Electrical resistivity anomaly in (Pr1−yMy)1−xCaxCoO3 epitaxial films (M=Y, Gd)fabricated by pulsed laser deposition
Y. Noda, H. Fujishiro, T. Naito, A. Ito, T. Goto, J. Hejtmanek, and Z. Jirak
Citation: AIP Advances 6, 025318 (2016); doi: 10.1063/1.4942558
View online: http://dx.doi.org/10.1063/1.4942558
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AIP ADVANCES 6, 025318 (2016)

Electrical resistivity anomaly in (Pr1−yMy)1−xCaxCoO3
epitaxial films (M=Y, Gd) fabricated by pulsed
laser deposition
Y. Noda,1 H. Fujishiro,1,a T. Naito,1 A. Ito,2 T. Goto,2 J. Hejtmanek,3and Z. Jirak3
1

Faculty of 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
Institute of Physics, ASCR, Cukrovarnicka 10, 162 00 Prague 6, Czech Republic
2(Received 27 November 2015; accepted 9 February 2016; published online 18 February 2016)(Pr1−yMy)1−xCaxCoO3 epitaxial films (M=Y, Gd) have been successfully fabricatedby pulsed laser deposition on the single crystal substrates with different lattice
constant. The polycrystalline bulk of this material shows a first-order metal-insulator(MI) transition below the critical temperature. Although ρ(T) of all the as-grownfilms shows a semiconducting behavior at entire temperature range, an anomalous ρ(T) upturn with wide hysteresis can be clearly seen for the film grown on
the SrLaAlO4 (SLAO) substrate, which applied the in-plane compressive stress tothe film. Such anomaly in ρ(T) is interpreted as a sign of the first-order phasetransition related with the spin-state (SS) transition, which was observed in thepolycrystalline bulk. C 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/). [http://dx.doi.org/10.1063/1.4942558]

I. INTRODUCTION

The perovskite cobaltites RECoO3 (RE=rare-earth element and Y) show a spin-state (SS) transition of Co3+ ions from a low spin state (LS; t 2g6eg0, S=0) to a high spin state (HS; t 2g4eg2, S=2)
with increasing temperature, followed by the formation of the metallic state of the intermediate
spin state (IS; t 2g5σ∗, S=1) at higher temperatures.1 The temperature induced SS transition indicates a small energy difference δE between the crystal-field splitting and Hund coupling energy.2,3The hole-doped systems such as La1−xSrxCoO3 generally show a temperature stable phase withitinerant cobalt states, which result presumably from the mixed IS Co3+/LS Co4+ or HS Co3+ /IS
Co4+ configurations, and undergo a ferromagnetic ordering at low temperatures. Most interestingly,
some Pr-based cobaltites exhibit a pronounced first-order transition to a low-temperature phaseof weakly paramagnetic character and reduced conduction. This effect was revealed for the firsttime on Pr0.5Ca0.5CoO3 at ∼ 90 K by the step-like resistivity jump and concomitant anomalies in
the magnetic susceptibility, heat capacity, lattice dilatation and photoemission spectroscopy.4–6 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 Co3+ and LS Co4+ (t 2g5eg0, S=1/2) states. Similar
metal-insulator (MI) transition was evidenced also in the less doped Pr1−xCaxCoO3 (x=0.3) underhigher hydrostatic pressures and in the (Pr1−xREy)1−xCaxCoO3 system (0.2<x<0.5) with a partial
substitution of Pr by smaller RE cations such as Sm, Eu, and Y under ambient pressure.7–9 This
peculiar transition appeared to be conditioned not only by the presence of both Pr and Ca ions, butalso by a larger structural distortion of the CoO6 network, depending on the average ionic radiusand size mismatch of perovskite A-site ions.9 Furthermore, the critical temperature TMI was founda corresponding author fujishiro@iwate-u.ac.jp

2158-3226/2016/6(2)/025318/9

6, 025318-1© Author(s) 2016.

