Optical Control of Ferromagnetism in Mn-Doped III-V V Semiconductors

Transcription

1 @ TAMU, October 22, 23 Optical Control of Ferromagnetism in Mn-Doped III-V V Semiconductors Junichiro Kono Department of Electrical & Computer Engineering, Rice University, Houston, Texas Rice University Jigang Wang Giti Khodaparast Michael Zudov Tokyo Institute of Technology Hiro Munekata Akira Oiwa University of Tokyo Noboru Miura Yasuhiro Matsuda University of Florida Gary Sanders Fedir Kyrychenko Yongke Sun Chris Stanton

2 Outline 1. Background: Control of spins, magnetism, and semiconductor spintronics 2. High-Field Cyclotron Resonance in InMnAs 3. Transient THz Absorption in InGaMnAs 4. Ultrafast Optical Studies & Control Two-Color Pump-Probe MOKE: Ultrafast Photoinduced Softening

3 Optical Control of Ferromagnetism e - Ultrafast control of magnetism in semiconductors through optical excitation Light-induced transient modifications in magnetic properties Ultrafast dynamics of correlated electrons with long-range order

4 Ultrafast Optics in Ferromagnetic Metals E. Beaurepaire et al., PRL 76, 425 (1996); PRB 58, (1998). Ni CoPt 3 Ultrafast demagnetization (~ hundred fs) and slow recovery Possible application to ultrafast magnetooptical recording

5 III-V V Ferromagnetic Semiconductors Low-temperature MBE grown III 1-x Mn x V: InMnAs: T c < 6 K GaMnAs: T c < 17 K Mn-Mn exchange: hole mediated Mn ions (Mn 2+ ) = acceptors & local magnetic moments (3d 5, S = 5/2) Carrier density tuning External control of ferromagnetism

6 Magneto-Electronics 1 st generation spintronic devices based on ferromagnetic metals already in commercial use GMR read-out heads in hard drives Magnetic tunneling junction (MTJ) or spin valve Nonvolatile MRAM: Microchips that never forget S. Parkin (199) Compatibility with Si and GaAs next phase: semiconductor spintronics

7 Recent Discoveries in Semiconductors A room temperature, optically induced, very long lived quantum coherent spin state in semiconductors that responds at Terahertz with no dissipation and can be transported by small electric fields (UCSB). Ferromagnetism in semiconducting GaMnAs at 12K (Japan, Europe, U.S.A.). DARPA Spins in Semiconductors Program (2 present)

8 Spin-Enhanced and Spin- Enabled Electronics Quantum Spin Electronics Tunneling/transport of quantum confined spin states Spin dependent resonant tunneling devices and spin filtering Spin FETs ( spin gating ) Spin LEDs, electroluminescent devices, and spin lasers Coherent Spin Electronics Optically generated coherent spin states and coherent control of propagating spin information - optical encoders and decoders Quantum Information Processing Qubits using coherent spin states a > + b 1>, a 2 + b 2 = 1 Spin based quantum computing, teleportation, code breaking and cryptography

9 Experiment I: Materials Characterization through Cyclotron Resonance

10 Issues & Questions (III,Mn)V Semiconductors Basic electronic structure not yet determined accurately Where are the carriers mediating exchange? Are free holes localized or delocalized, d-like or p-like? Poor mobilities ultrahigh B-fields necessary

11 Megagauss Cyclotron Resonance 2 1. FIR Magnetic Field (T) Time (µs) B fields up to 15 T Transmission Megagauss Laboratory, Univ. of Tokyo Transmission is recorded twice, both in the up- and down-sweep Sample survives many measurements on a single sample High B fields needed due to low mobilities in doped samples.

12 Samples Studied n-type InMnAs films (paramagnetic) In 1 x Mn x As, x =, 2.5, 5. and 12. %, m ~ 45 cm 2 /Vs p-type InMnAs films (ferromagnetic) In 1 x Mn x As/InAs/GaAs, x < 2.5 %, T C < 5 K n-type and p-type InMnAs/InAs superlattices 5nm/5nm 11 periods & 5nm/5nm 85 periods In.53 Ga.47 MnAs films (ferromagnetic) (In.53 Ga.47 ) 1-x Mn x As/In.53 Ga.47 As/InP, x < 5 %, T C < 12 K p-type InMnAs/GaSb heterostructures (ferromagnetic) In 1 x Mn x As/GaSb, x ~ 9 %, T C = 3-55 K

13 Electron Cyclotron Resonance in Megagauss Fields First observation of CR in (III,Mn)V DMS M. A. Zudov et al., Phys. Rev. B 66, 16137(R) (22). G. D. Sanders et al., Phys. Rev. B, Oct. 15 (23). The resonance shifts with Mn doping x mass decreases. Excellent agreement with theory determination of α & β. Calculated g* vs. T, B and x.

