Photo-induced ferromagnetism in bulk-Cd0.95Mn0.05Te via
exciton magnetic polarons
Y. Hashimoto, H. Mino, T. Yamamuro, D. Kanbara, T. MatsusueA, S. TakeyamaB
Graduate School of Science and Technology, Chiba University, Chiba, Japan
A
Faculty of Engineering, Chiba University, Chiba, Japan
B
The institute for Solid State Physics, University of Tokyo, Chiba, Japan
Abstract. Free exciton magnetic polarons in bulk CdMnTe with low Mn concentration have been investigated through
the time-resolved and spectral-resolved photo-induced Faraday rotation measurements. The time-resolved photo-induced
Faraday rotation measurements have revealed the spin relaxation dynamics and show a finite rotation angle lasting
longer than 13 ns. This previously unreported long decay is attributed to dark exciton magnetic polarons.
INTRODUCTION
In diluted magnetic semiconductors (DMS), there have
been many reports on “magnetic polarons” (MP), i.e.,
the exciton wearing ferromagnetically aligned Mn spin
clouds via the sp-d exchange interaction. After the first
report of free exciton magnetic polaron (FEMP) by
Gornik[1], much attention has been directed to the
DMS due to the possibility of the optical manipulation
of magnetism. A systematic study of localization
energy of the FEMP as a function of Mn concentration
has been performed on Cd1-xMnxTe with 0.1 < x <
0.35[2], and the maximum value of the energy is
expected to occur at a Mn concentration of 5 to 10%.
The lack of investigations on Cd1-xMnxTe in this low
Mn concentration regime arises from the difficulties in
growing high quality samples with a reduced amount
of bound exciton photoluminescence (PL) lines and
showing dominant free exciton luminescence.
However, recent development in molecular beam
epitaxy (MBE) has allowed the growth of high quality
bulk crystals in this low Mn concentration regime. In
this study, the spin relaxation dynamics and the
magnetic polaron formation process in bulk CdMnTe
with low Mn concentration have been investigated
through the time-resolved- (TR-) and spectralresolved- photo-induced Faraday rotation (SR-PIFR)
measurements.
EXPERIMENTAL SETUP
The samples of bulk-Cd1-xMnxTe were grown
on (100)-GaAs substrates with Cd1-yMgyTe buffer
layers by molecular beam epitaxy (MBE). The Mn
moler fraction of the samples was 5%. All the
measurements were performed at 1.4 K. In figure 1(a)
and 1(b), the experimental set-up for the spectralresolved and the time-resolved photo-induced Faraday
rotation (SR- and TR-PIFR) measurements are shown
Fig. 1. Schematics of (a) experimental set-up of
PIFR measurement and (b) Fourier transform
spectrum filter.
schematically. The excitation source was a modelocked Ti:sapphire laser producing 200 fs pulses with
80MHz. The pump pulse was circularly polarized and
the probe pulse was linearly polarized. They were
focused on the same position of the sample surface
with a spot size of 200m in diameter. The excitation
density was estimated to be 0.1J/cm2. In order to
spectral-resolve the PIFR signal, a Fourier transfer
spectrum filter (FTSF) shown in figure 1(b) was used.
The FTSF narrowed the spectral half width of the
probe pulse down to 0.7 nm and varies the energy of
the probe pulse by varying the slit position. The
rotation of the linearly polarized probe light was
measured by means of an optical bridge with a
precision better than a milli-degree (Fig. 1(a)).
RESULTS AND DISCUSSIONS
In figure 2(a), the temporal profiles of the
PIFR at 1.4 K are shown. The solid and dotted curves
were obtained by right-handed and left-handed
circularly polarized pumping, respectively. The photon
energy of the pump and probe pulse was set at the
band edge exciton (EX) resonance. The rise time of
the signal is limited by the laser pulse width, and the
decay contains three contributions of very different
relaxation time. The first decay time, less than 1 ps, is
attributed to a hole spin relaxation. The second decay
of 8 ps transient corresponds to an exciton spin
PIFR [a. u.]


PIFR amplitude
0
(a)
SUMMARY
1.4 K
10
20
Time delay [ps]
30
Negative delay time
(b)
0
relaxation. The third process has a long decay time and
appears even in negative delay region. This indicates
that the signal persisted more than 13 ns, which is the
period of the mode-locked laser used for the pumping.
We attribute this long decay to the dark exciton
magnetic polarons (DEMP) that remain long time due
to the prohibition of the radiative recombination. In the
following, we will have brief discussion about the
formation process of the DEMP. Assuming the righthanded circularly polarized excitation, the pump pulse
excites bright exciton with +1 angular momentum.
Individual spin relaxation of the hole and electron
spins results in the formation of the light hole dark
excitons with 0 angular momentum and the heavy hole
exciton with –2 angular momentum. In the diluted
magnetic semiconductor, the heavy hole dark exciton
will form the DEMP via the p-d exchange interaction.
The spin-polarized DEMP will act as a spin reservoir
that feeds the bright exciton states via electron and
hole spin flip. In order to clarify the nature of the long
decay signal in PIFR, we spectrum resolve the PIFR
signal at negative delay region. The spectral profile of
the PIFR signal in the negative delay region is shown
in figure 2(c). One can see that the PIFR spectrum
shows the maximum value at the band edge exciton
resonance. Theoretical analysis of the PIFR
spectrum[3] indicates that the observed PIFR spectrum
implies the energy splitting between the exciton states
with +1 and –1 angular momentum. This means the
ferromagnetically aligned Mn spins via the DEMP
formation cause the Zeeman splitting of the bright
exciton states. The ferromagnetic orientation of the
Mn spins via the DEMP formation can be interpreted
as the ferromagnetism mediated by the dark exciton.
FX resonance
Spin-dependent transient absorption and
spectrum- and time-resolved photo-induced Faraday
rotation measurements were performed. The temporal
profile of the PIFR shows the long decay of longer
than 13 ns. The PIFR spectrum at negative delay
region shows the maximum value at the band edge
exciton resonance and attributed to the Zeeman
splitting caused by the ferromagnetically aligned Mn
spins via the magnetic polaron.
1.670
1.675
Photon Energy [eV]
Fig. 2 (a) Photo-induced Faraday rotation for rightand left- cirularly polarized pump as a function of
the delay time between pump and probe pulse. (b)
Photo-induced Faraday rotation spectrum at negative
delay region
REFERENCES
1
A. Golnik et al., J. Phys. C16, 6073 (1983)
2
S. Takeyama, J. Cryst. Growth 184/185, 917 (1988)
3
W. Maslana et al., Phys. Rev. B 63, 165318 (2001)