation theory for strong U, J H for single Mn Method: Chao et al., PRB 18, 3453 (1978), related to Schrieffer-Wolff transformation  expand in   choose T (hermitian) to make linear order vanish  approximation: truncate after 2nd order  set  = 1  approximation: project onto ground state with N = 5, S = 5/2 in ( + ), out ( – ) unitary transformation does not change the physics projected energies: EFEF

7 Insertingone finds spin scattering with d 5 ! d 4 d 5 ! d 6 k dependent ↔ nonlocal in real space For small k, k´ only  =  ´ = p x,y,z  antiferromagnetic  correct order of magnitude

8 (d) RKKY interaction  two Mn impurities at 0 and R → canonical transformation  integrate out the carriers  oscillating  ferromagnetic at small R  highly anisotropic in real space (from bands)  anisotropic in spin space for larger R (spin-orbit) G G J J S1S1 S2S2

9 Can the spin anisotropy lead to non-collinear magnetization and reduction of average magnetization at T = 0 (Zaránd & Jankó)? For magnetization in z direction typical effective field in transverse ( x ) direction is given by With our results for J ij this is small even at x = 0.01 → no strong non-collinearity or reduction due to this mechanism Complementary ab-initio approach: Do LDA ( +U ) – usually without spin-orbit coupling – for supercell with 2 Mn impurities in various positions, extract J(R) from total energy → highly anisotropic in real space, isotropic in spin space (no spin-orbit!) Typically overestimates J(R) and thus T c (while we underestimate it)

10 Effect of weak disorder on the magnetization Results for magnetization and T c in Lecture 3 did not include disorder Inclusion of disorder: Coherent potential approximation (CPA) Takahashi & Kubo, PRB 66, (2002); also Bouzerar, Brey etc. impurity sites  only local Coulomb disorder potential (or zero in most other works)  (local J, simple semicircular DOS assumed) CPA: Approximate true scattering by multiple scattering at a single impurity embedded in an effective medium – good for low impurity concentration Cannot describe localization, difficult to treat extended scatterers (Coulomb)

11 Typical for CPA and DMFT calculations with point-like impurities: T c decreases for high hole concentrations (weak compensation) Origin: Impurity band broadens for higher spin polarization (since it is mostly due to large J ) → spin polarization unfavorable for band filling & 1/2 (Ga,Mn)As : Takahashi & Kubo x = hole per Mn increased magnetization Problem: Always requires unphysically large J to obtain reasonable T c, since no long-range Coulomb potentials → incorrect impurity-band physics

12 Disorder and magnetization: Strong disorder approach Method: mean-field theory with disorder Zener model with potential disorder: convex → concave for less disorder annealing C.T. et al., PRL 89, (2002)

13 Lightly doped, strongly disordered DMS: Percolation picture For low concentrations x of magnetic impurities in III-V DMS Kaminski & Das Sarma, PRL 88, (2002); PRB 68, (2003), Alvarez et al., PRL 89, (2002), etc.  at T = 0 : ferromagnetism if aligned clusters percolate  at T > 0 : two clusters align if the weak coupling J´ between them is & k B T → express by T -dependent BMP radius For lower T the aligned region growths and eventually percolates at T c (Kaminski & Das Sarma) weak link J´ Exponentially small for small hole concentration, but not zero (since no quantum fluctuations) – compare VCA/MF: T c / n h 1/3 n i

14 Global magnetization is carried by sparse cluster at T. T c : small Percolation theory: Kaminski & Das Sarma (2002) holes only total  Highly convex magnetization curves (upwards curvature)  MC: magnetization at T ! 0 strongly reduced compared to saturation Monte Carlo simulations: Mayr et al., PRB 65, (R) (2002) For clustered defects: large ordered clusters exist for T & T c, percolate at T c → more rapid (Brillouin-function-like) increase of bulk magnetization

