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 polaronby 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 [%]

Loc

aliz

ed e

ner

gy

10

Alloy Potential

5

Localized energy of Magnetic Polaron

Motivation

Alloy potential fluctuation : smallx = 5 ~ 10% → FEMP energy : Large

Method :

Aim :

Free exciton magnetic polaron (FEMP) in CdMnTe

Bulk-Cd1-xMnxTex = 5%

GaAs

Cd1-xMgxTe

Quartz disk

The opaque GaAs substrate was removedCdMgTe layer is transparency in CdMnTe

resonance of this sample

Sample

Cd1-xMnxTe

Absorption and Photoluminescence

PLLight source：He-Ne 633nm

Distinct PL line of the FEMP appear !!

PL position [eV] Binding energy [meV]

Absorption 1.6750

FX 1.6740

FEMP 1.6722 1.8

BMP 1.6657 8.3

BMP’ 1.6558 18.2

Photoluminescence

1.6801.6701.6601.650Photon energy [eV]

O. D.

FX

FEMP

BMP

Absorption

BMP'

PL4.2K

Mn concentration = 10 %

Ad

iab

atic

Pot

enti

al (

meV

)

1/aB

Inversion of the extension of the center of mass motion

Theoretical study ofthe self-trapped FEMP

Masakatsu Umehara

1.6801.6751.670Energy

B.S.

Time Delay

1.4 ~ 300K0 ~ 6.9T

Sample

λ/2 λ/4

λ/2

Ti:SapphireLaserProbe

Pump

Polarization Beam Splitter

Optical Bridge

Lock-in Amplifier

Experimental setup of PIFR

EX absorption

Laser spectrum

76MHz

Temporal profiles of PIFR

Faraday Rotation [a. u.]

151050Time [ps]

σ−

σ+5K

€

θF =−ωl2c(n+−n−)

n: refractive indexfrequency

l: sample thicknessc: light speed

1.6801.6751.670Energy

FWHM　 Pump： 6.2meV

(2.8nm)　 Probe： 1.6meV

(0.7nm)

Mirror

lens

slit

Grating

MirrorProbe beam

Fourier transfer spectrum filter

Laser spectrum

Band edge exciton resonance absorption

EX

Transmission [a. u.]

403020100Delay Time [ps]

FXFEMP

PIFR excitation energy dependence

Ferromagnetic spin alignment ofthe Mn ions via the FEMP !!!

FEMP： Exciton spin relaxation　　　　＋

Long decay process > 13ns

Mn spin relaxation ≈100nsT. Strutz et.al, Phys. Rev. Lett 68, 3912

Pump

Probe

FX： Exciton spin relaxation (8ps)

Energy

PIFR spectrum at 13ns

Exciton feels the effective magnetic field of the ferromagnetically aligned Mn spins

FEMP

Δt = 13ns

FXFEMP

Excitation energy

PIFR(

σ+

)- (PIFR

σ−

)[ . .]a u

1.6761.6741.6721.670 [ ]Photon energy eV

0

FEMP resonance

Long decay signal

FX resonance

No signal

Freeexciton

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)

SamplePolarization beam splitter

Balancing unit OUT

Experimental setup

Band edge exciton absorption spectrum

Excitation Laser spectrumIntensity

744742740738736

Wavelength [nm]

O. D.

PL [Arb. Units]

1.6801.6701.660

Energy [eV]

3.0

2.8

2.6

2.4

2.2

2.0

1.8

1.6

Absortption [O. D.]

FEMP

BMP

Absorption

4.2K

Absorption and Photoluminescence

Absorption ・ SpectrophotometerPhotoluminescence ・ 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

Rotation [arb. units]

3020100Time [ps]

FX resonance 1.678 eV

FMP resonance 1.673 eV

PIFR in Magnetic fieldTo remove the back ground, the PIFR measured with σ and σ 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