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to decrease in a quadratic dependence of applied magnetic field, so that the transition could becompletely suppressed in very high fields.10
An alternative scenario explaining the nature of such specific transition was proposed on thebasis of electronic structure calculations exploiting the temperature dependence of the structuralexperimental data for Pr0.5Ca0.5CoO3,11 in which the formal cobalt valence should change below
TMI from mixed-valence Co3.5+ towards pure Co3.0+ with strong preference for the LS state and,concomitantly, the praseodymium valence should increase simultaneously from Pr3+ towards Pr4+.
The theoretical hypothesis about the crucial role of variable praseodymium valence was experimentally supported by the observations of a Schottky peak in the low temperature specific heatof (Pr1−yYy)0.7Ca0.3CoO3 ( y=0.075 and 0.15),12 and by the x-ray absorption near-edge structure(XANES) spectra at the Pr L 3 edge directly.13–16
It is widely desirable to use a single crystal or an epitaxial film for the physical investigation. In
the La1−xSrxCoO3 system, single crystals and thin films have been fabricated and investigated.17–19
In the epitaxial thin film, the electrical conduction and the spin state change depending on thefilm thickness and fabrication condition.19 However, for the (Pr1−yMy)1−xCaxCoO3 system withthe simultaneous MI and SS transition, only polycrystalline bulk materials have been investigatedexperimentally in the previous studies and there is no investigations using a single crystal or an
epitaxial film because of the difficulty of the fabrication.
When the material is prepared in the thin film form, the physical properties are sensitive to crystal structure, metallic composition, oxygen content etc, mainly depending on the lattice mismatch
between the substrate and the material. Thin films experience a stress due to mismatch of thelattice and the thermal expansion to the substrate, in addition to the intrinsic internal stress. In
particular, the interface between the film and the substrate can play an important role, giving rise
to boundary regions with different structures and affecting the transport and magnetic properties
due to the induced strain especially in very thin films. The strain in the epitaxial thin films canbe easily manipulated by the film thickness and the lattice parameter of the substrate material. For
example, in the RNiO3 thin film, which shows the bandwidth controlled MI transition, the transition
temperature, TMI, was increased (decreased) by tensile (compressive) in-plane strain.20 The strain
effect is attributed to an increase (decrease) of the Ni-O-Ni bond angle, which affects to films inthe same way as the bulk under higher pressure.21 Thin films of these materials have potential forvarious applications such as the use in resistive switches and phase-change memory.
In the present work, we fabricated the (Pr1−yMy)1−xCaxCoO3 epitaxial films (M=Y, Gd) bypulsed laser deposition (PLD) on the single crystal substrates with different lattice constant. We
investigated the electrical resistivity anomaly, which seemed to be related with the spin-state transition shown in the bulk materials.
II. EXPERIMENTAL PROCEDURE(Pr1−yYy)1−xCaxCoO3 (PYCCO) and (Pr1−yGdy)1−xCaxCoO3 (PGCCO) thin films were preparedby the PLD method using a fifth harmonic wave of YAG laser with λ=213 nm and a repetition
rate of 10 Hz. The pulse duration is 5 ns and the power is 18 mJ. (Pr0.8Y0.2)0.6Ca0.4CoO3 and(Pr0.7Gd0.3)0.6Ca0.4CoO3 polycrystalline targets with nominal composition were prepared by a standard solid-state reaction. Pr6O11, Y2O3, Gd2O3, CaCO3, Co3O4 raw powders with purity higher
than 99.9% were used as starting materials. The powders were mixed, calcined at 1000◦C for
12 h, pressed into pellets with a diameter of 20 mm and a thickness of 5 mm and then sintered at
1200◦C for 48 h in flowing oxygen gas. Powder X-ray diffraction analysis revealed that the sinteredtargets were a single phase. The target and the substrate were mounted in the vacuum chamber andwere rotated at the speed of 7 rpm and 3 rpm, respectively, to reduce non-uniform erosion and todeposit uniformly. The deposition was carried out for 1 - 4 hours in a pure oxygen atmosphereof 0.1 to 1.0 Torr and at a substrate temperature of 600◦C and the film thickness was measured
to be about 50∼100 nm. The films were grown on (001) oriented SrTiO3 (STO: a=0.3905 nm),
(LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT: a=0.3868 nm), LaAlO3 (LAO: a=0.3790 nm) and SrLaAlO4(SLAO: a=0.3756 nm, c=1.2636 nm) single crystal substrates. The pseudo cubic lattice parameterof the (Pr1−yYy)1−xCaxCoO3 bulk (x=0.4, y=0.2) is a=0.3775 nm and the lattice mismatch using

