Magnetic properties of Ferromagnetic Semiconductor (Ga,Mn)As M. - PowerPoint PPT Presentation

Magnetic properties of Ferromagnetic Semiconductor (Ga,Mn)As M. Saw icki M. Saw icki Institute of Physics, Polish Academy of Sciences, Warsaw, Poland . � T. Dietl, et al., Warsaw In collaboration with: � B. Gallagher, et al., Nottingham � L.W. Molenkamp, et al., Wuerzburg � H. Ohno, et al., Sendai Support by: Japanese ERATO, EU FENIKS , Polish MNiI School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 1

Outlook • Introduction - motivation/history • T C and M S • Uniaxial magnetic anisotropy due to confinement and/or biaxial (epitaxial) strain - reorientation transition • Biaxial (cubic, 4-fold) in-plane anisotropy • Uniaxial in-plane anisotropy - reorientation transition - single domain behaviour Hole driven ferro-DMS, mostly (Ga,Mn)As School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 2

Spintronics Making spins to: • store and reveal information in a faster way • transmit information (supplementing charge and light) • process information (supplementing charge) Main applications: Spin valve (or MTJ) • magnetic field sensors • read heads Anti Ferro • galvanic isolators Ferro • Magnetoresistive RAMs Conductor/Oxide Ferro Why semiconductor spintronics? Substrate School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 3

Semiconductor Spin-electronics (Spintronics) Spin-related phenomena in semiconductors → an additional degree of freedom (spin + charge → spintronics ) spin including magnetism spintronics optics electronics School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 4

Ferromagnetic semiconductors May offer a possibility to replace of ‘All metal’ Spin-Based Electronic Devices • they posses both spins and mechanism that effectively couples spins with carriers. • technological compliance with semiconductor industry. School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 5

Towards ferromagnetic semiconductors • magnetic semiconductors magnetic semiconductors and insulators: short-range antiferromagnetic superexchange EuTe, ...., NiO, ... short-range ferromagnetic super- or double exchange EuS, ZnCr 2 Se 4 , La 1-x Sr x MnO 3 , ... EuS/KCl,... • diluted magnetic semiconductors Standard semiconductor + magnetic ion II-VI: Cd 1-x Mn x Te, ..., Hg 1-x Mn x Se, ... IV-VI: Sn 1-x Mn x Te, ..., Pb 1-x Eu x S III-V: In 1-x Mn x Sb, ..., Ga 1-x Er x N IV: Ge 1-x Mn x , ..., Si 1-x Ce x School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 6

History of DMS School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 7

Most of DMS: random antiferromagnet short range antiferromagnetic superexchange B School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 8

Evidences for antiferromagnetic interactions: magnetic susceptibility Curie-Weiss law χ = C /( T − Θ ) C = g µ B S ( S +1) xN o /3 k B Θ < 0 antiferro A. Lewicki et al. School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 9

Magnetisation of localized spins M(T,H) = g µ B Sx eff N o B S [ g µ B H / k B (T + T AF )] antiferromagnetic interactions x eff < x T AF > 0 Modified Brillouin function Y. Shapira et al. no spontaneous magnetisation … School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 10

Determination of sp-d exchange integrals: - giant splitting of exciton states ∆ E ~ M ~ B S ( H) c.b. σ + σ - σ + σ - ENERGY v.b. g eff > 10 2 J. Gaj et al., R. Planel,.. A. Twardowski et al. -- s-d: I sd ≡ α N o ≈ 0.2 eV G. Bastard, … no s-d hybridization => potential s-d exchange -- p-d: I pd ≡ β N o ≈ - 1.0 eV large p-d hybridization and large intra-site Hubbard U => kinetic p-d exchange School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 11

Effect of acceptor doping on magnetic susceptibility in Zn 1-x Mn x Te:P χ -1 vs. T 5 p -Zn 1-x Mn x Te 17 cm -3 p ≈ 10 p x = 0.023 4 -1 [ a.u. ] 3 18 cm -3 p ≈ 5 × 10 χ 2 1 T CW 0 0 5 10 15 Temperature [ K ] Sawicki et al. (Warsaw) pss’02 K ę pa et al. (Warsaw, Oregon) PRL’03 School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 12

Ferromagnetic temperature in p-(Zn,Mn)Te -3 ) Hole concentration (cm 17 18 19 20 eff (K) 20 10 10 10 10 5x10 (Zn,Mn )Te:N 30 30 F / x (Zn,Mn )Te:P Ferromagnetic Temp. T 10 10 Metallic Insulating 1 1 • ferromagnetism disappears in the absence of holes • ferromagnetism on both sides of metal-insulator transition Ferrand et al. (Grenoble, Warsaw) PRB’01 Sawicki et al. (Warsaw) pss’02 School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 13

Ferromagnetism in DMS – the origin -- carriers localized by impurities (BMP): inoperative Bhatt et al., Dugaev et al., Inoue et al., Das Sarma et al., Dagotto et al., ... -- delocalized carriers (Zener/RKKY model) Ryabchenko, et al., Dietl et al., MacDonald et al., Boselli et al., Petukhov, Sham et al., … hole world Mn Mn k k E F + E F Mn world T ≤ T C Mn Mn n Mn M Exchange spin splitting redistributes the carriers Mn Mn between spin subbands thus lowering their energy School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 14

