Electrical resistivity anomaly, valence shift of Pr ion, and magnetic behavior in epitaxial(Pr1-yYy)1-xCaxCoO3 thin films under compressive strain
H. Fujishiro, Y. Noda, K. Akuzawa, T. Naito, A. Ito, T. Goto, M. Marysko, Z. Jirak, J. Hejtmanek, and K. Nitta

Citation: Journal of Applied Physics 121, 115104 (2017); doi: 10.1063/1.4978747
View online: http://dx.doi.org/10.1063/1.4978747
View Table of Contents: http://aip.scitation.org/toc/jap/121/11
Published by the American Institute of Physics

JOURNAL OF APPLIED PHYSICS 121, 115104 (2017)

Electrical resistivity anomaly, valence shift of Pr ion, and magnetic behavior
in epitaxial (Pr1-yYy)1-xCaxCoO3 thin films under compressive strain
H. Fujishiro,1,a) Y. Noda,1 K. Akuzawa,1 T. Naito,1 A. Ito,2,3 T. Goto,2 M. Marysko,4 Z. Jirak,4
J. Hejtmanek,4 and K. Nitta5
1

Faculty of Science and Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan
3
Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai,
Yokohama 240-8501, Japan
4
Institute of Physics, ASCR, Cukrovarnicka 10, 162 00 Prague 6, Czech Republic
5Japan Synchrotron Radiation Research Institute, SPring-8, 1-1-1 Kouto, Sayo 679-5198, Japan
2(Received 22 October 2016; accepted 4 March 2017; published online 21 March 2017)
We have fabricated (Pr1yYy)1-xCaxCoO3 (PYCCO) epitaxial films with various thicknesses bypulsed laser deposition on the SrLaAlO4 (SLAO) substrate that applied an in-plane compressive
stress to the film, and investigated the temperature dependence of the electrical resistivity, q(T), of
the films. An anomalous q(T) upturn with a broad hysteresis could be clearly observed only for thethinnest film (d ¼ 50 nm), and the q(T) anomaly decreased by increasing film thickness, d. The temperature dependence of the X-ray absorption near-edge structure (XANES) spectra at Pr L2-edge
was measured for the films, and the valence states of praseodymium (Pr) ion were determined using
the analysis of the XANES spectra. As a result, the average valence of the Pr ion in the d ¼ 50 nm
film slightly increases with decreasing temperature from the common value of 3.0þ around room
temperature to 3.15þ at 8 K. The valence shift of Pr is thus similar to what was observed on the
PYCCO polycrystalline bulks with an abrupt metal-insulator transition, accompanied by a spinstate (SS) transition of Co ions. Furthermore, the low-temperature SQUID measurements evidenced
a paramagnetic behavior down to the lowest temperature, which suggests that the dominant part ofCo3þ ions in the film grown on the SLAO substrate tends to be in the low spin state characteristic
for the insulating ground state. These results strongly suggest that the anomalous q(T) upturn in thethin films on the SrLaAlO4 (SLAO) substrate is closely related to the SS transition of Co ions. On
the other hand, PYCCO films grown on the LaAlO3 (LAO) substrate that applied an in-plane tensilestress showed no valence shift of Pr ions and developed a long range ferromagnetic order, which
points to a complete suppression of the low-temperature transition. The behaviors of the epitaxial
films are discussed in terms of the in-plane stress exerted by different substrates and accumulated
elastic energy. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4978747]

I. INTRODUCTION

Perovskite cobaltite LaCoO3 and its rare-earth analogs
show marked evolutions of the electric and magnetic properties, which have a microscopic origin in the temperatureinduced spin-state (SS) crossovers of the octahedrally coordinated Co3þ ions of 3d6 electronic configuration.1–7 The
insulating ground state of these systems is based on Co3þions in the diamagnetic low spin state (LS; t2g6eg0, S ¼ 0),
while a paramagnetic insulating phase is gradually formedby local excitation of higher spin states, which takes place inrather broad temperature interval (40–120 K for LaCoO3).
The nature of such thermally activated species, the high spin
(HS; t2g4eg2, S ¼ 2) or intermediate spin (IS; t2g5eg1, S ¼ 1)
Co3þ ions, was under a long-time debate. The electromagnetic phase transition,8,9 metal-insulator (MI) transition,10
and the existence of various magnetic states, including ferromagnetic, cluster glass, and spin glass,11–13 have beenreported and discussed. Nonetheless, the recent experiments
and electronic structure calculations are strongly in favor ofa)E-mail: fujishiro@iwate-u.ac.jp

0021-8979/2017/121(11)/115104/8/$30.00

a mixed LS/HS phase, where HS sites experience mutual
repulsions.14,15 There is also large uncertainty about the transition to metallic phase at high temperatures (500 K for
LaCoO3). While the paramagnetic susceptibility has beenfound to be close to what is expected for homogeneous ISphase (cobalt ions in t2g5r* state),16 there is a growing evidence for a more complex disproportionation involving all
the three states (LS, IS, and HS), which is controlled not
only by the delicate balance of local energy terms (crystal
field splitting vs. Hund-rule coupling) but also by pair interactions between the local states (repulsive for HS and attractive for IS).14,17 Theoretical analysis performed on the basisof extensive dynamical mean-field theory (DMFT) calculations provides some view of what happens in such dynamic
coexistences—with increasing temperature, the HS states
gain progressively more weight over the LS ones, and the IS
states arise only as short-living fluctuations.15
The properties of mixed-valence cobaltites also attract a
lot of interest. The divalent substitution for La or rare-earth
site, in La1-xSrxCoO3 for instance, dopes hole carriers into
the system, which in turn results in stabilization of finite spinstates, formation of ferromagnetic droplets, and a phase

121, 115104-1

Published by AIP Publishing.

