Ferroelectric and dielectric characterization studies on relaxor- and ferroelectric-like strontium-barium niobates
Ferroelectric domain structure evolution induced by an external electric field was investigated by means of nematic liquid crystal (NLC) method in two strontium-barium niobate single crystals of nominal composition: Sr₀.₇₀Ba₀.3₀Nb₂O₆ (SBN:70 - relaxor) and Sr₀.₆a₀.₇₄Nb₂O₆ (SBN:26 - ferroelectric). O...
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Інститут фізики конденсованих систем НАН України
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| Цитувати: | Ferroelectric and dielectric characterization studies on relaxor- and ferroelectric-like strontium-barium niobates / K. Matyjasek, J. Dec, S. Miga, T. Łukasiewicz // Condensed Matter Physics. — 2013. — Т. 16, № 3. — С. 31701:1-10. — Бібліогр.: 29 назв. — англ. |
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irk-123456789-1208302017-06-14T03:04:24Z Ferroelectric and dielectric characterization studies on relaxor- and ferroelectric-like strontium-barium niobates Matyjasek, K. Dec, J. Miga, S. Łukasiewicz, T. Ferroelectric domain structure evolution induced by an external electric field was investigated by means of nematic liquid crystal (NLC) method in two strontium-barium niobate single crystals of nominal composition: Sr₀.₇₀Ba₀.3₀Nb₂O₆ (SBN:70 - relaxor) and Sr₀.₆a₀.₇₄Nb₂O₆ (SBN:26 - ferroelectric). Our results provide evidence that the broad phase transition and frequency dispersion that are exhibited in SBN:70 crystal have a strong link to the configuration of ferroelectric microdomains. The large leakage current revealed in SBN:26 may compensate internal charges acting as pinning centers for domain walls, which gives rise to a less restricted domain growth similar to that observed in classical ferroelectrics. Microscale studies of a switching process in conjunction with electrical measurements allowed us to establish a relationship between local properties of the domain dynamics and macroscopic response i.e., polarization hysteresis loop and dielectric properties. Еволюцiя сегнетоелектричної доменної структури, iндукованої зовнiшнiм електричним полем дослiджу-валась за допомогою методу нематичного рiдкого кристалу в двох монокристалах стронцiй-барiєвого нiобату номiнального складу: Sr₀.₇₀Ba₀.3₀Nb₂O₆ (SBN:70 — релаксорний) i Sr₀.₆a₀.₇₄Nb₂O₆ (SBN:26 — сегнетоелектричний). Нашi результати показують, що широкий фазовий перехiд i частотна дисперсiя, продемонстрованi кристалом SBN:70, мають тiсний зв’язок iз конфiгурацiєю сегнетоелектричних мiкро-доменiв. Великий струм стiкання, виявлений в SBN:26, може компенсувати внутрiшнi заряди, що дiють як центри пiнiнгу для доменних стiнок, що приводить до менш обмеженого росту доменiв подiбно до того, що спостерiгається в класичних сегнетоелектриках. Мiкромасштiбнi дослiдження процесу перемикання в поєднаннi з електричними вимiрюваннями дозволяють встановити спiввiдношення мiж локальними властивостями динамiки доменiв i макроскопiчним вiдгуком, а саме, гiстерезисною петлею поляризацiї i дiелектричними властивостями. 2013 Article Ferroelectric and dielectric characterization studies on relaxor- and ferroelectric-like strontium-barium niobates / K. Matyjasek, J. Dec, S. Miga, T. Łukasiewicz // Condensed Matter Physics. — 2013. — Т. 16, № 3. — С. 31701:1-10. — Бібліогр.: 29 назв. — англ. 1607-324X PACS: 77.80.Dj, 77.80.Fm, 77.80.Jk, 77.84.Dy, 77.22.Ej DOI:10.5488/CMP.16.31701 arXiv:1309.5726 http://dspace.nbuv.gov.ua/handle/123456789/120830 en Condensed Matter Physics Інститут фізики конденсованих систем НАН України |
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Ferroelectric domain structure evolution induced by an external electric field was investigated by means of nematic liquid crystal (NLC) method in two strontium-barium niobate single crystals of nominal composition: Sr₀.₇₀Ba₀.3₀Nb₂O₆ (SBN:70 - relaxor) and Sr₀.₆a₀.₇₄Nb₂O₆ (SBN:26 - ferroelectric). Our results provide evidence that the broad phase transition and frequency dispersion that are exhibited in SBN:70 crystal have a strong link to the configuration of ferroelectric microdomains. The large leakage current revealed in SBN:26 may compensate internal charges acting as pinning centers for domain walls, which gives rise to a less restricted domain growth similar to that observed in classical ferroelectrics. Microscale studies of a switching process in conjunction with electrical measurements allowed us to establish a relationship between local properties of the domain dynamics and macroscopic response i.e., polarization hysteresis loop and dielectric properties. |