14 Electron Cyclotron Resonance First observation of CR in (III,Mn)V DMS Transmission a). T = 3 K Experiment K 4 8 Magnetic Field (T) Theory The resonance shifts with Mn doping x mass decreases. Simple theory predicts no shift Excellent agreement with theory determination of α & β. Phys. Rev. B 66, 16137(R) (22).

15 Conduction Band g Factors At 3 K, without magnetic field and Mn concentration, g c -16 for InAs. Spin-up and spin-down Landau levels reverse with the increase of Mn doping, making g c change sign. Change dramatically with temperature, which is mainly due to the temperature dependent Brillouin function.

16 Hole States d d s s p-d hybridization E g p-like E g d-like d anti-bonding state (Mn impurity state) d d-like p-like bonding state p-like holes p-d exchange (Zener model) d-like holes double exchange CR : m *

17 Hole CR in p-in 1-x Mn x As at hn =.224 ev InMnAs #3 Mn.6% um h-active RT InMnAs R199 Mn 2.5% um h-active 15 K Transmission (5% per div.) α HH 11 K 75 K 52 K 37 K 19 K 16 K Transmission (2% per div.) α HH RT 16 K 7 K 46 K 17 K x =.6 B (T) x =.25 5 B (T) 1 15

18 Is the absorption band related to the Mn impurity band? Impurity cyclotron resonance (ICR)? with higher photon energy ICR CR E CR N=1 T ICR CR shift to a higher field N= B 2p+ B 1s ICR impurity band

19 Hole CR in p-in 1-x Mn x As at hn =.366 ev Transmission (5% per div.) In 1-x Mn x As (x=.6) HH µm (366 mev) hole-active 14 K K x =.6 B (T) No low field feature is observed! Feature observed at 224 mev is not ICR.

20 Hole Cyclotron Resonance G. D. Sanders et al., J. of Supercond. 16, 449 (23); J. Appl. Phys. 93, 6897 (23). Absorption due to high-order inter-landau-level transitions Lineshape depends on temperature and broadening. Temperature dependence Fermi surface sharpening. Provides info on hole density.

21 Polarization Dependent Hole CR e-active 5.53 µm h-active 16 K Transmission (5% per div.) 15 K x=.6 12 K x= 1.6 µm 52 K x=.6 37 K α HH HH 18 K HH 51K LH e-active (σ + ) circular polarization Due to valence band complexity HH x= LH 28K B (T)

22 Simulation of CR Spectra 8-band k?p method; J pd and finite k z effects are included + Fermi s golden rule 15 K 5.53 µm x =, B = 1 T (-1,1) (1,5) x=.6 12 K 16 K (,2) s - s + - Absorption (arb. units) x= 1.6 µm Experiment 52 K 51 K 18 K x = 5%, B = 1 T x=.6 28 K x= 37 K s - s B (T)

23 New Data on InMnAs/GaSb Transmission (5% per div.) e-active 167 K 8 K 15 K.117 ev h-active -1 1 Magnetic Field (T) T c = 35 K

24 Hole CR in the Ferromagnetic Phase Transmission (5% per div.) HH A Sample 1 LH B RT K K K K K K K 17 K A Sample 2 HH B LH RT 14 K 17 K 8 K 6 K 4 K 3 K 21 K 13 K Observed two lines with unusual T- dependence HH: abruptly shifts, narrows, and grows at low T LH: suddenly appears and grows at low T Magnetic Field (T) 1 15

25 Hole CR in the Ferromagnetic Phase new insight, puzzles, and speculations Clear Observation of Hole CR Existence of Delocalized p-like Holes in the Valence Bands Abrupt Changes in Position, Width, and Intensity at T = T * c (> T c ) Low temperature linewidth µ CR ~ 4-5 cm 2 /Vs!? (cf. µ DC ~ 4 cm 2 /Vs) Why is T * c > T c? Short-range magnetic order & local carrier spin polarization persists above T c [J. Schlieman et al. PRB 64, (21)]?? Appearance of LH Only at Low T Temp-dependent density? Strain? Ferromagnetisminduced?