Ferromagnetism in DMS – the origin -- carriers localized by impurities (BMP): inoperative Bhatt et al., Dugaev et al., Inoue et al., Das Sarma et al., Dagotto et al., ... -- delocalized carriers (Zener/RKKY model) Ryabchenko, et al., Dietl et al., MacDonald et al., Boselli et al., Petukhov, Sham et al., … T C = x eff N 0 S(S+1)J 2 A F ρ ( ε F )/12k F Mn Mn holes!!! = valence band k -- s-d: I sd ≡ α N o ≈ 0.2 eV E F no s-d hybridization T ≤ T C Mn Mn -- p-d: I pd ≡ β N o ≈ - 1.0 eV Exchange spin splitting large p-d hybridization redistributes the carriers between spin subbands thus lowering their energy School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 15

Why DMS, why (Ga,Mn)As? Carrier mediated ferromagnetism in semiconductors: More than 20 compounds showed x = 0.05, p = 3.5 × 10 20 cm -3 ferro- coupling so far C Si Ge Operational criteria: AlP AlAs GaN • Scaling of T C and M GaP GaAs with x and p GaSb InP InAs • Interplay between InSb semiconducting and ZnO ZnSe ferromagnetic ZnTe CdTe properties 10 100 1000 Curie temperature (K) T. Dietl, et al., Science 2000 School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 16

(Ga,Mn)As: single phase ferro-DMS T F = x eff N 0 S(S+1)J 2 A F ρ ( ε F )/12k F M REM (M Spontaneous ) [ a.u. ] 8% (Ga,Mn)As T C = 173 K T C ~ xp 1/3 χ -1 300 [ a.u. ] Remnant 200 Magnetisation T C ( K ) 140 150 160 170 180 190 200 Temperature [ K ] 0.10 8% (Ga,Mn)As M [110] (T) / M Sat (5K) [ r.u. ] 100 T = 172 K 0.05 T = 175 K 0.00 0 0 2 4 6 8 10 Total x Mn ( % ) -0.05 25 nm thick T. Dietl, H. Ohno, F. Matsukura, PRB ‘01 -1 0 1 2 3 T C = 173 K = -100 o C Magnetic Field [ Oe ] K-Y. Wang, et al., JAP ‘04 & ICPS’27 School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 17

Operational criteria for carrier-controlled ferromagnetic semiconductors Spin-LED Ferro-FET Y. Ohno et al., Nature’99 Also: H. Ohno et al., Nature’00 • Current induced domain wall switching J C ~10 5 A/cm 2 M. Yamanouchi, et al., Nature’04 • Electrically assisted magnetisation reversal D. Chiba, et al., Science’03 School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 18

Why DMS, why (Ga,Mn)As? Carrier mediated ferromagnetism in semiconductors: More than 20 compounds showed x = 0.05, p = 3.5 × 10 20 cm -3 ferro- coupling so far C Si Ge Operational criteria: AlP AlAs GaN • Scaling of T C and M GaP GaAs GaAs with x and p GaSb InP InAs • Interplay between InSb semiconducting and ZnO ZnSe ferromagnetic ZnTe CdTe properties 10 100 1000 Curie temperature (K) T. Dietl, et al., Science 2000 School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 19

T C in (Ga,Mn)As: prospects 350 300 300 Increase Mn Record T C (K) incorporation 250 High index surfaces? 200 150 173 K LT annealing 100 50 0 1990 1995 2000 2005 2010 Date School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 20

Mn in GaAs 3d 4 (A 0 ) + e+e+e Mn(...3d 5 4s 2 ) + GaAs = v.b. e e e e 3d e h e e e e 3d 3d 5 +h = „3d 4 ” A 0 J = 1 v.b. h e e e e e 3d 3d 4+1 = „3d 5 ” A - S = 5/2, L = 0 Mn = spin 5/2 + hole School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 21

Growth of (Ga,Mn)As Mn source Mn total hole + S=5/2 SUBSTRATE zero compensation limit 16 (Ga,Mn)As -3 ] 12 20 cm Something went wrong! 8 p [ 10 4 0 0 2 4 6 8 10 K. Yu, et al. x tot [ % ] School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 22

Mn interstitials c-RBS and c-PIXE reveal: in low-temperature MBE grown ferromagnetic (Ga,Mn)As Mn atoms occupy three distinct positions in the lattice substitutional Mn Ga , interstitial Mn I , and random (MnAs) in proportions depending on annealing. Mn ⁻ K. Yu, et al., PRB’02 Ga Mn ++ interstitial Mn I : � Double donor � Does not play ferro � AF bonds to Mn Ga As Blinowski, Kacman, PRB’03 Low temperature annealing!! Potashnik et al.,’02 School of Magnetism: M. Sawicki on (Ga,Mn)As - Constanta 9/09/2005 23