Fe 1-x Co x Si, a Silicon Based Magnetic Semiconductor

Transcription

1 Fe 1-x Co x Si, a Silicon Based Magnetic Semiconductor T (K) 1 5 Fe.8 Co.2 Si ρ xy (µω cm) J.F. DiTusa N. Manyala LSU Y. Sidis D.P. Young G. Aeppli UCL Z. Fisk FSU T C 1 Nature Materials 3, (24) H (T) University of Houston October 14, 24

2 Outline MnSi, FeSi, and CoSi Itinerant magnet, Kondo Insulator, Simple metal. Fe 1-y Co y Si Magnetotransport Electron-electron interactions Optical conductivity Less shiny in FM state The Anomalous Hall Effect Comparisons with ferromagnets, Heavy Fermions, and other magnetic semiconductors. Conclusions.

3 Magnetic Semiconductors Because of interest in Spintronics magnetic semiconductors have become popular. Advantages include potential for spin-polarized carrier sources and easy integration into semiconductor devices. Most thoroughly investigated is (GaMn)As, a Mn doped III-V semiconductor with T C < 15 K. Here we describe a different way to produce magnetic semiconductors Carrier doping of a strongly correlated, or Kondo insulator FeSi.

4 Phase Diagram of Fe 1-x,y Mn x Co y Si 1 T (K) T (K) metallic holes Helimagnetic Insulating metallic electrons σ (Ω cm) -1 M (G) 1e4 1e3 1e P S (µ B / FU).5.25 MnSi Fe.5 Mn.5 Si FeSi Fe.5 Co.5 Si CoSi one µ B per Co dopant

5 Magnetization 1. M (µ B per Co ion) Fe.85 Co.15 Si H (koe)

6 1 Pd + 1% Fe Pd + 1% Co Curie µ B / Saturated µ B 5 Pd+1%Rh+1%Fe ZrZn 2 Sc + 24%In Pd + 5%Ni MnSi Pd + 7%Ni Pd + 3%Fe Pd + 3%Co Pd + 5%Fe Pd + 7%Fe Pd + 7%Co Fe 1-y Co y Si Pd + 5%Co Ni + 35%Cu Ni + 69%Pd CoB Ni Ni + 58%Fe CrBr Fe 3 CrTe 1 EuO Gd MnB FeB MnSb 5 1 Curie Temperature (K)

7 Phase Diagram II Fe 1-y Co y Si Fully polarized Half metallic (y<.35) Lattice constant (A) 4.55 Vegard s Law n (1 22 / cm 3 ) 5 One hole per Mn dopant 1.5 One electron per Co dopant n (FU) -1 µ H (cm 2 / Vs) 4 2 T = 5 K No Obvious trends MnSi Fe.5 Mn.5 Si FeSi Fe.5 Co.5 Si CoSi

8 Results of Band Structure Calculations DOS [states / (ev unit-cell)] up down up down 1 2 E (ev) Fe.8 Co.2 Si -Fe 1-x Co x Si is predicted to be half metallic FeSi

9 Magnetoresistance Fe.8 Co.2 Si MnSi ρ (µω cm) T C T C ρ (µω cm) Positive MR in a ferromagnet T C T C ρ/ρ (%) ρ/ρ (%)

10 Transverse and longitudinal MR very similar Magnetoresistance 1 ρ/ρ (%) MR not an orbital effect 5 Most likely due to coupling to electron spins Fe.85 Co.15 Si H (koe)

11 σ (Ω -1 cm -1 ) Low Temperature Conductivity 3 2 Conductivity T and H dependent down to very low T 1 Fe.9 Co.1 Si FeSi.95 Al.5 Fe.92 Mn.8 Si T T =.25 K -1-2 σ (Ω -1 cm -1 ) H= 9 T.5 1 T 1/2 (K 1/2 ) H H 1/2 (T 1/2 )

12 Electron-Electron Interactions - σ/t 1/2 (Ω -1 cm -1 K -1/2 ) Si:B near the MI transition n (1 18 cm -3 ) 2 σ T and H/T Scaling of conductivity S. Bogdanovich et al. PRL 74, 2543 (1995). H 4 8 H/T (T/K) 12

13 Electron-Electron Interactions Altschuler-Aronov effect disorder and interactions When in proximity to the metal-insulator transition poor screening of electromagnetic fields. Diffusive motion of the carriers More than one carrier interaction during phasebreaking time Enhanced Coulomb interactions. Square-root singularity in the density of States

14 Electron-Electron Interactions For Paramagnetic FeSi 1-x Al x we find good agreement of MR with theory. [ σ ( H, T) σ ] = f ( g H / k T) µ B B f ( x) x 2 for x << 1; f ( x) x for x >> 1 There is no theory for e-e interaction effects in a FM. Simplest approach taken: H H = H + αm eff Best scaling of the data used to find α.

