スペクトルおよび 時間分解光誘起ファラデー回転による 磁気ポーラロンスピン配向過程 Spin polarization dynamics on magnetic polaron by means of spectrum- and time-resolved Faraday rotation 橋本 佑介、三野 弘文、山室 智文、神原 大蔵 、松末 俊夫 A 、嶽山 正二郎 B,C 千葉大院自然科学、千葉大工 A 、千葉大理 B 、 東大・物性研 C 21aTH-4

High quality sample with low Mn concentration Spectrum- and time-resolved Photo-induced Faraday rotation (SRPIFR, TRPIFR) Photo-induced Magnetism Mn Concentration [%] Localized energy 10 Alloy Potential 5 Localized energy of Magnetic Polaron Motivation Alloy potential fluctuation : small x = 5 ~ 10% → FEMP energy : Large Method : Aim : Free exciton magnetic polaron (FEMP) in CdMnTe

Bulk-Cd 1-x Mn x Te x = 5% GaAs Cd 1-x Mg x Te Quartz disk The opaque GaAs substrate was removed CdMgTe layer is transparency in CdMnTe resonance of this sample Sample Cd 1-x Mn x Te

Absorption and Photoluminescence PL Light source ： He-Ne 633nm Distinct PL line of the FEMP appear !! PL position [eV]Binding energy [meV] Absorption FX FEMP BMP BMP’ K

Mn concentration = 10 % Adiabatic Potential (meV) 1/a B Inversion of the extension of the center of mass motion Theoretical study of the self-trapped FEMP Masakatsu Umehara

B.S. Time Delay 1.4 ~ 300K 0 ~ 6.9T Sample λ/2 λ/4 λ/2 Ti:Sapphire Laser Probe Pump Polarization Beam Splitter Optical Bridge Lock-in Amplifier Experimental setup of PIFR EX absorption Laser spectrum 76MHz

Temporal profiles of PIFR 5K n: refractive index  frequency l: sample thickness c: light speed

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

FX FEMP PIFR excitation energy dependence Ferromagnetic spin alignment of the Mn ions via the FEMP !!! FEMP ： Exciton spin relaxation ＋ Long decay process > 13ns Mn spin relaxation ≈100ns T. Strutz et.al, Phys. Rev. Lett 68, 3912 Pump Probe FX ： Exciton spin relaxation (8ps)

PIFR spectrum at 13ns Exciton feels the effective magnetic field of the ferromagnetically aligned Mn spins FEMP Δt = 13ns FXFEMP Excitation energy FEMP resonance Long decay signal FX resonance No signal Free exciton Mn ion

The excitation energy dependence of the spectrum- and time-resolved photo-induced Faraday rotation have been observed. Under the FEMP resonance excitation, the PIFR signal shows the quite long decay signal that last more than 13ns. We attribute this long decay process to the Mn spin orientation via the FEMP formation. In the case of the FEMP resonance excitation, the PIFR spectrum at 13ns indicates the exciton energy splitting via the effective magnetic field that should be induced by the ferromagnetically aligned Mn spins. summary

Probe beam (linear polarized) Pump beam (circularly polarized) Sample Polarization beam splitter Balancing unit OUT Experimental setup Band edge exciton absorption spectrum Excitation Laser spectrum

Absorption and Photoluminescence Absorption ・ Spectrophotometer Photoluminescence ・ Ar-ion laser Excitation ・ 1/3 meter grating spectrometer and a charge-coupled device Experimental setup

Magnetic Polaron Localization only by sp-d exchange interaction Electron or hole localized to impurities, then formation of magnetic polaron Bound Magnetic Polaron (BMP) Free Exciton Magnetic Polaron (FMP) Mn spin e h Exciton spin e h E

PIFR in Magnetic field To remove the back ground, the PIFR measured with  and  pumping was subtracted Decay time of the signal in FMP resonance is longer than the repetition time of light source (13ns) The photoinduced ferromagnetic alignment of Mn spin by FMP formation The exciton life time is no more than 300 pico seconds The PIFR signal that remain in negative time region is appear 0.25T, 5K