SCIENCE CHINA
Physics, Mechanics & Astronomy
• Article •
October 2014 Vol. 57 No. 10: 1875–1878
doi: 10.1007/s11433-014-5401-9
Epitaxial growth and in-plane dielectric properties of
orthorhombic HoMnO3 films
GAO Ping, WANG WeiTian*, ZHANG Wei & SUN YuMing
Institute of Opto-Electronic Information Science and Technology, Yantai University, Yantai 264005, China
Received September 7, 2013; accepted October 12, 2013; published online April 17, 2014
Orthorhombic HoMnO3 (HMO) thin films were grown epitaxially on LaAlO3 (001) substrates by using pulsed laser deposition
technique. The films showed perfect orthorhombic crystallization and were well-aligned with the substrates. The in-plane dielectric constant and loss of HMO films were measured as functions of temperature (80–300 K) and frequency (120 Hz–100
kHz) by using coplanar interdigital electrodes. Two thermally activated dielectric relaxations were found, and the respective
peaks shifted to higher temperatures as the measuring frequency increased. The in-plane dielectric properties of epitaxial orthorhombic HMO films were considered as universal dielectric response behavior, and the dipolar effects and the hopping
conductivity induced by the charge carriers were used to explain the results.
dielectric properties, interdigital electrodes, HoMnO3 thin films, orthorhombic
PACS number(s): 81.15.Fg, 77.22.Gm, 77.55.+f
Citation:
Gao P, Wang W T, Zhang W, et al. Epitaxial growth and in-plane dielectric properties of orthorhombic HoMnO3 films . Sci China-Phys Mech Astron,
2014, 57: 18751878, doi: 10.1007/s11433-014-5401-9
1 Introduction
The stoichiometic rare earth manganites have attracted intensive theoretical and experimental research interest because of the various exotic properties, including ferroelectrics, ferromagnetism, colossal magnetoresistance effects
and multiferroics [1–4]. Many multifunctional-designed
devices made of these materials were reported to exhibit
fascinating dielectric, magnetic, and electric tunable functions. Among these rare materials, HoMnO3 (HMO) is a
prototypical material and shows unusual interplay of magnetic and electric properties. Most researches have focused
on the antiferromagnetism and ferroelectricity properties of
HMO bulk or single crystals at low temperature, and large
magnetoelectric effects were reported [5–7].
*Corresponding author (email: wtwang@ytu.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2014
Generally, HMO forms a stable hexagonal crystal structure. Concurrently, orthorhombic HMO (o-HMO) can also
be prepared by means of special synthesis methods although
it is not the thermodynamically stable phase at ambient
conditions [8–10]. Thus, HMO thin films can be fabricated
in hexagonal or orthorhombic structure expediently by choosing suitable substrates. Comparing with bulk materials,
HMO thin films exhibit unusual phenomena during certain
physical process. Moreover, the preparation of high-quality
films is an indispensable requirement for modern practical
applications [11,12].
Herein we report the growth of epitaxial o-HMO films on
LaAlO3 (LAO) substrates by using pulsed laser deposition
technique. The in-plane complex dielectric properties were
measured as function of frequency (120 Hz–100 kHz) with
interdigital electrodes (IDEs) to conduct a comprehensive
study of their surface dielectric properties. The universal
dielectric response (UDR) models were used to explain the
observed results.
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2 Experimental
The thin films were deposited using a KeF excimer laser
(=248 nm, =17 ns, 3 Hz repetition rate) focused onto the
high purity target of HMO ceramic which was prepared
through conventional solid-state reaction process using high
quality Ho2O3 and Mn2O3 raw materials. The X-ray diffraction (XRD) profile of the HMO ceramic target exhibited a
well-crystallized hexagonal structure (a=0.613 nm, c=1.14
nm) and no impurity phases could be detected in the X-ray
spectra. The typical laser energy density used was approximately 2.5 J/cm2, sufficiently high to ablate the ceramic
HMO. The target was mounted on a rotating holder, 40 mm
from the pseudo cubic LAO (001) substrates (a=0.3792 nm,
0.5 mm in thickness) which were maintained at 700°C under oxygen pressure of 30 Pa during the deposition process.