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FIG. 1. The relation between the lattice parameters of the (Pr0.8Y0.2)0.6Ca0.4CoO3 and those of the used single crystal
substrates.

the relation of ε=(aPYCCO-asub)/asub is +3.4% for STO, +2.5% for LSAT, +0.4% for LAO and -0.5%
for SLAO. The relation of the lattice parameters between the material and the substrates is shown in
Fig. 1, in which the in-plane tensile stress is applied to the film grown on the STO, LSAT and LAO
substrates and the in-plane compressive stress is applied in the film grown on the SLAO substrate. The
chemical composition of the grown films was measured by electron probe microanalysis (EPMA), in
which the measured composition of (x, y) of the film was decreased to (0.3, 0.13) from the nominal
composition of (0.4, 0.2) for the PYCCO sample. To clarify the crystalline quality of the grownfilms, X-ray diffraction analyses of θ-2θ and φ-2θ scans were performed, both of which identified theout-of-plane and in-plane alignments, respectively. The electrical resistivity ρ(T) was measured oncooling and heating runs in the 10 - 300 K range by a standard four-probe method for the as-grownfilms and heat-treated films under high-pressure oxygen gas (2.0 MPa) at 600◦C for 3 h.

III. RESULTS AND DISCUSSION

Figure 2 shows the out-of-plane XRD patterns (θ-2θ scan) of the PYCCO films fabricatedon the STO and SLAO substrates. The PYCCO films display clear (00l) reflections of the pseudo
cubic structure with no indications of impurities or miss-orientation together with the strong (00l)reflections from the substrates. These results demonstrate the out-of-plane orientation of PYCCO
thin films on the substrates.
Figure 3 presents the φ-2θ azimuth scan for the PYCCO film, fabricated on the SLAO substrate. Four sharp peaks of the (002) reflections were observed every 90 deg. and the in-planealignment can be confirmed, which were also confirmed for the films grown on the other substrates.
These results suggest that the PYCCO films are epitaxial films grown on the substrates.
Figure 4 shows the (002) intensity of the out-of-plane XRD patterns of the PYCCO films
on the STO substrate, as a function of the film thickness, d. The diffraction peak angle of thefilms decreases with increasing film thickness, d, and shifts toward the peak position for the bulk
crystal. These results suggest that the out-of-plane lattice constant of the epitaxial films on the STO
substrate is smaller than that of the bulk crystal and approaches the lattice constant of the bulk
material. This is because the tensile stress was induced in the film surface due to the positive latticemismatch ε (= +3.4%), and then was relaxed with increasing film thickness. On the other hand, forthe films grown on the SLAO substrate, the (002) peak was found at 2θ=47.70◦ for the d=100 nm
film (not shown), which was the lower angle than that of the bulk crystal (2θ=47.21◦). In this case,the in-plane compressive stress is induced in the film surface because of negative lattice mismatchε (= -0.5%).
Figure 5(a) shows the temperature dependence of the electrical resistivity, ρ(T), of the asgrown PYCCO films grown on the each substrate. The thickness of each film was about 50 nm.
ρ(T) of the target bulk is also shown in the figure, in which ρ(T) shows a jump with about twoorders of magnitude at TMI=140 K on the cooling run, and shows a sharp drop at TMI=150 K witha hysteresis. The absolute value of ρ(T) on the heating run was larger than that on the cooling

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FIG. 2. The out-of-plane XRD patterns (θ-2θ scan) of the PYCCO films grown on the (a) STO and (b) SLAO substrates in
the case of the target composition of (x=0.4, y=0.2).