115104-2

Fujishiro et al.

separated state of ferromagnetic cluster in a nonferromagnetic matrix for 0.04 < x < 0.22, and a ferromagnetic metal phase with itinerant cobalt states for x > 0.22.8
Most interestingly, some (Pr,Ca)-based cobaltites exhibit apronounced first-order transition to a low-temperature phasewith a weakly paramagnetic character and reduced electricalconductivity. This effect was revealed for the first time on
Pr0.5Ca0.5CoO3 at TMI  90 K by a step-like resistivity jumpand concomitant anomalies in magnetic susceptibility, heat
capacity, lattice dilatation, and photoemission spectroscopy.18–20 The mechanism of the transition was tentatively
ascribed to a spin-state crossover from the itinerant cobalt
states to an ordered mixture of localized LS Co3þ and LS
Co4þ (t2g5eg0, S ¼ 1/2) states. The MI transition was also evidenced in the less-doped Pr1-xCaxCoO3 (x ¼ 0.3) underhigher hydrostatic pressures,21,22 and in the (Pr1-yREy)1xCaxCoO3 system (0.2 < x < 0.5) with a partial substitutionof Pr by smaller RE cations such as Sm, Eu, and Y underambient pressure.22,23 This peculiar transition appeared to beconditioned not only by the presence of both Pr and Ca ions,but also by a larger structural distortion of the CoO6 network,
depending on the average ionic radius and the size mismatchof perovskite A-site ions.23 Furthermore, the critical temperature, TMI, was found to decrease in a magnetic field, and thetransition could be completely suppressed in very high magnetic fields.24
An alternative scenario explaining the nature of such a
specific transition was proposed on the basis of electronic
structure calculations exploiting the temperature dependenceof the structural experimental data for Pr0.5Ca0.5CoO3.25 The
calculations suggested that the formal cobalt valence wouldchange below TMI from a mixed-valence Co3.5þ towards apure Co3.0þ with a strong preference for the LS state and, concomitantly, the praseodymium valence would increase fromPr3þ towards Pr4þ. The theoretical hypothesis about the crucial role of variable praseodymium valence was experimentally supported by the observation of a Schottky peak in the
low temperature specific heat of (Pr1yYy)0.7Ca0.3CoO3(y ¼ 0.075 and 0.15),26 by X-ray absorption near-edge structure (XANES) spectra directly at the Pr L3-edge,27–30 and bysmall-angle neutron scattering measurements.31
It is generally desirable to use a single crystal or an epitaxial film for the physical investigation. In the epitaxial thinfilm, the electrical conduction and the spin state change,
which depend on the lattice mismatch between the cobaltite
and the substrate, the film thickness, and the fabrication condition.32 In particular, the interface between the film and thesubstrate can play an important role, giving rise to boundary
regions with different structures and affecting the transportand magnetic properties due to the induced strain, especiallyin very thin films. The experiments on LaCoO3 epitaxial
films have demonstrated very different behaviors for strained
films grown on (La,Sr)(Al,Ta)O3 (LSAT) substrate, applying
an in-plane tensile stress, and those grown on LaAlO3 (LAO)
substrate, applying a compressive stress. In the latter case, aparamagnetic ground state is obtained, which may signal thedominance of LS cobalt species as in bulk LaCoO3, whereas
the tensile stress stabilizes long-range ferromagnetic order
below 85 K.33 In the case of thicker films (d > 50 nm),

J. Appl. Phys. 121, 115104 (2017)

where epitaxial strain is relaxed to a high degree, the longrange ferromagnetic order is stabilized for the films grownboth on the LAO and LSAT substrates.
As the (Pr1-yREy)1-xCaxCoO3 system is concerned, nosingle crystal sample was reported and all previous experimental studies of the simultaneous MI-SS transition havebeen performed on polycrystalline bulk materials. Only
recently, we successfully fabricated (Pr1yMy)1-xCaxCoO3
epitaxial films (M ¼ Y, Gd) of 50 nm in thickness by pulsed
laser deposition (PLD) on single crystal substrates with different lattice parameters, and measured their electrical resistivity,q(T).34 An anomalous q(T) upturn with a wide hysteresis can
be clearly seen only for the film grown on a SrLaAlO4(SLAO) substrate, which applied the in-plane compressive
stress to the film. Such a q(T) anomaly is interpreted as a signof the first-order phase transition related to the SS transition of
Co ions, which was observed in the polycrystalline bulk.
However, there has been no direct evidence of the cause of
the q(T) anomaly, and no investigation of the thickness dependence of the q(T) anomaly has been attempted. Although theappearance of the MI transition with hysteresis and theincrease in the transition temperature for the Pr1-xCaxCoO3
bulk materials under hydrostatic pressure were reported previously,21,22 the influence of the epitaxial stress due to latticemismatch and the thermal stress due to the difference of thermal contraction coefficient between the film and substrate on
the MI transition has not been clarified so far.
In the present work, we report on the PLD growth of
50–300 nm thick (Pr1-yYy)1-xCaxCoO3 epitaxial films on the
SLAO substrate. To clarify the origin of the q(T) anomaly,
the Pr L2-edge XANES spectra and low-temperature magnetization are measured for the films. It appears that thevalence change at Pr sites and the spin-state crossover of Co
ions, known for polycrystalline samples, are realized also forthe compressively strained thinner films, but the transition isof diffuse character and spreads over a wide temperature
range. In thicker films showing different degrees of relaxation toward bulk (Pr1-yYy)1-xCaxCoO3 structure, the transition is inhibited. Absence of the transition is found also forfilms grown on the LaAlO3 (LAO) substrate that applies thein-plane tensile stress. These results suggest that substrates
have generally a blocking effect, which prevents the films toaccommodate 1% volume change below the transition tolower spin Co states, unless they are suitably strained at
higher temperature or, eventually, possess structural defects
with low-temperature mobility as reported elsewhere.33,35
II. EXPERIMENTAL PROCEDURE(Pr1-yYy)1-xCaxCoO3 (PYCCO) thin films were fabricated by the PLD method using a fifth harmonic wave of NdYAG laser with k ¼ 213 nm and a repetition rate of 10 Hz.
The pulse duration was 5 ns and the power was 18 mJ. A
polycrystalline target with nominal composition (x ¼ 0.4,
y ¼ 0.2) was prepared by a standard solid-state reaction. The
details of the fabrication method of the target bulk have been
described elsewhere.34 The target and the substrate were set
in a PLD apparatus and were counter-rotated at a speed of
7 rpm and 3 rpm, respectively, to reduce non-uniform erosion