After the deposition, the thin films were cooled at an atmospheric pressure of oxygen with a cooling rate of 5°C/
min. The thickness of the fabricated HMO thin films was
about 120 nm.
The crystalline structure of the deposited HMO films was
investigated by XRD with Cu K radiation at 1.54 Ǻ. The
low-frequency (120 Hz–100 kHz) surface dielectric measurements were performed with interdigital electrodes (IDEs)
by using a QuadTech1730 LCR Digibridge in a cooling run
from room temperature to 77 K, followed by a heating run
to room temperature. The data of both runs were reproducible and reversible.
3 Results and discussion
Figure 1 shows a typical -2 XRD pattern of an asprepared HMO film on a LAO (001) substrate. In addition
to the diffraction peaks from the substrate, the presence of
the two reflections along with their peak positions can be
consistently described by (110) and (220) peaks for the
o-HMO, indicating that the as-prepared HMO thin film is in
orthorhombic structure and in the preferable (110) orientation. The results are consistent with other reports [13,14].
To investigate the in-plane texture of the films, the X-ray 
scans in proximity the o-HMO (112) and LAO (112) reflections were measured. As shown in the inset of Figure 1,
fourfold symmetry peaks indicate that the o-HMO films are
epitaxial and in orthorhombic structure. The azimuthal 
scans of substrate are identical with that of the film, suggesting the film is well aligned with the substrate. At room
temperature, the lattice parameters of the bulk o-HMO
phase are a=5.255 Å, b=5.826 Å, and c=7.365 Å [15],
whereas lattice constants of two times along the [010] and
[100] directions of LAO substrate are 7.584 Å ×7.584 Å,
which is acceptable to accommodate the (110) plane of
o-HMO. The mismatches along [110] and [001] directions
of HMO thin films are 3.4% and 2.8%, respectively.
Figure 1 Typical XRD -2 patterns of o-HMO thin films grown on
LAO (001) substrates. The inset shows the azimuthal  scans of the (112)
peaks of o-HMO and substrte, displaying the alignment of crystallographic
orientations between film and substrate.
The characterization of the dielectric properties of HMO
thin films is important for a variety of potential electronic
designs. However, for many applications, it is the in-plane
electrical properties of the film that need to be characterized
[16,17]. For this purpose, the IDE structure is widely used
to electrically achieve the in-plane dielectric properties
[18–20]. The Au IDEs were deposited on the top surface of
HMO film by using a combination of magnetron sputtering
and lift-off photolithography. Figure 2(a) illustrates the
Scanning Electron Microscopy (SEM) image of the fabricated IDE pattern, and the schematic measurement is shown
in Figure 2(b). The top Au electrodes were approximately
0.1 m thick. The IDE patterns consisted of 50 fingers with
identical finger width (15 m) and spacing (20 m); the
fingers were 0.7 mm long. The dielectric constant of a thin
film with the IDE pattern can be calculated using the following expression [17],
 f  s 
C   K 1   s  

 4.6h  
K 1  exp 

 G  W 

,
where f and s are the film and substrate dielectric constants (s of the LAO substrate is 25), respectively, h is the
film thickness and K is a constant in units of picofarads. C
is the measured capacitance per unit finger length per electrode section of width. G and W are the finger width and
gap spacing, respectively.
Figure 3 shows the calculated in-plane dielectric properties of o-HMO films. The temperature dependence of the
real part ′ of the complex permittivity and loss tangent tan
were presented respectively. It is seen that ′ consists of two
distinct transitions, and the peak positions shifted to higher
temperatures as the measuring frequency increases. The first
peak of ′(T) at the low frequency reached a value as high as
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Figure 3 Temperature dependence of (a) the dielectric constant ′ and (b)
the loss tangent tan of a HMO film at different frequencies between 120
Hz and 100 kHz measured with IDE. The inset in (b) shows the fitting
result (solid curve), which is composed of two Gaussian peaks and an
exponential background (dashed curves).