run above TMI. The irreversibility in ρ(T) might come from the occurrence of micro cracks createdby the large volume contraction and expansion passing through the transition temperature.4 On theother hand, the ρ(T) curves for all the as-grown films show a semiconducting behavior without
pronounced anomaly, the absolute ρ(T) values change depending on the substrate used, and noirreversibility is detectable after the cooling and heating runs for the films grown on the STO, LSAT
and LAO substrates. Nonetheless, one may notice a distinct behavior of the film grown on the SLAO
substrate, namely the anomalous inflection point at 170 K and a wide hysteresis in ρ(T) below
this temperature that are seen in more detail Figure 5(b), where an alternative plot of logρvs.T −1/4,
commonly used for variable range hopping (VRH), is displayed. To understand this observation, we
refer to the fact that MI transition can be induced in polycrystalline Pr1−xCaxCoO3 (x=0.3) underhigh hydrostatic pressures and the critical temperature TMI increases with increasing pressure.7 The
compression stress in the present PYCCO film on SLAO substrate seems to have analogical effect toReuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. IP: 160.29.75.151 On: Fri, 19 Feb 2016 03:45:14

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FIG. 3. The in-plane XRD patterns (φ-2θ scan) of the PYCCO film fabricated on the SLAO substrate in the case of the target
composition of (x=0.4, y=0.2).

the hydrostatic pressure. In this sense, the anomalous ρ(T) upturn followed by a hysteresis region is
likely a sign of a similar first-order phase transition.
A question arises why the MI transition in the film grown on the SLAO substrate is lesspronounced compared to the bulk material. This is unexpected, especially in view of the fact that
actual mismatch conditions seem to be much in favor of the transition. Firstly, the out-of-planelattice constant of the film, 0.3820 nm, at room temperature (see Figure 6 below) combined with
the in-plane a=0.3756 nm imposed by SLAO substrate illustrates some deformation of the PYCCO
structure, but the unit cell volume remains exactly same as for bulk material. This is a signature that

FIG. 4. The (002) intensity in the out-of-plane XRD patterns of the PYCCO films, as a function of the thickness of the filmin the case of the target composition of (x=0.4, y=0.2).

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FIG. 5. Temperature dependence of the electrical resistivity, ρ(T ), at the cooling and heating runs: (a) The logρvs.T plot for
the as-grown (AG) PYCCO films on different substrates; (b) An alternative logρvs.T −1/4 plot with linear low-temperature
trend demonstrating the viability of the VRH scenario. The data shown by dashed lines refer to PYCCO films grown on LAO
and SLAO substrates, after a heat-treatment (HT) in high-pressure oxygen gas at 600◦C for 3 h.

film is likely well developed on the substrate without cracks. Secondly, it can be anticipated that theunit cell of PYCCO is shrank at the MI transition to the same extent as it is in the bulk material,
1.0% in volume or 0.3% in linear dimensions.22 Therefore, the deformation energy comprised in thefilm at room temperature will be removed below the transition to the low-temperature phase. There
should be evidently another reason for the less pronounced anomaly at TMI. It is worth mentioningthat early experiments on prototypical compound Pr0.5Ca0.5CoO3 revealed a critical sensitivity of
the MI transition to any oxygen deficiency. To check such hypothesis, the PYCCO films grownon the each substrate were post-annealed under high-pressure oxygen gas (2.0 MPa) at 600◦C for

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FIG. 6. The out-of-plane lattice constant a and the electrical resistivity for the PYCCO films at 300 K, ρ(300 K), of the
as-grown films on each substrate, as a function of lattice mismatch ε.