115104-3

Fujishiro et al.

and to deposit the film uniformly. The deposition was carriedout for 0.5–4 h in a pure oxygen pressure, PO2 ¼ 1.0 Torr,
and at a substrate temperature, Ts ¼ 873 K. The film thickness, d, was changed from 50 to 300 nm, which was determined by a contact profiler (Taylor Hobson Inc.). The
films were grown on the (001) oriented SrLaAlO4 (SLAO:
a ¼ 0.3756 nm, c ¼ 1.2636 nm) single crystal substrate of
0.5 mm in thickness. For comparison, the films were also
grown on the (001) oriented LaAlO3 (LAO: a ¼ 0.3790 nm)
single crystal substrate. The pseudo cubic lattice parameterof the PYCCO target bulk was a ¼ 0.3775 nm, and the latticemismatch, e ¼ (afilm  asub)/asub, was 0.5% for SLAO andþ0.4% for LAO, where afilm and asub were lattice parametersof PYCCO film and substrate, respectively. In this case, anin-plane compressive stress was applied to the films grownon the SLAO substrate, and an in-plane tensile stress wasapplied to the films grown on the LAO substrate. The chemical composition of the grown films was estimated by electronprobe microanalysis (EPMA), in which the composition ofthe (Pr1-yYy)1-xCaxCoO3 film was shifted to (x ¼ 0.3,
y ¼ 0.125) from the nominal target composition of (x ¼ 0.4,
y ¼ 0.2). To clarify the crystalline quality of the grown films,
X-ray diffraction analyses of a h/2h scan were performed
using Cu Ka radiation, which identified the out-of-planealignment. The electrical resistivity, q(T), was measured oncooling and heating runs in the 10–300 K range by a standard
four-probe method. In addition, test measurements of boththe electrical resistivity and thermoelectric power were performed under flowing oxygen at room and high temperatures, and the results were compared with data for bulk (Pr1yYy)0.7Ca0.3CoO3 and Pr1-xCaxCoO3 systems. Making use ofwell-known sensitivity of thermoelectric power to the cobalt
oxidation state and thus to oxygen stoichiometry,36 theclose-to-ideal oxygen stoichiometry of present films was
confirmed.
The temperature dependence of XANES spectra at thePr L2-edge was measured for the films at BL01B1 of SPring8 in Japan. Because of the overlap of the fine structure of La
L2-edge from the substrate with the Pr L3-edge from the film,
we selected to measure the Pr L2-edge XANES spectra,
although we usually measured Pr L3-edge XANES spectra in
the bulk samples.27,30 The X-ray beam was monochromatized using a Si(111) double-crystal monochromator. The
spectra were recorded in the fluorescence mode by 19-ch
solid-state detector (SSD) at various temperatures from 300
to 8 K using a cryocooler. The valence of all the Pr ions was
supposed to be 3.0þ for all the films around room temperature. To determine the mixed Pr3þ/Pr4þ content in the film at
low temperatures, comparative XANES measurements for
Pr6O11 (Pr3.667þ) and Pr2O3 (Pr3.0þ) were also carried out.
The recorded spectra were modeled by the sum of three
Lorentzian functions and one arctangent function representing the step-like edge of the continuum excitations, similarly
to the previous analyses.27,30 One Lorentzian function (peakA: 6442 eV) shows a fluorescence from L2 (2p1/2) to M4,
which represents Pr3þ ions. The other Lorentzian functions(peak B2: 6445 eV and peak B1: 6455 eV) show the excitations from 2p3/2 to 4f2L5d* and 4f15d*, with L being a ligand
hole in the O 2p orbital, both of which can be understood to

J. Appl. Phys. 121, 115104 (2017)

represent Pr4þ ions.37,38 The energy differences between
peaks A, B2, and B1 were fixed according to results of Hu
et al.39 The curve fittings were performed using Athenasoftware.40
The magnetic properties were probed on the films ofd ¼ 160 nm in thickness, by using a SQUID magnetometer.
The susceptibility data were taken in the 2–300 K range,
using the zero-field-cooled (ZFC) and field-cooled (FC)regimes and DC field of 10 kOe. The hysteresis loops up to
70 kOe were also scanned at 2 K. Moreover, in order to distinguish the magnetic response of the film from that of thesubstrate, careful chemical analysis of magnetic impurities
in the SLAO and LAO substrates was performed.
III. EXPERIMENTAL RESULTS AND DISCUSSION
A. Thickness dependence of the electrical resistivity,
q(T), of PYCCO films on SLAO and LAO substrates

Figure 1(a) shows the thickness dependence of the electrical resistivity, q(T), of PYCCO films (x ¼ 0.3, y ¼ 0.125)
with different thicknesses, grown on the SLAO substrate.
The q(T) of the polycrystalline bulk with the same composition is also shown in the figure, which shows an abrupt jumpof about two orders of magnitude at TMI ¼ 120 K on the cooling run, and shows only very small hysteresis on heating run.
In the PYCCO films, there is an anomalous upturn in the

FIG. 1. (a) Thickness dependence of the electrical resistivity, q(T), of the(Pr1-yYy)1-xCaxCoO3 (x ¼ 0.3, y ¼ 0.125) epitaxial films grown on the SLAO
substrate. q(T) of the polycrystalline bulk with the same composition is also
shown. (b) Out-of-plane XRD patterns of the (002) diffraction for the same
PYCCO films grown on the SLAO substrate.