15 Goldstone mode Stability of collinear magnetic state, magnetic fluctuations Question: Is the collinear state found by approximate (mean-field) methods stable against magnetic fluctuations? In particular with disorder? Expand energy around mean-field solution → density of states of magnetic excitations Schliemann & MacDonald, PRL 88, (2002) etc.: spin disorder, no Coulomb disorder  DOS at negative energies: state not stable  …collective spin excitations involving many spins (high participation ratio) Schliemann, PRB 67, (2003): 6-band k ¢ p model  solution not even stationary

16 C.T., J. Phys.: Cond. Mat. 15, R1865 (2003): Spin and Coulomb disorder, parabolic band  DOS at negative energies: state not stable  clustered defects: DOS shifts away from zero → stiffer magnetic order Collinear state unstable due to anisotropic magnetic interactions – but argument from RKKY interaction suggests that the deviation is small

17 Spin-charge coupling: possible stripe order C.T., cond-mat/ Carrier-mediated ferromagnetism → strong dependence of magnetism on hole concentration in (In,Mn)As, (Ga,Mn)As Landau theory for magnetization m and excess carrier concentration  n : Ohno et al. (2000) with magnetization charge Introduce electrostatic potential  : for p-type DMS

18 Spin- and charge-density waves (stripes) typical solutions lowest energy periodic, anharmonic magnetization and carrier density for

19 Magnetic order and transport: Resistive anomaly in dirty itinerant ferromagnets Zumsteg & Parks, PRL 24, 520 (1970) Ni Potashnik et al., APL 79, 1495 (2001) (Ga,Mn)As R(T)R(T) dR/dT ρ(T)ρ(T) TcTc TcTc

20  scattering from magnetic fluctuations  close to T c : critical slowing down → static, elastic Theories for paramagnetic regime, T > T c : (1) de Gennes & Friedel, J. Phys. Chem. Solids 4, 71 (1958) Approach equivalent to:  perturbation theory, similar to inverse quasiparticle lifetime, but transport rate involves factor  anomaly from small momentum transfers q Ornstein/Zernicke (sharp maximum)

21 (2) Fisher & Langer, PRL 20, 665 (1968) disorder damping for large length scales ↔ small q : electronic Green function decays exponentially on scale l (mean free path)  no de Gennes-Friedel singularity from small q  weak singularity from large q ¼ 2k F, have to go beyond Ornstein/Zernicke/Landau theory: Equivalent to Boltzmann equation approach (Lecture 4), disorder and magnetic scattering treated on equal footing Problem: fails for magnetic correlation length  (T) À l (mean free path), magnetization variations are explored by diffusive carriers α : small anomalous specific-heat exponent

22 (phase coherence length)  magnetization ~ constant in cells  conductivity of network:  spatial average h … i  large system: equivalent to average over quenched disorder magnetization (thermal) Beyond the Boltzmann approach C.T., Raikh & von Oppen, PRL 94, (2005) (a) Description of transport on large length scales 3D resistor network

23 (b) Two spin subbands: ↑,↓ UCF Correlation function spin ↑, ↓ carriers have different Fermi energies but see same disorder universal conductance fluctuations (UCF)

24 is a scaling function of x = eff. Zeeman energy £ diffusion time (Stone 1985, Altshuler 1985, Lee and Stone 1985) Correlations decrease with increasing Zeeman energy (c) With spin-orbit coupling: realistic case (d) Typical magnetization assuming Gaussian fluctuations: is a scaling function H(y) of y = eff. Zeeman energy £ spin-orbit time, increases by factor of 2 in strong effective Zeeman field long-wavelength modes

25 scaling function with For Gaussian fluctuations: Beyond Gaussian fluctuations: Stronger singularity than in Fisher/Langer and de Gennes/Friedel theories Condition: Transport disorder-dominated at T c (low T c, strong disorder) – (In,Mn)Sb ? maximum at T c