3 h. The resistivity data on two samples with minimum film/substrate mismatch were added to
the results on as-grown samples in Figures 5(a) and 5(b). For the film grown on the SLAO substrate, the anomalous ρ(T) upturn disappeared in the oxygenized sample and some irreversibility
in the cooling and heating runs arose. These changes suggest that, contrary to our expectation, thefirst-order phase transition vanished by the heat treatment. For the film grown on the LAO substrate,
the absolute ρ(T) value was only slightly increased by the post annealing.
Figure 6 presents the lattice constant a determined by out-of-plane measurements and theelectrical resistivity at 300 K, ρ(300 K), of the as-grown films on each substrate, as a function oflattice mismatch ε. The thickness of each film was about 50 nm. The out-of-plane lattice constanta monotonically decreases with increasing lattice mismatch ε, which suggests that the film was
extended in plane due to the tensile stress for the positive ε value. The ρ(300 K) value takes aminimum around ε=0, which is a reasonable trend. Namely, the deformation of CoO6 octahedra dueto the lattice mismatch affects the electron transport in the film.
Figures 7(a) and 7(b), respectively, show the temperature dependence of the electrical resistivity, ρ(T); the logρvs.T plot and the alternative logρvs.T −1/4 plot at the cooling and heating runs of
the as-grown (Pr1−yGdy)1−xCaxCoO3 (PGCCO) films on the each substrate. ρ(T) of the heat-treated
PGCCO films grown on the LAO and SLAO substrates are also shown. All the films, of which
the thickness was about 100 nm, were confirmed to be epitaxial films by XRD analysis. ρ(T) of
the target bulk is also shown in the figures, in which TMI was 70 K and 80 K on the cooling andheating runs, respectively. ρ(T) of all the as-grown films shows a semiconducting behavior at entire
temperature range. Similarly to the PYCCO film, an anomalous ρ(T) upturn with a wide hysteresis
can be clearly observed only for the as-grown film on the SLAO substrate, which is interpreted asa sign of the first-order phase transition. However, the ρ(T) upturn seems to remain after the heat
treatment as can be seen in Fig. 7(b). For the thinner PYCCO film on SLAO substrate about 50 nm
in thickness shown in Fig. 5, the transition was clearly suppressed by the heat treatment under thehigh-pressure oxygen gas, where the heat treatment removed the stress. On the other hand, in thepresent thicker PGCCO film on SLAO substrate with 100 nm in thickness, the heat treatment madeonly much smaller change. The appearance of the anomalous ρ(T) upturn and the relaxation after
the heat treatment change depending on the film thickness.

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FIG. 7. Temperature dependence of the electrical resistivity, ρ(T ), at the cooling and heating runs: (a) the logρvs.T plotand (b) the alternative logρvs.T −1/4 plot for the as-grown (AG) PGCCO films on different substrates. The data shown bydashed lines refer to the heat-treatment (HT) PGCCO films grown on LAO and SLAO substrates after the heat treatment inhigh-pressure oxygen gas at 600◦C for 3 h.

IV. SUMMARY

The (Pr1−yYy)1−xCaxCoO3 (PYCCO) and (Pr1−yGdy)1−xCaxCoO3 (PGCCO) films have been
fabricated by pulsed laser deposition (PLD) method on the single crystal substrates with different
lattice constants, and their crystallographic and electrical properties have been investigated. The
important experimental results and conclusions are summarized as follows.
(1) The obtained films have been confirmed to be epitaxial films grown on the single crystal
substrates with different lattice constants by XRD measurements.

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AIP Advances 6, 025318 (2016)(2) The electrical resistivity, ρ(T), of all the as-grown films shows a semiconducting behavior atentire temperature range. However, the anomalous ρ(T) upturn with a wide hysteresis can beclearly observed only for the films grown on the SLAO substrate, which applied the in-plane
compressive stress. The ρ(T) anomaly of the thin PYCCO film 50 nm in thickness on SLAO
vanished after the heat treatment in high-pressure oxygen gas. On the other hand, the ρ(T)upturn of the thick PGCCO film 100 nm in thickness on SLAO seems to remain after theheat treatment.
(3) The anomaly in ρ(T) is interpreted as a sign of the first-order phase transition accompaniedby the spin-state transition, which was observed in the polycrystalline bulk.

ACKNOWLEDGEMENTS

The authors thank Prof. N. Yoshimoto of Iwate University for his assistance in the in-plane
X-ray diffraction measurement. This work was partly supported by JSPS KAKENHI Grant No.
24540355 and was performed under the Inter-university Cooperative Research Program of theInstitute for Materials Research, Tohoku University.
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