Magnetic polarons

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
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
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 lt 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
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)
10
5
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
CdMnTes resonance.
7
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
1.4K
Distinct PL line of the FEMP appears !! FEMP
binding energy ? 1.8 meV
Absorption 4.2 K, PL 1.4K PL Light sourceHe-Ne
633nm
8
Temperature and magnetic field dependence of the
PL spectrum
Magnetic field
Temperature
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 Tisapphire laser
l 400 nm Synchronized Streak camera
FX
FEMP
BMP
Time ps
1.4K
FX
FEMP
Energy eV
BMP
Time ps
tBMP gt tFEMP gt tFX
10
Experimental setup of PIFR
Delay Stage
EX absorption
76MHz
TiSapphire Laser
B.S.
?/2
Laser spectrum
?/2
?/4
Probe
Pump
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
Fourier transfer spectrum filter
Band edge exciton resonance absorption
FWHM Pump6.2meV (2.8nm) Probe1.6meV
(0.7nm)
12
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
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) lt 1 ps
• tspin (electron) 8 ps
• tspin (Mn) gt 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 lt 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/zincblen
de.htm
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
Cd0.99Mn0.01TeIn low excitation regime
Excitation source He-Ne laser
MP and MP Line show the super-linear increase
against 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/(4pi(aex3)n))(1/3) print
rs JkBT/Ry DE(-3.24rs(-3/4))(10.0478(rs
3)(J2))(1/4) print DE
22
What is the meaning of the negative delay region?
0 ps
-13 ps
13 ps
TiS laser 76 MHz
23
(No Transcript)