26 Temperature Dependence of Shift Bowers-Yafet model with s(p)-d exchange interaction 1 2 ( α β) g x S z << E Temperature dependence of E CR should follow temperature dependence of magnetization x<s z > Mean-field theory

27 CR Shift with T in Ferromagnetic Phase Absorption (1 4 cm -1 ) p = 1 19 cm -3 hν = 117 mev hole-active γ = 4 mev Magnetic Field (T) 29 K 15 K 3 K 5 6 Calculated temperature dependence of the resonance field for peak A Temperature (K) p-inmnas x =.9 hν = 117 mev T c = 55 K 5 T c Shift in peak position as a function of temperature Resonance Field (T) 46 48

28 Mn doping superlattice B InAs In x Mn 1-x As x = 1.6% InAs 5 A 5 A 5 A InAs crystal periodically doped with Mn 5 A InAs / 5 A InMnAs superlattice n-type paramagnetic sample Mn concentration, x = 1.6% B field applied along [1] direction

29 Superlattice Confinement Potentials Spin & position dependent sp-d exchange in Mn doped superlattice confines spin up carriers in InAs and spin down carriers in InMnAs. B InMnAs InAs InMnAs InAs InMnAs Electron Heavy Hole

30 CR vs. Electron Concentration 35 T 35 T

31 Comparison of Theory & Experiment Transmission (arb. units) K K Experiment SL1 SL1 Theory K 5 B(Tesla) 1 Transmission (arb. units) K K 8.2 SL2 SL K 5 B(Tesla) 1

32 Experiments 1. High-Field Cyclotron Resonance in InMnAs 2. Transient THz Absorption in InGaMnAs 3. Ultrafast Optical Spectroscopy Two-Color Pump-Probe MOKE: Ultrafast Photoinduced Softening

33 Carrier States & Dynamics in Carrier-Induced (III,Mn)V( Ferromagnets (III,Mn)V: a new system for studying itinerant carriers interacting with localized spins Optical Conductivity s(w) provides a powerful probe for free carrier states and low energy dynamics E. Singley et al., PRL 89, 9723 (22) K. Hirakawa et al., PRB 65, (22) S. Katsumoto et al., Mat. Sci. Eng. B 84, 88 (21) Y. Nagai et al., J. Appl. Phys. 4, 6231 (21) Present work: We create transient carriers which influence the ferromagnetism and measure s(w,t) of these carriers

34 NIR-pump & THz-probe Setup Ge:Ga Detector FEL Sync. FIR probe Split-coil magnet T R Ge:Ga Detector Ti:S Pellicle beam combiner Off-axis parabolic mirror NIR pump Delay t d J. Kono et al., Appl. Phys. Lett. 75, 1119 (1999).

35 Sample: InGaMnAs/InP New member of (III,Mn)V family In.53 Ga.47 MnAs 5 nm In.53 Ga.47 As 1 nm InP (1) substrate Lattice matched to InP T c = 11 K p ~ 5 x 1 19 cm -3 at 4.2 K, m = 187 cm 2 /Vs Mn content = 13% T. Slupinski, A. Oiwa & H. Munekata, APL 8, 1592 (22).

36 THz Differential Transmission Dynamics at Room Temperature T / T T = 294 K 8 Time Delay (ps) 12 T < Photo-induced absorption Probe freq. = 5.7 THz (23 mev) Pump fluence:.1-1 mj/cm 2

37 Appearance of Secondary Peak in DT at Low Temperature T / T T = 4 K 8 Time Delay (ps) 12 Probe freq. = 5.7 THz Pump fluence:.1-1 mj/cm 2 Secondary peak at high pump fluence Plateau at the highest fluence

38 Temperature Dependence at High Fluence T / T T = 4K T = 8K T = 11K T = 14K T = 22K T = 294K Strong T dependence Anomalies (secondary peak, plateau) become more pronounced at lower T. 5 1 Time Delay (ps) 15 Longer decay time at T

39 Band Model In.53 Ga.47 As (In.53 Ga.47 ).87 Mn.13 As T<T c B= x= B= x=13% L Threshold L Threshold (111) (1) (111) (1)

40 Experiments 1. High-Field Cyclotron Resonance in InMnAs 2. Transient THz Absorption in InGaMnAs 3. Ultrafast Optical Spectroscopy Two-Color Pump-Probe MOKE: Ultrafast Photoinduced Softening

41 Advantages of Ferromagnetic Semiconductors over Ferromagnetic Metals Ultrafast pump primarily increases carrier density (rather than carrier temperature) enhance Mn-Mn exchange interaction Circularly-polarized pump spin polarized carriers Low-T MBE growth ultrashort lifetimes

42 InMnAs-Based Heterostructures 9-25 nm Type-II broken gap heterostructures with AlGaSb ~.4 ev ~.8 ev E C First ferromagnetic p- type films, T c = 7 K (1991), T c = 35 K in p- InMnAs/GaSb (1993) InMnAs x < 9.5% holes GaSb E F E V Light-induced ferromagnetism (1997) Electrical tuning of ferromagnetism (2)