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Article |
| author |
Matyjasek, K. Dec, J. Miga, S. Łukasiewicz, T. |
| spellingShingle |
Matyjasek, K. Dec, J. Miga, S. Łukasiewicz, T. Ferroelectric and dielectric characterization studies on relaxor- and ferroelectric-like strontium-barium niobates Condensed Matter Physics |
| author_facet |
Matyjasek, K. Dec, J. Miga, S. Łukasiewicz, T. |
| author_sort |
Matyjasek, K. |
| title |
Ferroelectric and dielectric characterization studies on relaxor- and ferroelectric-like strontium-barium niobates |
| title_short |
Ferroelectric and dielectric characterization studies on relaxor- and ferroelectric-like strontium-barium niobates |
| title_full |
Ferroelectric and dielectric characterization studies on relaxor- and ferroelectric-like strontium-barium niobates |
| title_fullStr |
Ferroelectric and dielectric characterization studies on relaxor- and ferroelectric-like strontium-barium niobates |
| title_full_unstemmed |
Ferroelectric and dielectric characterization studies on relaxor- and ferroelectric-like strontium-barium niobates |
| title_sort |
ferroelectric and dielectric characterization studies on relaxor- and ferroelectric-like strontium-barium niobates |
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Інститут фізики конденсованих систем НАН України |
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2013 |
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http://dspace.nbuv.gov.ua/handle/123456789/120830 |
| citation_txt |
Ferroelectric and dielectric characterization studies on relaxor- and ferroelectric-like strontium-barium niobates / K. Matyjasek, J. Dec, S. Miga, T. Łukasiewicz // Condensed Matter Physics. — 2013. — Т. 16, № 3. — С. 31701:1-10. — Бібліогр.: 29 назв. — англ. |
| series |
Condensed Matter Physics |
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AT matyjasekk ferroelectricanddielectriccharacterizationstudiesonrelaxorandferroelectriclikestrontiumbariumniobates AT decj ferroelectricanddielectriccharacterizationstudiesonrelaxorandferroelectriclikestrontiumbariumniobates AT migas ferroelectricanddielectriccharacterizationstudiesonrelaxorandferroelectriclikestrontiumbariumniobates AT łukasiewiczt ferroelectricanddielectriccharacterizationstudiesonrelaxorandferroelectriclikestrontiumbariumniobates |
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2025-07-08T18:41:47Z |
| last_indexed |
2025-07-08T18:41:47Z |
| _version_ |
1837105268539785216 |
| fulltext |
Condensed Matter Physics, 2013, Vol. 16, No 3, 31701: 1–10
DOI: 10.5488/CMP.16.31701
http://www.icmp.lviv.ua/journal
Proceedings Paper
Ferroelectric and dielectric characterization studies
on relaxor- and ferroelectric-like strontium-barium
niobates
K. Matyjasek1, J. Dec2, S. Miga2, T. Łukasiewicz3
1 Institute of Physics, Faculty of Mechanical Engineering and Mechatronics, West Pomeranian University of
Technology, Al. Piastow 48, 70–310 Szczecin, Poland
2 Institute of Materials Science, University of Silesia, 12 Bankowa St., 40–007 Katowice, Poland
3 Institute of Electronic Materials Technology, 133 Wólczyńska St. 01–919 Warsaw, Poland
Received October 3, 2012, in final form November 21, 2012
Ferroelectric domain structure evolution induced by an external electric field was investigated by means of
nematic liquid crystal (NLC) method in two strontium-barium niobate single crystals of nominal composition:
Sr0.70Ba0.30Nb2O6 (SBN:70 — relaxor) and Sr0.26Ba0.74Nb2O6 (SBN:26 — ferroelectric). Our results provide
evidence that the broad phase transition and frequency dispersion that are exhibited in SBN:70 crystal have
a strong link to the configuration of ferroelectric microdomains. The large leakage current revealed in SBN:26
may compensate internal charges acting as pinning centers for domain walls, which gives rise to a less restricted
domain growth similar to that observed in classical ferroelectrics. Microscale studies of a switching process in
conjunction with electrical measurements allowed us to establish a relationship between local properties of the
domain dynamics and macroscopic response i.e., polarization hysteresis loop and dielectric properties.
