Photo-induced ferromagnetism inPhoto-induced ferromagnetism in

bulk-Cdbulk-Cd0.950.95MnMn0.050.05Te via excitonTe via exciton

Y. Hashimoto, H. Mino, T. Yamamuro, D. Kanbara, AT. Matsusue, BS. TakeyamaGraduate School of Science and Technology, Chiba University, Chiba, Japan

AFaculty of Engineering, Chiba University, Chiba, JapanBThe institute for Solid State Physics, University of Tokyo, Chiba, Japan

magnetic polaronsmagnetic polarons

Magnetic polarons

Mn spin

Exciton spine

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

What is interesting about FEMP ?

Photo-induced magnetismvia the FEMP

Circular polarized light

FEMP

BMP

No magnetism via the BMP

Circular polarized light

Dark exciton magnetic polarons

Transient absorption with circularly polarized pump and probe pulses.

T [

a. u

.]

20151050

Time delay [ps]

() ( )

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

1

G

2 21

e

h

Hole spin flip < 1 ps

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 [%]

Localiza

tion

en

erg

y

105

Alloy Potential fluctuation

Localization energy of Magnetic Polaron

Sample

Bulk-Cd1-xMnxTe x = 5% GaAs substrate

Cd1-yMgyTe

Quartz disk

The opaque GaAs substrate was removed.

CdMgTe layer is transparent in the wavelength of CdMnTe’s resonance.

Cd0.95Mn0.05Te

Transparent buffer layer

Thickness: 0.5 m

Absorption and Photoluminescence spectrum

Peak position

[eV]

Binding Energy[meV]

Absorption

1.6750

FX 1.6740

FEMP 1.6722 1.8

Donor-BMP

1.6657 8.3

Acceptor-BMP

1.6558 18.2

Absorption: 4.2 K, PL: 1.4KPL Light source ： He-Ne 633nm

1.4K

Distinct PL line of the FEMP appears !! FEMP binding energy 1.8 meV

Temperature and magnetic field dependence of the PL spectrum

Magnetic field

Ph

otol

um

ines

cen

ce [

a. u

.]

1.6761.6721.668

Photon Energy [eV]

FX

FEMP

1.4K

10K

Temperature

Pho

tolu

min

esce

nce

[a. u

.]

1.6761.6721.6681.664

Photon Energy [eV]

FX

FEMP

0T

0.1T

0.2T

0.3T

1.4K

Time-resolved photoluminescence

1.67 1.68

0

100

200

Energy [eV]

Tim

e [p

s]

FXFEMPBMP

1.65 1.66 1.67 1.68 1.69

0

200

400

600

Energy [eV]

Tim

e [p

s]

Energy [eV]

Tim

e [

ps]

1

10

100

1000

Ph

otol

um

inec

ence

[A

rb.U

nit

s]

6004002000

Time [ps]

1.4KFXFEMP

BMP

BMP > FEMP > FX

Setup T = 1.4 K 76 MHz Ti:sapphire laser = 400 nm Synchronized Streak camera

Tim

e [

ps]

Experimental setup of PIFR

B.S.

Delay Stage

1.4 ~ 300K

0 ~ 6.9T

Sample

λ/2λ/4

λ/2Ti:Sapphir

eLaser

ProbePump

Polarization Beam Splitter

Optical Bridge

Lock-in Amplifier

76MHz

1.6801.6751.670Energy

EX absorption

Laser spectrum

Pump : Probe = 10 : 1Exciton density: 1.1 x 1016 / cm3

Fourier transfer spectrum filter

Mirro

r

lensslit

Grating

MirrorProbe beam

FWHM Pump ： 6.2meV (2.8nm) Probe ： 1.6meV (0.7nm)

Band edge exciton resonance absorption

1.6801.6751.670Energy

EX

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

PIF

R [

a. u

.]

3020100Time delay [ps]

1.4K

< 1 ps: hole spin relaxation

8 ps: exciton spin relaxation

Temporal profile Spectral profile

W. Maslana PRB 63 165318 (2001)

PIF

R (

) -

PIF

R (-

)

1.675

Photon Energy [eV]

0

EX resonance

Negative delay time

Possible nature of the long decay signal in PIFR

1, Ferromagnetic Mn spin orientation caused by the FEMP

Mn spin relaxation time in Cd0.95Mn0.05Te 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

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)

1.6740

1.6739

1.6738

1.6737En

ergy

[eV

]

20151050Magnetic Field [mT]

Bright exciton Dark exciton

Bright-exciton dark-exciton level crossing

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

e

he

h

Dark excitonic effect ?

Transient absorption shows very long decay

Transient absorption spectrum

Red shift (~ 0.3 meV)

Radiative decay time < 300 ps

Dark exciton ?

DX

EX

Do dark excitons cause band gap renormalization ?

T

[a.

u.]

5000Time delay [ps]

0

= 139 ps

0.5

0.0

-0.5

T/T

1.691.681.67Photon energy [eV]

1.3 ps

13 ns

341 psBGR?

FEMP structure in CdMnTe

Hole wave function: 14.4 AElectron wave function: 64 A

In the hole wave function: NMn ~ 1In the electron wave function: NMn ~ 100

Hole wave function Electron wave function

MASAKATSU UMEHARA, PRB 67, 035201 (2003)

Mott density: 9.1 x 1017/cm3

(In the present case, rs = 4.4)

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

3283310 1075.2)105.6( mm

327328

1063.31075.2

1

m

m

CdTe unit cell : 6.482 A

CdTe unit cell volume :

Number of the CdTe unit cell:

Super linear increase of the PL intensity in Cd0.99Mn0.01TeIn low excitation regime

conventional Gaussian type

inverse-Boltzman type

FX

MP

inverse-Boltzman typeMP’

Ph

otol

um

ines

cen

ce [

a.u

.]

1.6151.6101.6051.6001.595

Energy [eV]

11.1mW 8.0mW 5.0mW 3.5mW 2.0mW 1.4mW

FX

MP

MP’

1.4 1.4 KK

MP and MP’ Line show the super-linear increase against the excitation power

8000

6000

4000

2000

0

Ï•

ª‹ “x

[a.u

.]

108642

Excitation Power [mW]

Inte

gra

ted Inte

nsi

ty [

Arb

. U

nit

s.]

MPMP’

MP ∝ I1.3

MP’ ∝ I1.3

Excitation source: He-Ne laser

Out line

1. What is free exciton magnetic polaron ?2. Sample3. Results & Discussion

PL & absorptionPhoto-induced Faraday rotation

4. Conclusions

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

412343

31

3

)0478.01()24.3(

)1

)(4

3(

JrrE

Ry

TkJ

nar

ss

B

ex

s

Ti:S laser76 MHz

PIF

R [

a. u

.]

0 ps-13 ps +13 ps

What is the meaning of the negative delay region?

1

G

2 21