26 Recent experiments – new puzzles Ferromagnetism in superdilute magnetic semiconductors Dhar et al., PRL 94, (2005); Sagepa et al., cond-mat/ GaN:Gd with Gd concentrations from 7 £ to 2 £ cm -3 ( x = 8 £ to 2 £ )  wurtzite structure  formal valence Gd 3+ : isovalent, configuration 4d 7, local spin S = 7/2  high concentration of native donors ( N vacancies) expected Observations:  room-temperature ferromagnetism ( T c » 360K for x = 8 £ )  highly insulating sweep field-cooled zero-field-cooled

27  giant magnetic moment per Gd for low x moment per Gd m absolute Magnetization must be carried by “something else” – native defects? How does very little Gd induce magnetic ordering?  effective field acting on VB is reversed photoluminescence

28 Sn 1-x X x O 2 Two species of substitutional Mn in (Ga,Mn)As : XAS, XMCD results Kronast et al. (BESSY II Collaboration), submitted  Mn 2+ with ~ 3d 5 configuration, large moment, orders magnetically  Mn 3+ with ~ 3d 4 configuration, strong valence fluctuations, large moment, does not order Questions: What mechanism lifts the d 5 ! d 4 transition by several eV? Why does high-spin Mn 3+ not participate in the ordering? “ d 0 ferromagnetism” in oxides Coey et al., Nature Mat. 4, 173 (2005)  ferromagnetism in HfO 2 (no partially filled shells?)  magnetization extraplotates to nonzero value for strongly diluted DMS

29 Magnetic semiconductors – what next? Goals for DMS experiments:  control of growth dependence, reproducability  unconventional DMS (oxides etc.), concentrated magnetic semiconductors  other magnetic probes: NMR/NQR/  SR and neutron scattering  dynamics: optical pump-probe and noise  crossover to antiferromagnetism, superconductivity, QHE… …and theory:  study of crossover between weak doping (BMP‘s) and band picture  unconventional DMS (oxides etc.): different mechanisms?  better ab-initio methods to get hydrogenic impurity level of Mn in GaAs  detailed simulation of DMS growth to find defect distribution  selfconsistent theory of scattering and carrier-mediated magnetism

30 DMS/nonmagnetic semiconductor heterostructures Transport, disorder & magnetism layers, single layer vs. superlattice metallic or insulating? magnetic properties? 2.FNF structure: RKKY coupling between layers, control by gate voltage – unlike metal structures 3.DMS quantum dots many local moments few local moments 4.DMS/nonmagnetic interfaces, spin injection MC simulation of interdiffusion cf. experiment: Kawakami et al., APL 77, 2379 (2000)

31 Electronic correlations & quantum critical points  electronic correlations: (Ga,Gd)N ?  at least two quantum critical points: ferromagnetic end point metal-insulator transition  …with overlapping critical regions  Griffiths-McCoy singularities: rare regions relevant for poperties Galitski et al., PRL 92, (2004) Vision: DMS are ideal materials to study the interplay of disorder and electronic correlations: both are important and can be tuned Possible parallels to cuprates: indications that dopand-induced disorder is important in cuprates, McElroy et al., Science 309, 1048 (2005)

32 I am grateful for discussions and collaborations with G. Alvarez, W.A. Atkinson, M. Berciu, L. Borda, G. Bouzerar, L. Brey, H. Buhmann, K.S. Burch, S. Dhar, T. Dietl, H. Dürr, S.C. Erwin, G.A. Fiete, E.M. Hankiewicz, F. Höfling, P.J. Jensen, T. Jungwirth, P. Kacman, J. König, J. Kudrnovský, A.H. MacDonald, L.W. Molenkamp, W. Nolting, H. Ohno, F. von Oppen, C. Paproth, M.E. Raikh, F. Schäfer, J. Schliemann, M.B. Silva Neto, J. Sinova, C. Strunk, G. Zaránd and others Diluted Magnetic Semiconductors Prof. Bernhard Heß-Vorlesung 2005

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