Key words: relaxor ferroelectrics, niobates, domain walls, switching process, SBN
PACS: 77.80.Dj, 77.80.Fm, 77.80.Jk, 77.84.Dy, 77.22.Ej
1. Introduction
Strontium-barium niobates, SBN (SrxBa1−xNb2O6, 0.25 É x É 0.75), are ferroelectric crystals that have
received a great interest in their applications in optoelectronics [1]. The controlled manipulation of the
domain structure via external electric field involves an important issue of potential applications, such as
optical-frequency conversion and high density optical data storage [2]. Thus, efforts are directed towards
producing SBN crystals with high quality ferroelectric domain reversible structures. Polarization switch-
ing, which proceeds by nucleation and growth of domains, has attracted much attention also from the
viewpoint of statistical physics related to solid-state transformation.
Only a fewworks focusing on SBN domain structure dynamics can be found in literature. Mostly, high-
resolution domain structure studies (nanoscale domains) have been performed for a congruently melting
composition Sr0.61Ba0.39Nb2O6 (SBN:61) by piezoresponse force microscopy (PFM) [3–8]. The microscale
domain structure kinetics in SBN:61 was investigated using an electro-optic imaging microscope [9, 10]
and NLC method [11–13]. The observed specific features of the switching process were accounted for
freezing or pinning domain walls connected with disordered structure of SBN crystals.
SBN has an open tetragonal tungsten bronze structure, in which only five of six available positions
of Sr
2+
and Ba
2+
cations are occupied [14]. The origin of relaxor behaviour in SBN can be attributed
to the development of a quenched random fields associated with the composition/structural disorder
[15, 16]. The ferroelectric properties of SBN system change with the composition [17, 18]. To the best of
our knowledge, there is no report on the domain structure dynamics in other SBN compositions, in spite
© K. Matyjasek, J. Dec, S. Miga, T. Łukasiewicz, 2013 31701-1
http://dx.doi.org/10.5488/CMP.16.31701
http://www.icmp.lviv.ua/journal
K. Matyjasek et al.
of its significant effect on the physical properties. Temperature dependence of dielectric permittivity
ε(T ), at various frequencies has shown a gradual crossover from typical relaxor behaviour in SBN:75
into classical ferroelectrics as observed in the case of SBN:40 [18, 19]. It is reflected by a broad peak of
ε(T ) function, which was strongly dependent on frequency for SBN:75 crystal; while a sharper peak, with
the position of the maximum of ε(T ) function independent of frequency, was observed for SBN:40.
In this report, we have used the NLC method to study the domain creation process in static electric
fields, in SBN:70 and SBN:26 compositions. The NLC method involves the averaging over macroscopic
scale and this enables us to correlate the domain structure dynamics with macroscopic characterization
techniques such as electric displacement – electric field (D−E ) hysteresis loop, switching currents and di-
electric permittivity measurements. Investigating the change from relaxor (SBN:70) to classical ferroelec-
tric behaviour (SBN:26) is thus helpful for better understanding the physics of the relaxor ferroelectrics.