115104-4

Fujishiro et al.

J. Appl. Phys. 121, 115104 (2017)

q(T) curve having an inflection at around 170 K and by broadhysteresis spreading to much lower temperatures that can beseen, however, for the film of d ¼ 50 nm only. For the films
thicker than d ¼ 100 nm, the inflection in q(T) decreases withincreasing film thickness, d, and no hysteresis can be
distinguished.
Figure 1(b) presents the out-of-plane XRD patterns ofthe (002) diffraction for the same films. The peak intensity
and the peak angle increased with increasing film thickness,d. The full width at half maximum (FWHM) value also
decreased with increasing d, which was 1.40 for d ¼ 50 nm
and 1.07 for d ¼ 300 nm, respectively. These results indicatethat the 50 nm film is elastically compressed on the SLAO
substrate, and this property is likely the reason for theobserved hysteresis in the resistivity measurement. On theother hand, a partial relaxation takes place with increasing
film thickness, for which the average lattice parameter of thec-axis approaches that of the bulk crystal and the epitaxial
strain becomes effective only close to the interface.
Figure 2 shows the thickness dependence of the q(T) of
PYCCO films (x ¼ 0.3, y ¼ 0.125) with different thicknessesgrown on the LAO substrate. For all these films, q(T) slightly
increases with decreasing temperature, which is a clear contrast to the notable upturn in PYCCO films on the SLAO
substrate. No anomaly in q(T) can be detected, even for thefilms as thin as d ¼ 100 nm. And the absolute value of q(T)decreases with increasing film thickness, d.
B. XANES spectra

Figure 3(a) shows the temperature dependence of the
XANES spectra at the Pr L2-edge for the d ¼ 50 nm filmgrown on the SLAO substrate. The two main peaks situatedat 6442 and 6455 eV (named peaks A and B1) originate fromthe Pr L2 to M4 transitions. At 290 K, Pr3þ sites essentiallycontribute to the peak A, with a little bump around 6455 eV
caused presumably by multiple scattering, which is commonly introduced in the theoretical calculation of XANES.29
The small bump was also observed at the Pr L3-edge in
the Pr2O3 and Pr0.7Ca0.3CoO3 polycrystals,27 which is
not related to the valence change of Pr ions. When the

FIG. 2. Thickness dependence of the electrical resistivity, q(T), of the (Pr1-yYy)1-x
CaxCoO3 (x ¼ 0.3, y ¼ 0.125) epitaxial films grown on the LAO substrate. q(T) of the polycrystalline bulk with the same composition is also
shown.

FIG. 3. Temperature dependence of the XANES spectra at the Pr L2-edge
for the (a) d ¼ 50 nm film grown on the SLAO substrate and (b) d ¼ 300 nm
film grown on the LAO substrate. The inset of each figure shows the magnification of the spectra around 6455 eV.

temperature decreases, the shape of the XANES spectrachanges markedly. The intensity of peak B1 slightly increases
and, at the same time, the intensity of peak A decreases and a
new component (peak B2 at 6445 eV) can be resolved on its
high-energy slope. The B1 and B2 peaks are manifestations ofPr4þ states.37–39 These results suggest that, for the d ¼ 50 nm
film, the valence of the Pr ions increases from 3þ toward 4þ
with decreasing temperature, which is consistent with thereports for the PYCCO and Pr0.5Ca0.5CoO3 polycrystals.29,30,41
Figure 3(b) shows the temperature dependence of the
XANES spectra at the Pr L2-edge for the d ¼ 300 nm filmgrown on the LAO substrate. Because of the strong signal
from the thicker film, the spectra are not noisy comparedwith those for the d ¼ 50 nm film shown in Fig. 3(a). Apartfrom the main peak A, there is again a small bump at around6455 eV, originating in the multiple scattering as mentioned
above. The intensities of peaks A and B1 are almost thesame, even if the temperature decreases down to 8 K, whichsuggests that the valence of the Pr ions is almost constant of
3.0þ at the entire temperature range.
In order to determine the valence shift quantitatively,
the XANES spectra were fitted to a sum of three Lorentzianfunctions (peak A for Pr3þ and peaks B1 and B2 for Pr4þ)and one arctangent function representing the continuum
excitations, as indicated in Section II. Figures 4(a) and 4(b)115104-5

Fujishiro et al.

FIG. 4. XANES spectrum at the Pr L2-edge of the (a) Pr6O11 and (b) Pr2O3
samples at 300 K, for which the fitting was performed. The inset in (a) shows
the calibration line for the determination of the valence of the Pr ion in thefilms used in the study.

show the XANES spectrum at the Pr L2-edge of Pr6O11 andPr2O3 at 300 K, which are, respectively, regarded as a standard for Pr3.667þ and Pr3.0þ. In the fitting of the temperaturedependent XANES spectra for the PYCCO films, the intensity ratio of B1:B2 was fixed to be 1.0:2.0. The calibrationline was used for the determination of the valence of the Pr
ion shown in the inset of Fig. 4(a).
Figure 5(a) shows an example of the fitting curve of the
XANES spectrum at 8 K for the d ¼ 50 nm film grown on the
SLAO substrate, where the valence change achieves a maximum. It is seen that the XANES spectrum can be well fitted
within the energy range including the related peaks of Pr3þand Pr4þ. The B2 component is also resolved and appears
notable so that the resemblance with the Pr3þ/Pr4þ mixture
in Pr6O11 is obvious. The temperature course of the valenceof the Pr ion is shown in Fig. 5(b) for the films. For thed ¼ 50 nm film on the SLAO substrate, the average Pr
valence increases gradually on cooling from 300 K, andreaches a final value of 3.15þ at 8 K. It is worth mentioningthat in our previous study of the (Pr1yYy)0.7Ca0.3CoO3 bulk
materials, the Pr valence varied from 3.15þ in y ¼ 0.075 up to
3.27þ in y ¼ 0.15 at 8 K.27 The Pr valence of 3.15þ for thepresent d ¼ 50 nm film (x ¼ 0.3, y ¼ 0.125) on the SLAO substrate is lower than expected for bulk material of this composition, 3.23þ. This may signal that the in-plane compressive
strain, which is the driving force for low-temperature transition