Figure 2 (a) SEM image of an IDE formed by photolithography; (b) the
schematic model of dielectric measurement with IDE.
260, corresponding to the peak in loss tangent with the same
development with the frequency, while the other one can
not be discerned in tan because of the exponential-like
increasing background. Two Gaussian peaks superimposed
upon an exponential function can be used to simulate the
experimental data of tan. Typical results for frequency 120
Hz are shown in the inset of Figure 3(b). The solid curve is
the fitting result, which is the sum of the three dashed
curves representing two Gussian peaks and exponential
background, respectively. This indicates that there exist two
sets of thermally activated relaxations in HMO films, which
resemble the dielectric behavior of other rare earth manganites with comparable crystal structure [21]. The similar
in-plane dielectric results were obtained in HMO thin films
with different thickness (120 nm, 150 nm, 190 nm), which
suggests that the film thickness (>100 nm) or internal stress
has little effects on the surface dielectric properties of epitaxial HMO films on LAO substrates.
The exponential-like increasing background in loss tangent implies that the background is associated with the hopping conductivity indued by the charge carriers. The localized charge carriers hopping between spatially fluctuating
lattice potentials can also result in dipolar effects. Such behavior was first reported by Jonscher [22] for such a wide
variety of materials that this behavior was denoted as the
UDR. According to this model, the imaginary part of the
complex dielectric constant can be expressed as '' 
A s  0 , where A and s are temperature dependent constants and ε0 is the electric permittivity of free space, =2f
is the angular frequency. As a consequence of the KramersKronig relation, the real part of the complex dielectric conA
stant is given by       tan( sπ / 2) s , where  
0
is the high-frequency value of  ( ). According to these
two equations, a straight line with the slope of s can be obtained in the plot of log(f′) vs. logf at a given temperature,
and the same characteristic can be observed in the log(f′′)
vs. logf pattern.
Figure 4 shows the inference in log-log graph. Approximate straight lines can be obtained in Figure 4(a) for the
frequency dependence of f′ with s~0.76, which is consistent with the assumption given by Jonscher [22]. With
decreasing temperature, the activity of the localized carriers
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dipoar effects and the hopping conductivity were used to
explain the results.
This work was supported by Shandong Province Natural Science Foundation of China (Grant No. ZR2011AM014).
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Figure 4 (Color online) Log-log plot of frequency dependence of (a) f′
and (b) f′′ of o-HMO films at given temperatures.
is suppressed, which reduces the dipolar effects, resulting in
the gradual deviation from the straight line. Figure 4(b)
shows the frequency dependence of ′′. The imaginary
component of the complex dielectric constant ′′(T) was
determined from the loss tangent (tanδ) data. The data of
log(f′′) fall on a straight line with logf with the slope of
s=0.1–0.3. In general, the exponential increasing background in loss tangent implies that the relaxation is associated with the hopping of carrier ions with appropriate relaxation of surrounding ions. Below 180 K, the points of f′′
deviate from the straight line at high frenquency. Perhaps
some other energy dissipation mechanisms arising from
relaxation is stimulated into action in the high-frenquency
range.
