In situ observation of the ferroelectric-paraelectric phase transition in a triglycine sulfate
single crystal by variable-temperature electrostatic force microscopy
E. Z. Luo,* Z. Xie, J. B. Xu, and I. H. Wilson
Department of Electronic Engineering and the Materials Science & Technology Research Center, The Chinese University of Hong Kong,
New Territories, Hong Kong, People’s Republic of China
L. H. Zhao
Applied Physics Department, Hunan University, Changsha 410082, People’s Republic of China
Received 15 June 1999
The ferroelectric-paraelectric phase transition of triglycine sulfate single crystal was investigated by
variable-temperature electrostatic force microscopy. Near the Curie temperature T
C
, the evolution of ferro-
electric domains with temperature was observed in situ. We have found that the domain structures are not
thermally reversible until T
C
-2 °CT T
C
, within which the domain density N diverges reversibly via
( T
C
-T )
-
, with =0.540.05, larger than is predicted by mean-field theory. The spontaneous polarization
P in individual domains decreases continuously and reversibly to zero and eventually vanishes at T
C
. Quan-
titative analysis reveals that P
2
( T
C
-T ).
I. INTRODUCTION
Triglycine sulfate TGS:NH
2
CH
2
COOH
3
H
2
SO
4
is a
well-known ferroelectric material and is of special interest
because it is one of the few ferroelectric crystals having typi-
cally a second-order phase transition ferroelectric-
paraelectric at its Curie point T
C
49 °C.
1–4
Below the Cu-
rie point TGS exists as a typical uniaxial ferroelectric phase,
the spontaneous polarization P is along the crystallographic
b axis with antiparallel 180° configurations. The most inter-
esting issue in a second-order phase transition is the critical
phenomena near T
C
.
1–4
Macroscopic measurements have
proved that the material can be well described by Landau’s
mean-field theory.
5,6
The critical behavior of the order pa-
rameter is Landau type with the critical exponent =
1
2
, i.e.,
P
T T
C
, 1
where =( T
C
-T )/ T
C
is the reduced temperature. Another
interesting phenomenon in a second-order phase transition is
domain evolution with temperature. The critical behavior of
the equilibrium domain density N can be written as
1–4
N
-
=
1
4
, T T
C
. 2
The exponent =
1
4
has rarely been measured. By using
nematic liquid-crystal method, Nakatani measured the tem-
perature dependence of domain width, which is the inverse
of N, in TGS during cooling down from above T
C
.
7
The
thermodynamic reversibility was not studied in Ref. 7. From
the data presented in Ref. 7, one can calculate that is close
to 1 when T is very close to T
C
. Similar results have also
been observed by Wang et al. in La
1 -x
Nd
x
P
5
O
14
LNPP
single crystals,
8
which exhibit a strong thermal hyteresis ef-
fect in domain density. However, it decreases rapidly as T
deviates from T
C
. Therefore, from both experimental and
theoretical points of view, it is highly desirable to visualize
in situ the domain evolution during the phase transition on a
nanometer scale. It is also of great interest to experimentally
verify the phenomenological mean-field theory at the single
domain scale. This demands a technique with capability of
imaging ferroelectric domains with high resolution and in an
environment with high-temperature stability and accuracy.
Significant progress has been achieved in imaging do-
mains in ferroelectric single crystals and thin films by using
various forms of scanning force microscopes SPM’s,
9
in-
cluding conventional atomic force microscopy AFM,
10
lat-
eral force microscopy LFM,
11,12
and electrostatic force mi-
croscopy EFM in contact and noncontact mode.
13–16
Recently, Hong and co-workers pointed out that EFM work-
ing in dynamic contact mode they term it DC-EFM has
superior domain contrast to that by noncontact EFM.
17,18
There have been some observations of domain evolution in
TGS.
11,13,17
However, little attention has been paid to ther-
modynamics. In this paper, we present a study of the
ferroelectric-paraelectric phase transition of TGS by using
DC-EFM. For this purpose, we have designed a special heat-
ing stage with high-temperature stability and accuracy.
19
By
controlling the sample temperature carefully, the temperature
dependence of domain configuration, as well as of the spon-
taneous polarization P in domains, has been studied.
II. EXPERIMENT
The experiments were carried out in a commercial AFM
Nanoscope III, Digital Instruments. A TiN coated tip was
used. Here the basic principle of DC-EFM is introduced be-
low. An ac modulation V =V
ac
cos(t) was applied between
the conducting tip and the bottom electrode of the sample.
The component of electrostatic force F
e
induced by the
surface charge is given by
18
F
e
=
b
C /2
0
V
ac
cos
t , 3
where
b
is the bounded surface charge density, in the case
of ferroelectric domains
b
= P ,
0
is the dielectric constant
in vacuum, and C is the tip-sample capacitance. The EFM
works in contact mode. Therefore the electrostatic force is
proportional to
b
and V
ac
. The electrostatic force is de-
tected by a lock-in amplifier and imaged simultaneously with
PHYSICAL REVIEW B 1 JANUARY 2000-I VOLUME 61, NUMBER 1
PRB 61 0163-1829/2000/611/2034/$15.00 203 ©2000 The American Physical Society