J. Appl. Phys. 121, 115104 (2017)

FIG. 5. (a) Example of the fitting of the XANES spectrum at 8 K for thed ¼ 50 nm film on the SLAO substrate. For the fitting, one arctangent function and three Lorentzian functions are used. (b) Temperature dependence ofthe valence of Pr ions in the films (d ¼ 50 nm on SLAO and d ¼ 300 nm on
LAO) estimated using the XANES spectra and curve fitting.

in PYCCO epitaxial films, was exhausted before the transition
could be completed. On the other hand, for the d ¼ 300 nm
film on the LAO, there was no change in the valence of the Pr
ion down to 8 K, which suggested that the valence of the Pr
ion remained at 3.0þ. The ambiguity of the estimated valencevalues is 0.03, which results from the arbitrariness of theparameters in the arctangent and Lorentzian functions used.
The XANES spectra thus provide strong arguments that theq(T) anomaly for the d ¼ 50 nm film on the SLAO substrate
results from the spin-state (SS) transition of Co ions, similar to
the PYCCO polycrystalline bulks.
C. Magnetization of PYCCO films on SLAO and LAO
substrates

The magnetic behavior was investigated for the thick(Pr1-yYy)1-xCaxCoO3 (x ¼ 0.3, y ¼ 0.125) films (d ¼ 160 nm)
grown on the SLAO and LAO substrates at Ts ¼ 873 K andPO2 ¼ 0.01 Torr for 1 h. Although it is the best to measure
the same films (d ¼ 50 nm film on SLAO and d ¼ 300 nm
film on LAO) as shown in Fig. 3, the thicker films must be
measured to enhance the intrinsic magnetic signal of thefilms. Because a single crystal SLAO substrate grown in an
iridium (Ir) crucible contained the minute contamination ofIr4þ species with magnetic moment, j ¼ 1/2, which affect tothe low temperature paramagnetic tail superposed to thedominant diamagnetic response of the PYCCO film. For this115104-6

Fujishiro et al.

reason, the thicker d ¼ 160 nm films were used for the magnetic investigation. Figure 6(a) shows the temperature dependence of the magnetic susceptibility v(T) of the films during
field cooling (FC) and zero-field cooling (ZFC) runs underthe magnetic field of l0H ¼ 10 kOe. For comparison, thesame v(T) plots for the reference bulk samples are shown in
Fig. 6(b), in which the PYCCO bulk (x ¼ 0.3, y ¼ 0.15), abbreviated as “PrY15,” exhibits a clear MI transition characterized
by a sharp drop of magnetic susceptibility at 134 K,26 and thepure Pr0.7Ca0.3CoO3 bulk, abbreviated as “PrCa30,” does notundergo any MI-SS transition and shows instead a marked
upturn of susceptibility at 80 K, which signals the

J. Appl. Phys. 121, 115104 (2017)

spontaneous FM ordering at Tc ¼ 55 K.26,42 Figure 6(c) shows
the magnetization loops measured at 2 K for the films, compared with the PrY15 and PrCa30 bulks. It should be noted thatthe magnetic data on the present PYCCO films are subjected toa large uncertainty due to small intrinsic signals of the layer
itself, superimposed to the paramagnetic contribution of substrates. The eventual inaccuracy in the film thickness determination and, importantly, certain paramagnetic contamination of
Ir in the SLAO substrate that are of the Curie-type temperaturedependence and interfere thus with diamagnetism of the films,
further complicates the quantitative analysis.43
Nonetheless, in spite of these limitations, we would liketo note that the ZFC/FC susceptibility curves observed for
PYCCO/SLAO film remind the Curie-type behavior of thePrY15 bulk, although a reduction of magnetic susceptibility
cannot be observed, which may come from the diffuse character of MI transition. However, the narrow hysteresis loop
at 2 K is of a similar curvature and magnitude as found in the
PrY15 bulk with similar chemical composition undergoing
the MI transition. Even for the moderate MI transition in the
PYCCO/SLAO film (d ¼ 160 nm), we can unambiguously
exclude the persistence of the metallic phase down to lowtemperatures. We deduce that the dominant part of Co3þions in the PYCCO film grown on SLAO substrate with asmall negative mismatch e ¼ 0.5% occurs likely in the LS
state, which is an essential characteristic of the insulatingstate.
On the other hand, the magnetic properties of PYCCO/LAO film with e ¼ þ0.4% are distinct both by the presenceof FM upturn in ZFC/FC susceptibility curves below 80 K
and by a large coercivity in hysteresis loop at 2 K. The magnetization curve remarkably resembles the results on
Pr0.7Ca0.3CoO bulk, though the absolute value of the remnant moment is reduced (see Fig. 6(c)). Magnetic characterization (not shown) of the PYCCO film grown on SrTiO3(STO) substrate with the largest mismatch of e ¼ þ3.4%
have shown that ZFC/FC susceptibility curves exhibit similar
FM upturn as PYCCO/LAO film and large coercivity in hysteresis loop at 2 K as well. Moreover, the low temperatureremnant moment was even higher than that detected in the
PYCCO/LAO films. We thus conclude that PYCCO filmgrown on substrates with a positive mismatch retains the ferromagnetic metallic phase of t2g5r* character with delocalized carriers in the antibonding eg band down to the lowest
temperatures.
IV. DISCUSSION

FIG. 6. (a) The ZFC/FC susceptibility curves of the (Pr1-yYy)1-xCaxCoO3(x ¼ 0.3, y ¼ 0.125) epitaxial films 160 nm in thickness, grown on the
SLAO and LAO substrates. The measurement was done at 10 kOe. (b) The
similar ZFC/FC susceptibility curves of the (Pr1-yYy)1-xCaxCoO3 bulks(x ¼ 0.3, y ¼ 0.15; “PrY15” and x ¼ 0.3, y ¼ 0; “PrCa30”) referred from
Ref. 26, which are examples of the presence and absence of MI transition,
respectively. (c) The hysteresis loops of the films at 2 K. Diamagnetic contribution of both substrates together with low temperature paramagnetism
associated with magnetic impurities in case of SLAO substrate were subtracted (see text).

