Transcript Slide 1
Anisotropic magnetoresistance effects in ferromagnetic semiconductor and metal devices Tomas Jungwirth Institute of Physics ASCR Alexander Shick, Jan Mašek, Josef Kudrnovský, František Máca, Karel Výborný, Jan Zemen, Vít Novák, Kamil Olejník, et al. Hitachi Labs., UK & Japan Jorg Wunderlich, Byong-Guk Park, Andrew Irvine, David Williams, Akira, Sugawara, et al. University of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, Kevin Edmonds, Andrew Rushforth, Chris King et al. University of Texas and Texas A&M Allan MacDonald, Jairo Sinova University of Wuerzburg Polish Academy of Sciences Laurens Molenkamp, Charles Gould Tomasz Dietl, et al. Tohoku University Hideo Ohno, et al. Outline 1. Intro - basic micromagnetics in DMSs 2. DMS materials science 3. AMR effects in DMSs and metals – devices and physics (Ga,Mn)As: an archetypical dilute moment FM semiconductor Ga SW-transf. Jpd SMn . shole As-p-like holes Mn As Mn Mn-d-like local moments Dilute Mn-doped SC: sensitive to doping; 100smaller Ms than in conventional metal FMs Mn-Mn coupling mediated by holes in SO-coupled SC valence bands: sensitive to gating, comparable magnetocrystalline anisotropy energy and stiffness to metal FMs For not too strong p-d hybridization: kinetic-exchange (Jpd) & host SC bands provides simple yet often semiquantitative description MF-like M(T); square hysteresis loops 1 mm 500 nm Macro (100’s m) domains; 8K 10-100 nm domain walls (~A/K) reflecting combined T-dependent uniaxial and cubic anisotropies 22 K One Strain controlled micromagnetics and current induced DW dynamics tunable 100x smaller critical currents than in metals Huge hysteretic MR tunable by gate due to CBAMR spintronic transistor (b) 0.1-1 m … plus weak dipolar crosslinks prospect for dense integration of magnetic microelements One Outline 1. Intro - basic micromagnetics in DMSs 2. DMS materials science 3. AMR effects in DMSs and metals – devices and physics coupling strength / Fermi energy Magnetism in systems with coupled dilute moments and delocalized band electrons band-electron density / local-moment density (Ga,Mn)As GaAs VB GaAs:Mn extrinsic semiconductor Mn-acceptor level (IB) GaMnAs disordered VB 2.2x1020 cm-3 VB-IB VB-CB Short-range ~ M . s potential - additional Mn-hole binding - ferromagnetism - scattering MIT in GaAs:Mn at order of magnitude higher doping than quoted in text books MIT in p-type GaAs: - shallow acc. (30meV) ~ 1018 cm-3 - Mn (110meV) ~1020 cm-3 Mobilities: - 3-10x larger in GaAs:C - similar in GaAs:Mg or InAs:Mn > 2% Mn: metallic but strongly disordered Mn spacing Model: SO-coupled, exch.-split Bloch VB & disorder - conveniently simple and increasingly meaningful as metallicity increases - no better than semi-quantitative MnGa solubility limit Covalent SCs do not like doping self-compensation by interstitial Mn Interstitial MnInt is detrimental to magnetic order charge and moment compensation defect + Mnsub As MnInt Mnsub Can be annealed out MnInt Tc 95K in as-grown (9% Mn) theory & exp. to 173 in annealed (6% Mnsub) Ga but MnGa < nominal Mn Weak hybrid. Delocalized holes long-range coupl. Search for optimal III-V host: optimal combination of hole delocalization, InSb, InAs, GaAs d5 p-d coupling strength, low self-compensation Strong hybrid. Impurity-band holes short-range coupl. GaP, AlAs d5d4 no holes d GaN d4 I-II-Mn-V ferromgantic semiconductors III = I + II Ga = Li + Zn • GaAs and LiZnAs are twin semiconductors • Prediction that Mn-doped are also twin ferromagnetic semiconductors • No limit for Mn-Zn (II-II) substitution • Independent carrier doping by Li-Zn stoichiometry adjustment Outline 1. Intro - basic micromagnetics in DMSs 2. DMS materials science 3. AMR effects in DMSs and metals – devices and physics Anisotropic, SO-coupled, exchange-split hole bands M || <100> M || <111> Chemical potential CBAMR M Tunneling DOS TAMR M I Impurity scattering rates AMR I Coulomb blockade AMR – anisotropic chemical potential Source Q VD Drain Gate VG Q( M ) U dQ'VD ( Q' ) e 0 [110] [010] M F Q [100] [110] [010] ( Q Q0 )2 ( M ) C U & Q0 CG [ VG VM ( M )] &VM 2C e CG electric & magnetic control of Coulomb blockade oscillations Worth trying to look for 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 ferromagnet change of |(M)|~100K • Room-T conventional SET (e2/2C >300K) possible Tunneling AMR – anisotropic TDOS TAMR in GaMnAs Au GaMnAs Au Anisotropc tunneling amplitudes M perp. Resistance AlOx Magnetisation in plane ~ 1-10% in metallic GaMnAs M in-plane Huge when approaching MIT in GaMnAs TAMR in metals theory experiment Anisotropic magnetoresistance EXPERIMENT THEORY Semiquantitative numerical understanding in GaMnAs Qualitative physical (analytical) picture SO & polarized scatterers anisotropic scattering