43 CW Optical Control of Ferromagnetism S. Koshihara et al., PRL 78, 4617 (1997); A. Oiwa et al., APL 78, 518 (21). Light-induced ferromagnetism Light-induced coercivity decrease Persistent photoeffect

44 Persistent Photoeffect ~.4 ev µ ~.8 ev Electron-hole separation Hole density increases persistently InMnAs GaSb Slow Not reversible

45 Two-Color MOKE in InMnAs/GaSb Selective pumping of InMnAs by fs MIR pulses Photogenerated, transient spinpolarized carriers Probing timedependent ferromagnetism NIR probe MIR pump ~15 fs InMnAs holes GaSb C µ V Mn ions: ~1 21, background carriers: ~1.1 x 1 19, Transient carriers: cm -3

46 Experimental Setup R/R Time Delay MOKE Signal Time Delay MOKE Signal t 1 t 2 t 3 θ H

47 Carrier Dynamics 1 R/R (%) -1-2 R/R (%) Time Delay (ps) 1 Pump 1.26 mm Probe 775 nm 4 8 Time Delay (ps) 12

48 Origin of the Two Regimes.5 (1) R/R (%) (2) (2) C.B (1) V.B (1) carrier trapping (~ 2 ps) (2) recombination of trapped carriers Pump 2 µm,.5 mw Probe 775 nm Time Delay (ps)

49 Carrier Dynamics in InMnAs 2 1 (1) carrier trapping R/R (%) -1-2 (1) (2) Pump = 126 nm Probe = 775 nm (~ 2 ps) (2) Carrier recombination of trapped carriers Time Delay (ps)

50 Oscillatory Behavior 3x1-2 2 mw T R/R mw 4.1 mw 5.5 mw R/R (%) 2 1 T InGaMnAs, T=2 K, pump 1.26 micro probe 775 nm K22d-InMnAs, T=14 K 5 1 Time Delay (ps) Time Delay (ps) Possibly due to coherent phonons, plasmons & spin waves

51 Ultrafast MOKE Dynamics in InMnAs Photo-induced MOKE Signal (V) σ + pumping σ - pumping T = 122 K Not related to ferromagnetism This is due to carrier spin Carriers lose spin coherence upon trapping (?) Time Delay (ps) 6

52 Time & B-Scans B at Low T MOKE Signal (V) -8x K σ + pumping σ - pumping Time delay (ps) x1-5 x1-5 x t = t = σ + pumping t = -4 ps σ - pumping Magnetic Field (T)

53 Ultrafast Photoinduced MOKE J. Wang et al., J. Supercond. 16, 373 (23); Physica E, in press; cond-mat/3517 T K T K B (T) MOKE Signal (deg.) T = 16 K T c = 55 K H = Oe Pump: 2 µm Probe: 775 nm 2 4 Delay Time (ps) 6

54 Ultrafast Photoinduced Softening J. Wang et al., J. Supercond. 16, 373 (23); Physica E, in press; cond-mat/3517 MOKE Signal (1-6 V) MOKE Signal (1-6 V) MOKE Signal (1-6 V) Magnetic Field (T) Magnetic Field (T) Magnetic Field (T) t ~ 4 ps t ~ ps t ~ 11 ps Loop shrinks horizontally and then comes back! First demonstration of ultrafast optical manipulation of coercivity

55 Temperature Dependence of CW MOKE MCD Signal (V) K 45 K 35 K 2 K 1 K -6x Magnetic field (T) In.91 Mn.9 As(25nm)/GaSb(82nm) on GaAs(1) T c = 55 K

56 Origin of Ultrafast Softening What determines coercivity? -- Anisotropy & Exchange E Ksin θ JS = cosφ 2 2 i i ij ij CW Anisotropy (K) does NOT change with carrier density Exchange increases Decreased domain wall energy Smaller coercivity (1-5 V) MOKE Signal Ultrafast Magnetic Field (T)

57 Nonthermal Magnetooptical Recording M ~1 fs t > M B H c B H c mod H c mod < H ex < H c t = Spin flipping ultrafast information recording

58 Summary Accomplished: Characterized electronics states in InMnAs films and heterostructures using high magnetic fields Demonstrated the existence of delocalized p-like nature of holes in InMnAs Anomalous relaxation in transient THz absorption at low T possibly due to intervalley scattering enhanced by ferromagnetic order Observed very short carrier lifetimes due to low-t growth Photogenerated transient carriers modify magnetic properties in InMnAs first demonstration of ultrafast softening: H c (coercivity) decreases ( hard soft )