2. Experimental details
SBN crystals, as uniaxial ferroelectric materials, exhibit only 180
◦
domains because the paraelectric
phase has a tetragonal symmetry (4/mmm), and the order parameter in the ferroelectric phase (4mm)
occurs along [001] direction [14]. Details concerning the crystal fabrication by Czochralski method are
given elsewhere [18]. Two large single crystals of nominal composition Sr0.70Ba0.30Nb2O6 (SBN:70) and
Sr0.26Ba0.74Nb2O6 (SBN:26) have been grown. Using an Inductively Coupled Plasma – Optical Emission
Spectroscopy (ICP–OES), a real composition of the grown single crystals was determined to be
Sr0.70Ba0.26Nb2O5.96 and Sr0.35Ba0.69Nb2O6.04, respectively. The obtained single crystals seem to be slightly
non-stoichiometric. Despite an obvious difference between the nominal and real compositions, the sam-
ples are labelled throughout the paper by their nominal stoichiometry. Platelet-shaped samples were cut
perpendicular to the [001] direction and polished to the optical quality.
The NLC mixture of p-methoxybenzylidene-p-n-butylaniline (MBBA) and pethoxybenzylidene-p-n-
butylaniline (EBBA) was used to observe optically indistinguishable 180
◦
domain walls. The NLC method
makes possible a continuous observation of the domain pattern during polarization reversal in an electric
field if a cover glass coated with a conducting layer of tin oxide is used. The reversed regions, where re-
orientation of domains still occurs, look somewhat darker than the “grey” surrounding domains, because
a certain electrohydrodynamic instability, particularly dynamic scattering, takes place in these regions
[20]. The (D −E ) hysteresis loops were recorded using a modified Sawyer-Tower circuit by using an ac
field of frequency 50 Hz and a digital oscilloscope. Switching current transients were measured using
a wave-form function generator, a small standard resistor and digital oscilloscope. The complex linear
susceptibility χ=χ′−iχ′′ was measured using a Solartron 1260 Impedance Analyzer together with a 1296
Dielectric Interface (SBN70) and an Agilent E4980A Precision LCR Meter (SBN26) applying a weak prob-
ing ac electric field of the order of 2 Vcm−1
. For electric and dielectric studies, the SBN samples were
prepared as plates with dimensions 5×5×0.5mm3
with Cu-Au electrodes evaporated onto the principal
(001) faces. Thin copper interface is to improve the adhesion of gold. The temperature protocols required
by experiment were managed using a Lake Shore Model 340 temperature controller.
3. Experimental results
3.1. Ferroelectric characteristics
Figure 1 presents the temperature dependences of real parts of electric susceptibility of SBN:70 and
SBN:26 measured at different frequencies. The SBN:70 displays a typical relaxor behavior where the
temperature position of the broad maximum of susceptibility and its height strongly depend on the fre-
quency of the driving electric field. On the other hand, SBN:26 shows much higher susceptibility peaks
(χ′ = 115000) taking place at a fixed temperature, T = 461 K. These maxima might be considered as a
sign of a phase transition between an ordered ferroelectric and paraelectric states. Since the electric con-
ductivity (leakage current) of SBN:26 increases distinctly at higher temperatures, the measurements of
31701-2
Dielectric studies on strontium-barium niobates
Figure 1. (Color online) Temperature dependences of the real part of the electric susceptibility of SBN:70
and SBN:26 measured at different frequencies, 10−1 É f É 105
Hz; due to an enhanced electric conduc-
tivity (leakage current) of SBN26 only data measured at two highest frequencies are reliable.
susceptibility were carried out only at higher frequencies of the probing field (104
and 105
Hz). Under
this condition the effect of the leakage current on the dielectric response may be neglected.
Figure 2 shows the room temperature D −E loops for SBN:70 (a) and SBN:26 (b) compositions. These
loops were obtained by applying a sinusoidal signal at a frequency of 50 Hz. The polarization charges
accumulated on the crystal sample result from various effects of the crystal bulk, such as changes in the
domain structure and leakage currents. TheD−E loops of the SBN:26 sample, present an oval-like profile
owing to the large leakage current in the crystal sample. It is difficult to determine the coercive field, Ec
which increases steadily with an increase of the electric field amplitude. These results suggest that even
at high electric fields, there are frozen regions that do not participate in the switching polarization and/or
that a large contribution to the polarization is due to the sidewise domain wall motion, which is a very
slow mechanism.