In the bulk (Pr1-xREy)1-xCaxCoO3 systems under ambientpressure, which show the simultaneous MI-SS transition, a
volume contraction of 1% took place below TMI  100 K.44
The entropy change makes DS  5 JK1mol1,45,46 whichmeans that the internal energy of the low-temperature phaseis about DSTMI  500 Jmol1 lower, compared with thehigh-temperature phase. The present PYCCO films are fixed
on the substrate and the lattice parameters and the unit cellvolume are difficult to change. Even if a suitable condition
for the MI-SS transition were obtained, it would be at some

115104-7

Fujishiro et al.

expense of elastic energy due to strains that are frozen at low
temperatures.
In the PYCCO films on SLAO, the lattice mismatch,e ¼ 0.5%, exists at room temperature and thermal stress isalso applied to the films during low temperature q(T) measurements, which results from the difference of thermal contraction coefficient between the PYCCO film and SLAO
substrate. Since the thermal contraction coefficient of
PYCCO is about twice as large as that of the SLAO substrate,47,48 the lattice mismatch is effectively decreased at
low temperatures, to about e ¼ 0.25% at 50 K. The in-planestress, which is exerted on the film, can be roughly estimatedby r ¼ eY(1  )1 with Y and  being the Young modulus
and the Poisson ratio, respectively.48,49 Assuming for cobaltites, Y ¼ 150 GPa50 and  ¼ 0.3,51 we arrive to the conclusionthat the compressive in-plane stress makes re ¼ 1.07 GPa at
300 K and re þ rt ¼ 0.535 GPa at 50 K, which is a favorable
condition to start the MI-SS transition in PYCCO even at
higher temperatures than in polycrystalline bulks. However,this compressive strain is gradually spent since the complete transition to the low-temperature state is characterized
by rather large volume contraction of 1%, which would
make an additional in-plane strain of tensile character,eMI ¼ þ0.33%. The corresponding in-plane tensile stressrMI ¼ þ0.715 GPa thus cancels the compressive in-plane stressre þ rt ¼ 0.535 GPa that exists in the high-temperaturephase.
In the case of thick PYCCO films that can be considered
as nearly relaxed at room temperature, the different thermal
contraction between the film and substrate brings a tensile inplane stress at low temperatures, rt ¼ þ0.535 GPa at 50 K,
which would be increased, upon the MI-SS transition tort þ rMI ¼ 1.250 GPa as derived from the strain (e þ eMI¼ þ0.58%). Neglecting the inhomogeneities close to the substrate–film interface, the corresponding elastic energy of uniformly strained films (per unit volume) can be written as E ¼ 1/
2(r11e11 þ r22e22) with only in-plane variables r11 ¼ r22 ande11 ¼ e22, while the other terms become zero either because of
the absence of sheer strain (e12 ¼ 0) or because of the absenceof out-of-plane stress (r33 ¼ 0). For rt þ rMI ¼ 1.250 GPa,et þ eMI ¼ þ0.58% and actual molar volume, the formula gives
an estimate that the elastic energy accummulated during the
MI-SS transition might be as high as E ¼ 950 Jmol1.
Although this value might be partly reduced at the presence of
defects, it is clear that the rise of energy due to frozen strainsexceeds or is at least comparable to the above mentioned dropof energy, DSTMI  500 Jmol1. In other words, there is no
driving force for MI-SS transition in relaxed PYCCO films.
We conclude that the discussed elastic properties may
explain well what is actually observed, namely the diffuse
character of MI-SS transition in compressively strained
50 nm thick PYCCO film on SLAO substrate and the absenceof transition in thicker relaxed films or thin tensile-strained
films.

J. Appl. Phys. 121, 115104 (2017)

structurally modified by content of small size yttrium
y ¼ 0.125, have been fabricated by the pulsed laser deposition (PLD) method on the SrLaAlO4 (SLAO) and LaAlO3(LAO) single crystal substrates. The thickness dependenceof electrical resistivity, q(T), has been investigated and thecompressive stress of the films on the SLAO substrate has
been analyzed. The most important results and conclusions
are summarized as follows:(1) An anomalous q(T) upturn with wide hysteresis can beclearly observed only for the thinnest film (d ¼ 50 nm)
grown on the SLAO substrate. The hysteretic q(T) anomaly vanished for the films thicker than d ¼ 100 nm andinflections in q(T) around 170 K decreased with increasing film thickness, d. These results indicate a gradualloss of the full compressive stress with increasing d.
(2) On the other hand, the q(T) of the film grown on the
LAO substrate, which applied the tensile stress to thefilm, shows no anomaly, and the absolute value of q(T)decreases with increasing the film thickness, d.
(3) From the measurements of the XANES spectra at the Pr
L2 for the films, the average valence of the Pr ion in thed ¼ 50 nm film on the SLAO substrate slightly increases
with decreasing temperature from the common value
3.0þ at around room temperature to 3.15þ at 8 K. On
the other hand, there is no valence shift of Pr in the filmon the LAO substrate in the entire temperature range.
These results strongly suggest that the anomalous q(T)upturn in the thin film on the SLAO substrate is closely
related to the MI-SS transition seen in PYCCO polycrystalline bulks.
(4) The Curie-type susceptibility v(T) curve and the narrowhysteresis loop at 2 K in the PYCCO thin film grown on
SLAO substrate are the similar trends to the PYCCO
bulk with MI transition. These results suggest an essential characteristic behavior of the insulating state at low
temperatures, in which LS Co3þ ionic states dominate.
On the other hand, PYCCO films grown on LAO substrate retain the ferromagnetic metallic phase of t2g5r*
character with delocalized carriers in the antibonding egband down to the lowest temperatures.
(5) The strain exerted by substrates on epitaxial films hinders to develop volume change required for the lowtemperature spin-state transition. The epitaxial compressive strain is gradually spent during the transition, so thatthe remaining regions of the high temperature state willneed lower and lower temperatures to transform into the
low temperature state. Therefore, the transition in theepitaxial thin film on SLAO is diffuse and spreads over alarge temperature range. On the other hand, thick films
experience generally a tensile strain that increases withdecreasing temperature. This is completely unfavorable
situation since large elastic energy would be accumulated upon the MI-SS transition.

