1. Photo-induced ferromagnetism in bulk-Cd 0.95 Mn 0.05 Te via exciton Y. Hashimoto, H. Mino, T. Yamamuro, D. Kanbara, A T. Matsusue, B S. Takeyama Graduate School of Science and Technology, Chiba University, Chiba, Japan A Faculty of Engineering, Chiba University, Chiba, Japan B The institute for Solid State Physics, University of Tokyo, Chiba, Japan magnetic polarons

2. Magnetic polarons Mn spin Exciton spin e h e h E Free Exciton Magnetic Polaron (FEMP) Bound Magnetic Polaron (BMP) Localization only by sp-d exchange interaction A Golnic, et. al. J. Phys. C16, 6073 (1983) M. Umehara, Phys. Rev. B 68, 193202 (2003) Local magnetic order surrounding an impurity bound exciton

3. What is interesting about FEMP ? Photo-induced magnetism via the FEMP Circular polarized light FEMP BMP No magnetism via the BMP Circular polarized light

4. Dark exciton magnetic polarons Transient absorption with circularly polarized pump and probe pulses. Hole spin relaxation Exciton spin relaxation Individual spin relaxation of the electron and hole Dark exciton may form dark exciton magnetic polaron via the strong p-d exchange interaction Dark exciton formation e h Hole spin flip  < 1 ps 

5. Free exciton magnetic polaron (FEMP) in CdMnTe High quality CdMnTe sample with low Mn concentration CW and time-resolved Photoluminescence Time- and spectral-resolved photo-induced Faraday rotation (TR- and SR-PIFR) Current work : Alloy potential fluctuation : Small x = 5 ~ 10% → FEMP energy : Large S. Takeyama, J. of Crys. Growth, 184-185 (1998) 917-920 Mn Concentration [%] Localization energy 105 Alloy Potential fluctuation Localization energy of Magnetic Polaron

6. Sample Bulk-Cd 1-x Mn x Te x = 5% GaAs substrate Cd 1-y Mg y Te Quartz disk The opaque GaAs substrate was removed. CdMgTe layer is transparent in the wavelength of CdMnTe’s resonance. Cd 0.95 Mn 0.05 Te Transparent buffer layer Thickness: 0.5  m

7. Absorption and Photoluminescence spectrum Peak position [eV] Binding Energy [meV] Absorption1.6750 FX1.6740 FEMP1.67221.8 Donor-BMP1.66578.3 Acceptor- BMP 1.655818.2 Absorption: 4.2 K, PL: 1.4K PL Light source ： He-Ne 633nm 1.4K Distinct PL line of the FEMP appears !! FEMP binding energy  1.8 meV

8. Temperature and magnetic field dependence of the PL spectrum Magnetic field Photoluminescence [a. u.] FX FEMP 1.4K 10K Temperature FX FEMP 0T 0.1T 0.2T 0.3T 1.4K

9. Time-resolved photoluminescence FXFEMPBMP Energy [eV] Time [ps] 1.4K FX FEMP BMP  BMP >  FEMP >  FX Setup T = 1.4 K 76 MHz Ti:sapphire laser = 400 nm Synchronized Streak camera Time [ps]

10. Experimental setup of PIFR B.S. Delay Stage 1.4 ~ 300K 0 ~ 6.9T Sample λ/2 λ/4 λ/2 Ti:Sapphire Laser ProbePump Polarization Beam Splitter Optical Bridge Lock-in Amplifier 76MHz EX absorption Laser spectrum Pump : Probe = 10 : 1 Exciton density: 1.1 x 10 16 / cm 3

11. Fourier transfer spectrum filter Mirror lens slit Grating Mirror Probe beam FWHM Pump ： 6.2meV (2.8nm) Probe ： 1.6meV (0.7nm) Band edge exciton resonance absorption EX

12. Photo-induced Faraday rotation Long decay process Longer than the repetition time of the excitation source 13 ns PIFR spectrum at 13 ns shows the maximum value at the EX resonance Zeeman splitting 1.4K < 1 ps: hole spin relaxation 8 ps: exciton spin relaxation Temporal profile Spectral profile W. Maslana PRB 63 165318 (2001)

13. Possible nature of the long decay signal in PIFR 1, Ferromagnetic Mn spin orientation caused by the FEMP Mn spin relaxation time in Cd 0.95 Mn 0.05 Te  100 ns 2, Dark exciton magnetic polaron e h Mn spins are ferromagnetically aligned via the FEMP formation T. Strutz et.al, Phys. Rev. Lett 68, 3912 (1992) The relaxation time of the dark exciton is much longer than the bright exciton Mn spins are ferrpmagnetically aligned via the DEMP formation

14. Future work Resonant spin amplification Direct observation of the ferromagnetically aligned Mn spins by means of Resonant Spin Amplification The origin of the long PIFR signal J. M. Kikkawa, PRL 80 4313 (1998) Bright-exciton dark-exciton level crossing

15. Summary Performed first time-resolved Faraday rotation on CdMnTe which shows clear FEMP PL  Spin dynamics of holes, electrons and Mn ions t spin (hole) < 1 ps t spin (electron) ~ 8 ps t spin (Mn) > 13 ns  Possible evidence of photo-induced magnetism via FEMP and DEMP formation e h e h

16. Dark excitonic effect ? Transient absorption shows very long decay Transient absorption spectrum Red shift (~ 0.3 meV) Radiative decay time < 300 ps Dark exciton ? Do dark excitons cause band gap renormalization ? BGR?

17. FEMP structure in CdMnTe Hole wave function:14.4 A Electron wave function:64 A In the hole wave function: N Mn ~ 1 In the electron wave function:N Mn ~ 100 Hole wave function Electron wave function MASAKATSU UMEHARA, PRB 67, 035201 (2003) Mott density: 9.1 x 10 17 /cm 3 (In the present case, r s = 4.4)

18. Crystal structure of CdTe http://www.uncp.edu/home/mcclurem/lattice/zincblende.htm Crystal structure of the CdTe: Zinc Blend In one unit cell, Cd: 4 peaces Te: 4 peaces CdTe unit cell:6.482 A CdTe unit cell volume: Number of the CdTe unit cell:

19. Super linear increase of the PL intensity in Cd 0.99 Mn 0.01 Te In low excitation regime conventional Gaussian type inverse-Boltzman type FX MP inverse-Boltzman type MP’ FX MP MP ’ 1.4 K MP and MP’ Line show the super-linear increase against the excitation power Integrated Intensity [Arb. Units.] MP MP’ MP ∝ I 1.3 MP’ ∝ I 1.3 Excitation source: He-Ne laser

20. Out line 1. What is free exciton magnetic polaron ? 2. Sample 3. Results & Discussion PL & absorption Photo-induced Faraday rotation 4. Conclusions

21. Estimation of the dark exciton density and lifetime rs=(3/(4*pi*(aex^3)*n))^(1/3) print rs J=kB*T/Ry DE=(-3.24*rs^(-3/4))*(1+0.0478*(rs^3)*(J^2))^(1/4) print DE

22. Ti:S laser 76 MHz 0 ps-13 ps+13 ps What is the meaning of the negative delay region?

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