3.2. Domain switching in SBN:70 crystal
To investigate the domain dynamics, the crystal sample was preliminary poled into a single domain
state with a sufficiently high electric field E > Ec, and then an electric field of the opposite direction was
applied to the sample. The formation of domains is possible if the amplitude of dc field exceeds some
threshold value at a given site of the crystal, which depends on the state of the aging of the sample. The
static domain structure gave no contrast. Figure 3 illustrates the domain pattern evolution observed in
SBN:70 crystal sample at room temperature when external electric field is switched on. The dark areas
Figure 2. The family of D −E hysteresis loops obtained at room temperature by applying ac-field at a
frequency of 50 Hz: (a)— SBN:70 and (b)— SBN:26 crystal.
31701-3
K. Matyjasek et al.
Figure 3. Domain configurations developing at different stages of polarization reversal process, ob-
served on the (001) plane of SBN:70 crystal sample, during switching in electric field from 1 kVcm
−1
to
1.5 kVcm
−1
. (a)— initial single domain state. Time from the moment of applying E = 1 kVcm−1
: (b) 0.4 s,
(c) 0.8 s, (d) 2 s, (e) 5 s— the domain walls are rather difficult to distinguish because of their slow velocity,
(f) distinct variations of the domain pattern are noticeable following the application of E = 1.3 kVcm−1
,
(g) the picture of delineated domain walls 4 s after electric field application of 1.3 kVcm
−1
, (h) the switch-
ing process was completed after application of E = 1.5 kVcm−1
.
correspond to the areas which actually reversed their polarization direction while clear areas are the
regions where the switching process has not started [initial polarization state in figure 3 (a)] or has al-
ready been completed. The switching process is realized by the nucleation and growth of domains. We
use the term “nucleation” to describe the emergence of new antiparallel domains within the original do-
main as they appear in the video image. However, it is an open question whether the initial domain state
is single-domain or contains the nano-scale residual domains not resolved by the NLC method. It should
be emphasized that the thickness of the visible “wall” is not a real physical thickness of the domain wall,
which is much smaller (of the order of several unit cells). The sidewise movement of the domain walls in
SBN:70 crystals is strongly perturbed by the random field environment related to the relaxor properties
and to the resulting domain wall pinning effect [15, 16]. Local structure disorder gives rise to quenched
random fields whose fluctuations are the source of local enhancements of the coercivity. As a result of
the pinning effect, some wall segments change their position from image to image, whereas others stay
immobile for a long time. As a consequence, a maze type domain pattern is formed. A close inspection re-
vealed that individual domain walls expand with a great resistance under the dc field and finally clamp
the entire dynamics. The pronounced slowing down of the domain walls is reflected in a poor contrast of
NLC above the slowly moving domain walls, as can be seen in figure 3 (d) and more clearly in figure 3 (e),
where domain walls are hardly seen. The variations of the domain pattern are noticeable following the
application of the higher electric field of 1.3 kV/cm. Simultaneously, a fine domain structure appears, as
can be seen in figure 3 (f). With pinning centres present in the crystal, large domains are broken up into
smaller ones since certain areas of the domains are incapable of switching, and consequently this process
is accompanied by an increase of the domain density. However, the polarization is fully reversed in the
observed surface area after a further poling in E = 1.5 kV/cm. The switching process is completed first
in the central part of the image in figure 3 (h) (where no obvious domain walls can be identified), then
in the outer region of the video scan. This field is also sufficient to reverse the polarization state in the
entire volume of the examined SBN:70 crystal sample.
The fast polarization switching process was investigated by measuring the switching currents in re-
sponse to squarewave electric pulses. A set of square pulses (of 10ms duration) was applied by combining
two positive and two negative pulses in series. A true switching current was then obtained by subtracting
the nonswitching current from the switching one. Figure 4 presents the switching curves obtained for
various amplitudes of the electric fields for SBN:70 crystal sample. The rate of polarization switching at
constant E can be found from the switching current i (t ) by integration of i (t ) from t = 0 to the instant t .