V. SUMMARYACKNOWLEDGMENTS(Pr1yYy)1-xCaxCoO3 (PYCCO) epitaxial films, withhole doping controlled by the calcium content x ¼ 0.3 andThe synchrotron radiation experiments were performedat the BL01B1 of SPring-8 with approval of the Japan

115104-8

Fujishiro et al.

Synchrotron Radiation Research Institute (JASRI) (Proposal
Nos. 2013A1093 and 2016B1089). The authors thank Dr. T.
Ina of JASRI for his experimental assistance for XRF
measurements. This work was partly supported by JSPS
KAKENHI (Grant No. 24540355) and was performed underthe Inter-university Cooperative Research Program of theInstitute for Materials Research, Tohoku University. The
work at Institute of Physics, Prague, was supported by
Project No. 17-04412S of the Czech Science Foundation.
1

P. M. Raccah and J. B. Goodenough, Phys. Rev. 155, 932 (1967).
M. A. Korotin, S. Y. Ezhov, I. V. Solovyev, V. I. Anisimov, D. I.
Khomskii, and G. A. Sawatzky, Phys. Rev. B 54, 5309 (1996).
3
K. Asai, A. Yoneda, O. Yokokura, J. M. Tranquada, G. Shirane, and K.
Kohn, J. Phys. Soc. Jpn. 67, 290 (1998).
4
T. Kyomen, Y. Asaka, and M. Itoh, Phys. Rev. B 71, 024418 (2005).
5
S. W. Biernacki, Phys. Rev. B 74, 184420 (2006).
6
P. G. Radaelli and S.-W. Cheong, Phys. Rev. B 66, 094408 (2002).
7
D. Phelan, D. Louca, S. Rosenkranz, S.-H. Lee, Y. Qui, P. J. Chupas, R.
Osborn, H. Zheng, J. F. Michell, J. R. D. Copley, J. L. Sarrio, and Y.
Moritomo, Phys. Rev. Lett. 96, 227201 (2006).
8
C. He, S. El-Khatib, J. Wu, J. W. Lynn, H. Zheng, J. F. Michell, and C.
Leighton, Eur. Lett. 87, 27006 (2009).
9
D. Phelan, D. Louca, K. Kamazawa, S.-H. Lee, S. N. Ancona, S.
Rosenkranz, Y. Motome, M. F. Hundley, J. F. Michell, and Y. Moritomo,Phys. Rev. Lett. 97, 235501 (2006).
10
H. M. Aarbogh, J. Wu, L. Wang, H. Zheng, J. F. Michell, and C. Leighton,
Phys. Rev. B 74, 134408 (2006).
11
M. Itoh, I. Natori, S. Kubota, and K. Motoya, J. Phys. Soc. Jpn. 63, 1486
(1994).
12
D. N. Nam, K. Jonason, P. Nordblad, N. V. Khiem, and N. X. Phuc, Phys.
Rev. B 59, 4189 (1999).
13
P. L. Kuhns, M. J. R. Hoch, W. G. Moulton, A. P. Reyes, J. Wu, and C.
Leighton, Phys. Rev. Lett. 91, 127202 (2003).
14
K. Knizek, Z. Jirak, J. Hejtmanek, P. Novak, and W. Ku, Phys. Rev. B 79,
014430 (2009).
15
V. Krapek, P. Novak, J. Kunes, D. Novoselov, Dm. M. Korotin, and V. I.
Anisimov, Phys. Rev. B 86, 195104 (2012).
16
Z. Jirak, J. Hejtmanek, K. Knızek, and M. Veverka, Phys. Rev. B 78,
014432 (2008).
17
A. Doi, J. Fujioka, T. Fukuda, S. Tsutsui, D. Okuyama, Y. Taguchi, T.
Arima, A. Q. R. Baron, and Y. Tokura, Phys. Rev. B 90, 081109(R)(2014).
18
S. Tsubouchi, T. Ky^omen, M. Itoh, P. Ganguly, M. Oguni, Y. Shimojo, Y.
Morii, and Y. Ishii, Phys. Rev. B 66, 052418 (2002).
19
S. Tsubouchi, T. Ky^
omen, M. Itoh, and M. Oguni, Phys. Rev. B 69,
144406 (2004).
20
T. Saitoh, Y. Yamashita, N. Todoroki, T. Kyomen, M. Itoh, M.
Higashiguchi, M. Nakatake, and K. Shimada, J. Electron. Spectrosc. Relat.
Phenom. 144–147, 893 (2005).
21
T. Fujita, T. Miyashita, Y. Yasui, Y. Kobayashi, M. Sato, E. Nishibori, M.
Sakata, Y. Shimojo, N. Igawa, Y. Ishii, K. Kakurai, T. Adachi, Y. Ohishi,
and M. Takata, J. Phys. Soc. Jpn. 73, 1987 (2004).
22
T. Fujita, S. Kawabata, M. Sato, N. Kurita, M. Hedo, and Y. Uwatoko,
J. Phys. Soc. Jpn. 74, 2294 (2005).
23
T. Naito, H. Sasaki, and H. Fujishiro, J. Phys. Soc. Jpn. 79, 034710 (2010).
2