15 Scaling of the Conductivity (σ - σ)/t 1/2 (Ω -1 cm -1 K -1/2 ) Fe.7 Co.3 Si a + b(h eff / T) 2.2 < T < 1 K & < H < 32 T c + d(h eff / T) 1/2 Manyala et al. Nature 44, 581 (2) H + αm / T (T / K) 1

16 Scaling of the Conductivity 4.5 σ Fe.7 Co.3 Si σ (mω -1 cm -1 ) 4. H eff = model c + d(h eff / T) 1/ T (K) a + b(h eff / T) 2 1

17 Density of States 1 Fe.85 Co.15 Si Energy in mev M Γ DOS (1 23 / ev cm 3 )

18 Conclusions for MR Fe 1-y Co y Si has a large positive magnetoresistance unlike most ferromagnets. Conductivity obeys H/T scaling when magnetization properly accounted for. Quantum interference effects are important for understanding transport Important contribution to σ for Temperatures up to ~1 K.

19 DOS 1/eV Formula Unit 1 1 Intrinsic semiconductor T = Optical properties of.5 E (ev) Semiconductors Electron Doped Semiconductor Spin Polarized Electron Doped Semiconductor.5.5 E (ev) E (ev)

20 (GaMn)As Optical Conductivity Low frequency spectral weight increases for x<.52 New resonance appears at 2 cm -1. Singley et al PRL 89, 9723

21 (GaMn)As Temperature Dependence Low frequency spectral weight increases below T C = 72 K Scattering rate and m* decrease below T C. Singley et al PRL 89, 9723

22 Optical Properties of Magnetic Semiconductors DOS 1/eV Formula Unit 1 1 Spin Polarized doped semiconductor.5 E (ev) (GaMn)As Fe 1-x Co x Si.5.5 E (ev) E (ev)

23 Gap fills with temperature σ(ω) is flat gap is gone by for k B T > ½ Spectral Sum rule not satisfied for ω < 8 σ (ω) (1 3 Ω -1 cm -1 ) K 1 K 15 K 3 K FeSi 1 1, 1, ω / 2πc [cm -1 ]

24 σ (ω) (1 3 Ω -1 cm -1 ) Drude Spectral weight decreases below T C. Large discrepancy with band structure calculation. 1 K 8 15 K 3 K 4 Fe.8 Co.2 Si T C = 36 K 1 1, 1, ω / 2πc [cm -1 ]

25 Becomes less shiny below T c Fe.8 Co.2 Si 1 K 15 K 3 K

26 Change in the Optical Conductivity with Doping σ 1 (ω,t) Fe.8 Co.2 Si σ 1 (ω,t) FeSi Shows where added carriers reside ω / 2πc [cm -1 ] Ω 1 cm 1 T C Loss of spectral weight T (K)

27 Results of Band Structure Calculations DOS [states / (ev unit-cell)] up down up down 1 2 E (ev) Fe.8 Co.2 Si -Fe 1-x Co x Si is predicted to be half metallic FeSi -Larger DOS in PM phase than in FM phase

28 Scattering Rate and Resistivity Calculated Γ -the width of the Drude peak as a function of Temperature Compare with DC resistivity Γ [cm -1 ] Conclude that the carrier scattering rate increases below T C. M [G] CoSi T C.8 T MnSi Fe.7 Co.3 Si T C T C 5 T Fe.8 Co.2 Si Fe.8 Co.2 Si Γ ρ DC T (K) ρ DC [µω cm]

29 Electron-Electron Interactions; A Quantum Interference Effect Electron electron interactions DOS M, H > H= -3 3 E (mev) Carriers interact more than once during a Phase breaking scattering time. This increases the Coulomb interaction between carriers Result=singularities in the DOS at Fermi Energy

30 Conclusions for Optics The low frequency σ(ω) evolves in a manner that is inconsistent with standard semiconductors, (GaMn)As, and itinerant magnets. Doping produces optical response throughout the gap region- carriers donated to the conduction band of the insulator rather than an impurity band. Optical reflectivity decreases upon entering spinpolarized state rise in scattering rate caused by Coulomb interactions in a disordered system F.P. Mena et al. to be published.