The results are presented in figure 4. The observed “partial” saturation of polarization may confirm the
fact that there are slowly switching regions that do not contribute to the switching current signals even
31701-4
Dielectric studies on strontium-barium niobates
Figure 4. The switching current and the switched polarization versus time for SBN:70 crystal sample. The
broken curves correspond to the fit of the polarization data with a KWW function.
in E > Ec. This should be related to the “pinning effect” of the domain walls in SBN:70 crystal sample.
The values of polarization obtained by the pulsed field technique are comparable to that obtained from
hysteresis-loop (H-L) measurements at the same pulse amplitude of the ac-field at frequency 50 Hz. The
maximum value of polarization ∼ 8 µC/cm2
found by H-L at frequency f = 50 Hz and E = 8 kVcm−1
is
small compared to the total polarization of about 20 µC/cm
2
obtained from the measurements at a very
low-frequency of E in which all domains can be aligned [21, 22]. It was found from pyroelectric mea-
surements that a total polarization of a polydomain crystal requires the field exposure times that can
range up to several tens of seconds even for the fields well above the coercive field [21]. In contrast to the
stable H-L measured at high frequency of ac-field, a non-coincidence of trajectories of H-L was obtained
during quasistatic loop registration in the course of a repeated field cycling [21, 22]. This process was
accompanied by a considerable decrease in the amplitude of polarization (the so-called fatigue effect).
The relaxation of polarization is well described by a Kohlrausch,Williams andWatts (KWW) stretched
exponential function P (t ) = P0
{
1−exp[− (t/t0)n]
}
with 0 < n < 1 widely used in dielectric relaxation
studies. The fits are represented by broken lines in figure 4. The KWW function is usually used to describe
a complex relaxation and can reflect a very broad distribution of relaxation times [23, 24]. This is closely
related to a distribution of random electric fields inherent to relaxor crystals. It is interesting to note that
KWW function described the switching kinetics in SBN:61 [13] as well as the relaxing domains on the
nanoscale, with PFM imaging of the domain configuration in SBN:61 doped with cerium [5].
3.3. Domain switching in SBN:26
Microscopic observations of the domain pattern show that SBN:26 crystals usually contain intrinsic
defects in concentration high enough to effect the rate of domain nucleation and growth in certain re-
gions of the crystal sample. As is shown in figure 5, the distribution of nucleation sites is not random.
Such spatially non-uniform distribution of domain nuclei suggests the presence of a frozen polarization
component or built-in field, which favours one direction of spontaneous polarization in certain regions.
The fact that a similar picture of delineated domains has been observed for a positive [figure 5 (a)] and
a negative [figure 5 (b)] polarization state, demonstrates the presence of a frozen polarization state, pos-
sibly due to locally accumulated defects. The symmetry of the hysteresis loop for SBN:26 crystal sample
also indicates a lack of built-in directional field, which could stabilize the domain structure in a prefer-
ential direction. We observed a very slight variation in the domain size during the switching process in
low electric fields due to the domain wall pinning effect. When the external field exceeds some threshold
value (1 kV/cm for the crystal sample examined), the domain walls begin to move from the crystal edge,
31701-5
K. Matyjasek et al.
Figure 5. Inhomogeneous distribution of domains observed on (001) plate of SBN:26 crystal sample in the
electric field: (a) +0.6 kVcm−1
, (b) −0.6 kVcm−1
.
as is shown in figure 6. The evolution of the domain pattern takes place via expansion of the domain
front formed after coalescence of newly created domains at the edge of the crystal sample [figures 6 (b)–
(d)]. After a sufficient waiting time (∼ 0.4 s) along with the progress in switching, the macroscopically
visible domains arise in the area before the moving domain front (in the region where nucleation pro-
cess is suppressed). The new domains are clearly seen in figure 6 (d). The domain wall front mobility is
strongly inhomogeneous over the crystal surface. A marked slowing down of the domain wall velocities
was observed during its propagation. Very approximate estimations give values from 5 ·10−3
m/s at the
initial stage to 8 ·10−6
m/s at the end of the switching process in E = 1.0 kV/cm. At higher electric fields,
the nucleated domains have been observed almost on the whole crystal surface, and the switching speed
increases.