J. Appl. Phys. 121, 115104 (2017)24

T. Naito, H. Fujishiro, T. Nishizaki, N. Kobayashi, J. Hejtmanek, K.
Knizek, and Z. Jirak, J. Appl. Phys. 115, 233914 (2014).
25
K. Knizek, J. Hejtmanek, P. Novak, and Z. Jirak, Phys. Rev. B 81, 155113(2010).
26
J. Hejtmanek, E. Santava, K. Knizek, M. Marysko, Z. Jirak, T. Naito, H.
Sasaki, and H. Fujishiro, Phys. Rev. B 82, 165107 (2010).
27
H. Fujishiro, T. Naito, S. Ogawa, N. Yoshida, K. Nitta, J. Hejtmanek, K.
Knizek, and Z. Jirak, J. Phys. Soc. Jpn. 81, 064709 (2012).
28
J. L. Garcia-Munoz, C. Frontera, A. J. Baron-Gonzalez, S. Valencia, J.
Blasco, R. Feyerherm, E. Dudzik, R. Abrudan, and F. Radu, Phys. Rev. B
84, 045104 (2011).
29
J. Herrero-Martın, J. L. Garcıa-Munoz, S. Valencia, C. Frontera, J. Blasco,
A. J. Baron-Gonzalez, G. Subias, R. Abrudan, F. Radu, E. Dudzik, and R.
Feyerherm, Phys. Rev. B 84, 115131 (2011).
30
H. Fujishiro, T. Naito, D. Takeda, N. Yoshida, T. Watanabe, K. Nitta, J.
Hejtmanek, K. Knizek, and Z. Jirak, Phys. Rev. B 87, 155153 (2013).
31
D. Phelan, K. P. Bhatti, M. Taylor, S. Wang, and C. Leighton, Phys. Rev. B
89, 184427 (2014).
32
M. Kriener, C. Zobel, A. Reichl, J. Baier, M. Cwik, K. Berggold, H.
Kierspel, O. Zabara, A. Freimuth, and T. Lorenz, Phys. Rev. B 69, 094417(2004).
33
V. V. Mehta, N. Biskup, C. Jenkins, E. Arenholz, M. Varela, and Y.
Suzuki, Phys. Rev. B 91, 144418 (2015).
34
Y. Noda, H. Fujishiro, T. Naito, A. Ito, T. Goto, J. Hejtmanek, and Z.
Jirak, AIP Adv. 6, 025318 (2016).
35
A. Gulec, D. Phelan, C. Leighton, and R. F. Klie, ACS Nano 10, 938(2016).
36
Y. Wang, Y. Sui, P. Ren, L. Wang, X. Wang, W. Su, and H. J. Fan, Inorg.
Chem. 49, 3216 (2010).
37
H. Yamaoka, H. Oohashi, I. Jarrige, T. Terashima, Y. Zou, H. Mizota, S.
Sakakura, T. Tochio, Y. Ito, E. Ya. Sherman, and A. Kotani, Phys. Rev. B
77, 045135 (2008).
38
A. Bianconi, A. Kotani, K. Okada, R. Giorgi, A. Gargano, A. Marcelli,and T. Miyahara, Phys. Rev. B 35, 806 (1987).
39
Z. Hu, S. Bertram, and G. Kaindl, Phys. Rev. B 49, 39 (1994).
40
B. Ravel and M. Newville, J. Synchrotron Radiat. 12, 537 (2005).
41
J. Hejtmanek, Z. Jirak, O. Kaman, K. Knizek, E. Santava, K. Nitta, T.
Naito, and H. Fujishiro, Eur. Phys. J. B 86, 305 (2013).
42
Z. Jirak, J. Hejtmanek, K. Knizek, M. Marysko, P. Novak, E. Santava, T.
Naito, and H. Fujishiro, J. Phys. Condens. Matter 25, 216006 (2013).
43
E. Talik, A. Pajaczkowska, A. Guzik, P. Zajdel, J. Kusz, A. Klos, and A.
Szysiak, Mater. Sci. Eng. B 182, 74 (2014).
44
K. Knizek, J. Hejtmanek, M. Marysko, P. Novak, E. Santava, Z. Jirak, T.
Naito, and H. Fujishiro, Phys. Rev. B 88, 224412 (2013).
45
M. Marysko, Z. Jirak, K. Knizek, P. Novak, J. Hejtmanek, T. Naito, H.
Sasaki, and H. Fujishiro, J. Appl. Phys. 109, 07E127 (2011).
46
V. Hardy, F. Guillou, and Y. Breard, J. Phys.: Condens. Matter 25, 246003(2013).
47
T. Naito, H. Sasaki, H. Fujishiro, and M. Yoshizawa, J. Phys. Conf. Ser.
200, 012137 (2010).
48
D. Fuchs, E. Arac, C. Pinta, S. Schuppler, R. Schneider, and H. V.
L€ohneysen, Phys. Rev. B 77, 014434 (2008).
49
A. H. Cottrell, in The Mechanical Properties of Matter (John Wiley andSons, Inc., New York-London-Sydney, 1964), pp. 115 and 136.
50
T. Vogt, J. A. Hriljac, N. C. Hyatt, and P. Woodward, Phys. Rev. B 67,
140401(R) (2003).
51
N. Orlovskaya, K. Kleveland, T. Grande, and M. A. Einarsrud, J. Eur.
Ceram. Soc. 20, 51 (2000).