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Photo-induced ferromagnetism in

Photo-induced ferromagnetism in. 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

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Photo-induced ferromagnetism in

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  1. Photo-induced ferromagnetism in Y. Hashimoto, H. Mino, T. Yamamuro, D. Kanbara, AT. Matsusue, BS. Takeyama Graduate School of Science and Technology, Chiba University, Chiba, Japan AFaculty of Engineering, Chiba University, Chiba, Japan BThe institute for Solid State Physics, University of Tokyo, Chiba, Japan magnetic polarons bulk-Cd0.95Mn0.05Te via exciton

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

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

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

  5. x = 5 ~ 10% → FEMP energy : Large Alloy potential fluctuation : Small Free exciton magnetic polaron (FEMP) in CdMnTe Localization energy of Magnetic Polaron Current work : Alloy Potential fluctuation Localization energy 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) 5 10 Mn Concentration [%] S. Takeyama, J. of Crys. Growth, 184-185 (1998) 917-920

  6. Sample Bulk-Cd1-xMnxTe x = 5% GaAs substrate Transparent buffer layer Cd1-yMgyTe Thickness: 0.5 mm Cd0.95Mn0.05Te Quartz disk The opaque GaAs substrate was removed. CdMgTe layer is transparent in the wavelength of CdMnTe’s resonance.

  7. Absorption and Photoluminescence spectrum 1.4K Distinct PL line of the FEMP appears !! FEMP binding energy  1.8 meV Absorption: 4.2 K, PL: 1.4K PL Light source：He-Ne 633nm

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

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

  10. Experimental setup of PIFR Delay Stage EX absorption 76MHz Ti:Sapphire Laser B.S. λ/2 Laser spectrum λ/2 λ/4 Pump Probe Pump : Probe = 10 : 1 Exciton density: 1.1 x 1016 / cm3 Sample 1.4 ~ 300K 0 ~ 6.9T Lock-in Amplifier Polarization Beam Splitter Optical Bridge

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

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

  13. h e　 Possible nature of the long decay signal in PIFR 1, Ferromagnetic Mn spin orientation caused by the FEMP Mn spins are ferromagnetically aligned via the FEMP formation Mn spin relaxation time in Cd0.95Mn0.05Te 100 ns T. Strutz et.al, Phys. Rev. Lett 68, 3912 (1992) 2, Dark exciton magnetic polaron Mn spins are ferrpmagnetically aligned via the DEMP formation The relaxation time of the dark exciton is much longer than the bright exciton

  14. Future work Resonant spin amplification The origin of the long PIFR signal Direct observation of the ferromagnetically aligned Mn spins by means of Resonant Spin Amplification 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 • tspin (hole) < 1 ps • tspin (electron) ~ 8 ps • tspin (Mn) > 13 ns • Possible evidence of photo-induced magnetism via FEMP and DEMP formation h h e　 e　

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

  17. FEMP structure in CdMnTe Hole wave function: 14.4 A Electron wave function: 64 A Mott density: 9.1 x 1017/cm3 (In the present case, rs = 4.4) In the hole wave function: NMn ~ 1 In the electron wave function: NMn ~ 100 Hole wave function Electron wave function MASAKATSU UMEHARA, PRB 67, 035201 (2003)

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

  19. MP 1.4 K FX MP’ MP FX MP’ MP MP’ Integrated Intensity [Arb. Units.] Super linear increase of the PL intensity in Cd0.99Mn0.01TeIn low excitation regime Excitation source: He-Ne laser MP and MP’ Line show the super-linear increaseagainst the excitation power conventional Gaussian type inverse-Boltzman type MP ∝ I1.3 MP’ ∝ I1.3 inverse-Boltzman type

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

  23. s+

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