The pulse switching technique for studying the switching behaviour in higher electricfields for SBN:26
crystals is inappropriate because the conduction current obscures the displacement current. Kinetics of
polarization relaxation was determined in low electric fields by measuring the fraction of the switched
area as a function of time since the domain structure is naturally linked to the polarization state of the
crystal. Optical microscopy inspection on the opposite polar faces showed a nearly complete penetration
of the domains throughout the crystal bulk. It means that the forward domain growth along the polar-
ization direction takes place very quickly, and the sidewise domain growth determines the kinetics of
polarization switching. Thus, the domain growth becomes a two-dimensional problem. Therefore, the
area of the switched domains normalized to the total scan area is expected to be close to the normalized
reverse polarization. The recording of the domain patterns was performedwith a digital camera. In order
Figure 6. Domain pattern evolution observed in SBN:26 crystal sample in the electric field of 1 kVcm
−1
.
(a) initial single domain state. Time from the moment of applying E (b) 0.4 s, (c) 4 s, (d) 6 s, (e) 7 s, (f) 8 s.
Polarization axis is normal to the image plane.
31701-6
Dielectric studies on strontium-barium niobates
Figure 7. Time dependence of the switched area for two different electric fields for domain configurations
as shown in figure 6.
to obtain a quantitative description of the polarization evolution, the data have been processed using an
image analysis program. Figure 7 exemplarily illustrates the evolution of the switched areas as a function
of time under two different applied fields. The data points are well fitted (solid lines) with KWW function,
where the values of the stretching exponent, n, are given in figure 7.
4. Discussion
It is interesting to note that the domain switching process in SBN:70 single crystals is similar to that
observed in Ni doped SNB:61 single crystals [13]. It was shown that Ni doping additionally deteriorates
the homogeneity of the SBN:61 crystals. In both type of crystals, only the electric field E > Ec is sufficient
to complete the microscale domain switching in the entire volume of the crystal sample. In the doped
SBN:61 and SBN:70 single crystals, slow and fast domain walls coexist, which could play an important
role in the ferroelectric phase transition broadening, assuming the formation of polar regions with locally
different Curie temperature. On the other hand, the domain switching in SBN:26 crystal in the high-field
regime may qualitatively resemble the switching observed in classical ferroelectric crystals, in which
the growth of the existing domains is more favourable than the creation of new ones due to a relatively
low domain-wall energy [25]. Thus, the nucleated domains expand with no or little resistance under
an electric field. The SBN:26 crystal exhibits a high electrical conductivity. There has been reported a
considerable increase of the leakage current density since the applied field exceeds a critical value in
SBN crystals with low Sr content [17, 26]. Once the domains are nucleated in the high-field regime, the
field induced charge carriers may compensate the sources of random fields giving rise to the depinning
of the domain walls in SBN:26 crystals. It has been reported that having raised the sample conductivity
by illumination, the pinning centres were eliminated and the polarization dynamics was fully restored
in doped SBN:61 crystals [27].
The relaxation of polarization during the switching process can be described in the examined single
crystals by KWW stretched exponential function. It is interesting to note that our results are consistent
with a general concept saying that dynamical heterogeneity was established as a possible source of a
stretched exponential relaxation [23].
The domain walls make a considerable contribution to the total dielectric response of the SBN system
investigated. One can presume that a weakmeasuring ac field in dielectric measurements does not affect
the domain configuration but creates nuclei on the domain walls. Our results may confirm that the com-
position disorder could play an important role in the dielectric response in SBN:70. A broad distribution
of the heights of local pinning barriers should yield a distribution of activation energies for the nucleation
of reversed steps on the existing domain walls. Consequently, SBN:70 crystals show much broader tem-
perature dependence and lower values of dielectric susceptibility than those observed in SBN:26 crystals,
31701-7
K. Matyjasek et al.
in which the motion of domain walls is less restricted by a random field environment. Dielectric mea-
surements as well as the domain switching observations have evidenced the change from typical relaxor
behaviour in SBN:70 into classic ferroelectric behaviour in the case of SBN:26 crystal. It is interesting to
note that the crossover from relaxor to ferroelectric behaviour was also observed in solid solutions of
BaTi1−xSnxO3, and was evidenced by dielectric spectroscopic data [28].