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4 Conclusions
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Epitaxial o-HMO films were fabricated on LAO (001) substrates by pulsed laser deposition. The in-plane dielectric
properties were investigated using Au IDE patterns. Two
peaks of the real part of the complex permittivity were obtained and the peak positions shifted to higher temperatures
as the measuring frequency increases, corresponding to the
peaks in loss tangent with exponential-like increasing background. The in-plane dielectric properties of epitaxial oHMO films can be described by the URD model, and the
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Eerenstein W, Mathur N D, Scott J F. Multiferroic and magnetoelectric materials. Nature, 2006, 442: 759–765
Lottermoser T, Lonkai T, Amann U, et al. Magnetic phase control by
an electric field. Nature, 2004, 430: 541–544
Zhang Y T, Wang C C, Liu W B, et al. Spin-glass like behaviors in
La1xTbxMnO3 perovskite. Sci China Ser G-Phys Mech Astron, 2009,
52: 1893–1897
Zhang C G, Zhang X Z, Sun Y H, et al. Atomistic simulation of dynamical and defect properties of multiferroic hexagonal YMnO3. Sci
China-Phys Mech Astron, 2011, 54: 836–840
Mathews S, Ramesh R, Venkatesan T, et al. Ferroelectric field effect
transistor based on epitaxial perovskite heterostructures. Science,
1997, 275: 238–240
Suzuki S, Yamamoto T, Suzuki H, et al. Fabrication and characterization of Ba1−xKxBiO3/Nb-doped SrTiO3 all-oxide-type Schottky
junctions. J Appl Phys, 1997, 81: 6830–6837
Yang C C, Chung M K, Li W H, et al. Magnetic instability and oxygen deficiency in Na-doped TbMnO3. Phys Rev B, 2006, 74(9):
094409
Lorenz B, Wang Y Q, Sun Y Y, et al. Large magnetodielectric effects
in orthorhombic HoMnO3 and YMnO3. Phys Rev B, 2004, 70(21):
212412
Zhou J S, Goodenough J B. Unusual evolution of the magnetic interactions versus structural distortions in RMnO3 perovskites. Phys Rev
Lett, 2006, 96: 47202
Lorenz B, Wang Y Q, Chu C W. Ferroelectricity in perovskite
HoMnO3 and YMnO3. Phys Rev B, 2007, 76: 104405
Murugavel P, Lee J H, Lee D, et al. Physical properties of multiferroic hexagonal HoMnO3 thin films. Appl Phys Lett, 2007, 90: 142902
Kim J W, Schultz L, Dorra K, et al. Growth and multiferroic properties of hexagonal HoMnO3 films. Appl Phys Lett, 2007, 90: 012502
Lin J G, Han T C, Wu C T, et al. Directional growth and characterizations of orthorhombic HoMnO3 films. J Cryst Growth, 2008, 310:
3878–3880
Lin T H, Hsieh C C, Shih H C, et al. Anomalous magnetic ordering in
b-axis-oriented orthorhombic HoMnO3 thin films. Appl Phys Lett,
2008, 92: 132503
Brinks H W, Rodriguez-Carvajal J, Fjellvag H, et al. Crystal and
magnetic structure of orthorhombic HoMnO3. Phys Rev B, 2001, 63:
094411
Kumar A K S, Paruch P, Triscone J M, et al. High-frequency surface
acoustic wave device based on thin-film piezoelectric interdigital
transducers. Appl Phys Lett, 2004, 85: 1757–1759
Dimos D, Raymond M V, Schwartz R W, et al. Tunability and calculation of the dielectric constant of capacitor structures with interdigital electrodes. J Electroceram, 1997, 1: 145–153
Hoerman B H, Ford G M, Kaufmann L D, et al. Dielectric properties
of epitaxial BaTiO3 thin films. Appl Phys Lett, 1998, 73: 2248–2250
Song Z T, Chan H L W, Choy C L, et al. Dielectric and ferroelectric
properties of in-plane lead lanthanum titanate thin films. Microelectron Eng, 2003, 66: 887–890
Pond J M, Kirchoefer S W, Chang W, et al. Microwave properties of
ferroelectric thin films. Integr Ferroelectr, 1998, 22: 317–328
Wang C C, Cui Y M, Zhang L W. Dielectric properties of TbMnO3
ceramics. Appl Phys Lett, 2007, 90: 012904
Jonscher A K. The ‘universal’ dielectric response. Nature, 1977, 267:
673–679