Temporal decoupling of spin and crystallographic phase transitions in Fe(ptz)
6
(BF
4
)
2
Hiroshi Watanabe,
1
Hideki Hirori,
2
Gabor Molnár,
3,4
Azzedine Bousseksou,
3,4
and Koichiro Tanaka
1,2,
*
1
Department of Physics, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
2
Institute for Integrated Cell-Material Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
3
Laboratoire de Chimie de Coordination, CNRS, 205 Route de Narbonne, 31077 Toulouse, France
4
UPS, INPT, LCC, Université de Toulouse, F-31077 Toulouse, France
Received 15 April 2009; published 12 May 2009
We have studied relaxation dynamics of the thermally and photoinduced metastable states in
Feptz
6
BF
4
2
near the spin transition temperature using magnetic-susceptibility measurements and optical-
absorption spectroscopy as function of the time. We observed two-step relaxations from the photoinduced
high-spin to the ground low-spin state. The time-resolved measurements of the spin-relaxation processes allow
us to decouple the spin transition and the crystallographic phase transition and evaluate their intrinsic transition
temperatures as 122 and 132 K, respectively.
DOI: 10.1103/PhysRevB.79.180405 PACS numbers: 75.30.Wx, 64.60.-i, 78.47.-p
The photoinduced phase-transition PIPT phenomenon
has been extensively studied in a variety of systems because
of its fundamental importance in physics and its potential
applications in optical data storage and processing
devices.
1–3
The cooperative interaction plays an important
role in PIPT and induces nonlinear phenomena, such as the
existence of a threshold light intensity and an incubation
period.
3–5
This is particularly true for ironII spin-crossover
coordination complexes, where the photoirradiation causes a
spin transition between the
5
T
2
high-spin HS state and the
1
A
1
low-spin LS state.
The thermal spin transition strongly coupled with the
crystallographic phase transition is among the most intrigu-
ing phenomena in spin-crossover complexes.
6–9
A delicate
balance between two order parameters of spin and structure
gives us possibilities to drive the materials into various meta-
stable states by external stimuli. Even in other types of
phase-transition phenomena in solids, e.g., the multiferroic
systems,
10
perovskite manganite,
11
prussian blue analogs,
12
and charge-transfer molecular crystals,
13,14
the coupling and
decoupling of charge, spin and lattice degrees of freedom are
key matters for understanding and controlling of material
phases. However, the strong coupling between different or-
der parameters renders the respective phase-transition phe-
nomena difficult to study, and direct selective observation of
strongly coupled phase transitions is desirable. In systems
which are described by several order parameters, time-
resolved measurements may allow us to observe respective
phase transitions from photoinduced metastable states to elu-
cidate the underlying functional response.
In this Rapid Communication, we report the relaxation
dynamics in the thermal-quenched and photoinduced HS
PIHS states in the spin-crossover complex Feptz
6
BF
4
2
near the thermal phase-transition temperature using time-
resolved magnetic-susceptibility and optical-absorption spec-
troscopy measurements. Feptz
6
BF
4
2
is well-established
model system for studying the phase-transition phenomena
with competing order parameters.
7,8,15–22
We observed two-
step relaxations in the PIHS state from the HS to LS state
through an intermediate state. Using infrared IR spectros-
copy we show that the intermediate state has the same spin
and crystallographic structure as the quenched state. The
temperature dependence of the HS fraction and the IR-
absorption spectra show that a spin transition without a crys-
tallographic phase transition in the intermediate state should
occur at 122 K. Furthermore, the critical slowing down phe-
nomena indicates that the crystallographic phase transition
with the spin transition occurs at 132 K.
Single crystals of Feptz
6
BF
4
2
were prepared as de-
scribed previously,
23
with a typical crystal size of
2 2 0.1 mm. First, we made the magnetic-susceptibility
measurement using a superconducting quantum interfer-
ence device SQUID magnetometer MPMS-5S Quantum
Design, which has a 20 s time resolution. We estimated the
HS fraction of the samples from the product of the magnetic
susceptibility and temperature T.
Figure 1a shows the temperature dependence of the HS
fraction on slow temperature change 1 K/min and exhibits
an abrupt thermal spin transition accompanied by a
hysteresis loop. The spin transition temperatures where the
HS and LS fractions are equal to 0.5 upon heating
T
1/2↑
LS → HS and cooling T
1/2↓
HS → LS are obtained as
137 and 125 K, respectively. The measured transition tem-
peratures depend on the heating and cooling rate. It is known
that this spin transition competes with a crystallographic
phase transition.
8
This compound exhibits two different crys-
tallographic phases:
16
rhombohedral at high temperature
HT or a disordered phase at low temperature LT. When
the sample is cooled slowly 1K / min, the HS → LS spin
transition is accompanied by a crystallographic phase transi-
tion from the HT to LT structure phase. On the contrary,
quenching
10 K / min to 80 K brings about the LS state without the
crystallographic phase transition.
In order to elucidate the mechanism underlying the phase
transitions with competing order parameters, we observed
the relaxation dynamics of the HS fraction at various tem-
peratures, as indicated by the lower arrows in Fig. 1a. Fig-
ure 1b shows the relaxation curves of the HS fraction,
HS
,
from the upper branch of the hysteresis loop to the LS state,
exhibiting a sigmoidal behavior. As shown in Fig. 1b, the
HS state relaxes to the LS state below 131 K and the relax-
ation time increases with increasing temperature. At 132 K,
the initial HS fraction
HS
= 0.85 is persistent over 20 h.
As shown in Fig. 1b, the relaxation curves of the HS
fraction can be observed well above 127 K. However, we
PHYSICAL REVIEW B 79, 180405R2009
RAPID COMMUNICATIONS
1098-0121/2009/7918/1804054 ©2009 The American Physical Society 180405-1