It must be noted that visualization of the domain structure by NLC method may not thoroughly re-
flect the domain structure evolution in electrical switching at the same pulse amplitudes. Due to a slow
response time (of the order of several ms), liquid crystal molecules may not follow the changes of the do-
main structure in the regions that exhibit high speed switching. Moreover, the results of the works show
a strong effect of an interface (electrode and ferroelectric surface) conductivity on the kinetics of the
switching process [29]. In a metal contact, electrical charges can freely move and can effectively screen
the depolarization field accompanying the ferroelectric polarization, which decreases the switching pro-
cess. However, visualization of a domain structure by NLC-method can be used in checking the quality
of the crystal for the purpose of choosing high quality SBN samples with a homogeneous distribution of
domain nuclei.
5. Conclusions
The NLC decoration technique is a relatively simplemethod to visualize the distribution and evolution
of the microscale domains during the switching process, which obviously reflects the internal disorder of
a crystal. The domain structure dynamics confirm the dielectric measurements stating that the ferroelec-
tric properties of SBN system change with the composition from strong relaxor behaviour in SBN:70 into
classical ferroelectrics as observed in the case of SBN:26. Characteristic features of the domain structure
dynamics in SBN crystal of different composition (SBN:70 and SBN:26) can serve as a direct verification of
the concept of local random fields that exist due to a structural disorder of the SBN crystal. Fluctuations
of random fields neither effect the entire domains nor form a macroscopic bias field, but act as pinning
centres for the domain walls in SBN:70 crystal. For SBN:26 crystal, the domain walls can only be pinned
in low electric fields. At higher fields, the field- induced charge carriers may compensate the sources of
random fields giving rise to a less restricted domain growth. The polarization switching process in both
crystals can be described by a stretched exponential function widely used in dielectric relaxation studies.
Acknowledgements
Thanks are due to the Polish Ministry of Science for a partial financial support under grant
No. N N507 455034. Professor W. Kleemann from the University of Duisburg-Essen is acknowledged for
making accessible the Solartron Impedance Analyzer.
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Сегнетоелектричнi i дiелектричнi дослiдження
стронцiй-барiєвих нiобатiв релаксорного i
сегнетоелектричного типу
К.Матиясек1, Я. Дец 2, С.Мiґа2, Т. Лукасєвiч3
1 Iнститут фiзики, факультет механiчної iнженерiї i мехатронiки,
Захiдно-померанський технологiчний унiверситет,Щецiн, Польща
2 Iнститут матерiалознавства, Сiлезький унiверситет, Катовiце, Польща
3 Iнститут технологiї матерiалiв електронiки, Варшава, Польща
Еволюцiя сегнетоелектричної доменної структури, iндукованої зовнiшнiм електричним полем дослiджу-
валась за допомогою методу нематичного рiдкого кристалу в двох монокристалах стронцiй-барiєвого
нiобату номiнального складу: Sr0.70Ba0.30Nb2O6 (SBN:70 — релаксорний) i Sr0.26Ba0.74Nb2O6 (SBN:26 —сегнетоелектричний). Нашi результати показують, що широкий фазовий перехiд i частотна дисперсiя,
продемонстрованi кристалом SBN:70, мають тiсний зв’язок iз конфiгурацiєю сегнетоелектричних мiкро-
доменiв. Великий струм стiкання, виявлений в SBN:26, може компенсувати внутрiшнi заряди,що дiють як
центри пiнiнгу для доменних стiнок, що приводить до менш обмеженого росту доменiв подiбно до того,
що спостерiгається в класичних сегнетоелектриках. Мiкромасштiбнi дослiдження процесу перемикання
в поєднаннi з електричними вимiрюваннями дозволяють встановити спiввiдношення мiж локальними
властивостями динамiки доменiв i макроскопiчним вiдгуком, а саме, гiстерезисною петлею поляризацiї i
дiелектричними властивостями.
Ключовi слова: релаксорний сегнетоелектрик, нiобати, доменнi стiнки, процес перемикання, SBN
31701-10
Introduction
Experimental details
Experimental results
Ferroelectric characteristics
Domain switching in SBN:70 crystal
Domain switching in SBN:26
Discussion
Conclusions
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