Suppression of the metal-insulator transition by magnetic field in (Pr1 y Y y)0.7Ca0.3CoO3 (y=0.0625)
Tomoyuki Naito, Hiroyuki Fujishiro, Terukazu Nishizaki, Norio Kobayashi, Jií Hejtmánek, Karel Knížek, andZdenk Jirák
Citation: Journal of Applied Physics 115, 233914 (2014); doi: 10.1063/1.4884435
View online: http://dx.doi.org/10.1063/1.4884435
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JOURNAL OF APPLIED PHYSICS 115, 233914 (2014)

Suppression of the metal-insulator transition by magnetic fieldin (Pr12yYy)0.7Ca0.3CoO3 (y 5 0.0625)
Tomoyuki Naito,1,a) Hiroyuki Fujishiro,1 Terukazu Nishizaki,2 Norio Kobayashi,2nek,3 Karel Knızek,3 and Zdene
k Jira k3
Jirı Hejtma
1

Faculty of Engineering, Iwate University, Morioka 020-8551, Japan
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
3
Institute of Physics, ASCR, Cukrovarnick
a 10, 162 00 Prague 6, Czech Republic
2(Received 20 February 2014; accepted 2 June 2014; published online 19 June 2014)
The (Pr1yYy)0.7Ca0.3CoO3 compound (y ¼ 0.0625, TMISS ¼ 40 K), at the lower limit for
occurrence of the first-order metal-insulator (MI) and simultaneous spin-state (SS) transitions, hasbeen studied using electrical resistivity and magnetization measurements in magnetic fields up to
17 T. The isothermal experiments demonstrate that the low-temperature insulating phase can bedestabilized by an applied field and the metallic phase returns well below the transition temperature TMISS . The reverse process with decreasing field occurs with a significant hysteresis. The
temperature scans taken at fixed magnetic fields reveal a parabolic-like decrease in TMISS withincreasing field strength and a complete suppression of the MI-SS transition in fields above 9 T.
C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4884435]
V
I. INTRODUCTION

The Co3þ ions in the LaCoO3 cobaltite and its rare-earth
analogs have been extensively studied for the past five decades as controversy still remains over whether their spin-state(SS) transition occurs between the low-spin (LS) state (t62g e0g ,S ¼ 0) and the intermediate-spin (IS) state (t52g e1g , S ¼ 1) or
high-spin (HS) state (t42g e2g , S ¼ 2).1,2 Still lesser understanding concerns the nature of the ferromagnetic metallic phasein mixed-valence cobaltites of the La1xSrxCoO3 type. It isargued that localized models cannot describe appropriately
all the cobaltite complexity since the Co eg states interact
with O 2p states and are spread over a large energy range.3
More recent dynamic mean field theory (DMFT) calculations
thus suggest for La1xSrxCoO3 a complex global state thatcan be locally understood as a dynamic fluctuation over different cobalt valences and spin states,4 but with certain simplification, the metallic nature of these phases and theirferromagnetic ground state can be related to an electrontransfer between neighbors of the IS-Co3þ/LS-Co4þ or
HS-Co3þ/HS-Co4þ kinds, eventually also LS-Co3þ/LS-Co4þ.5
To account for the magnetic properties actually observed,
in particular, the low-temperature spontaneous moments andeffective moments at intermediate temperatures, one may conclude on a domination of the IS-Co3þ/LS-Co4þ pairs in
La1xSrxCoO3.6 The situation is changed in systems in which
the large La and Sr cations are substituted by smaller rareearth and calcium cations. Still within the simplified model of
localized states, these systems show on cooling a gradualspin-state crossover toward a LS-Co3þ/LS-Co4þ mixture.7 A
distinct behavior takes place in some Pr-based mixed-valencecobaltites, in which the crossover to lower spin states acquires
a form of first-order transition of metal-insulator (MI) type.
This transition was observed for the first time in oxygen stoichiometric Pr0.5Ca0.5CoO3 and was reported as having aa)Electronic mail: tnaito@iwate-u.ac.jp

0021-8979/2014/115(23)/233914/5/$30.00

pronounced peak in heat capacity and sharp changes in magnetic susceptibility, electrical resistivity, thermopower, andunit cell volume at TMI–SS  90 K.8,9 The same phenomenon
was evidenced later for analogous systems with lower calcium
content, provided that they were either subjected to high pressures (physical pressure)10 or the praseodymium sites weresuitably doped by yttrium or smaller rare-earth cations like
Sm, Gd, and Tb (chemical pressure).10,11 Some empiricalrules for the appearance of the MI transition were derived
with regard to the values of cationic radii at the perovskite
A-sites and their variance (size mismatch).11
It was found that the key to this specific MI-SS transition was a significant shift from mixed-valence Co3þ/Co4þ
ions to pure Co3þ ion, enabled by the simultaneous change
in valence of some Pr3þ ions into Pr4þ ions. For the prototypical compound Pr0.5Ca0.5CoO3 this valence shift was confirmed and determined quantitatively with the identificationof the Pr4þ-related Schottky peak in heat capacity measurements12 and by X-ray absorption spectroscopy at the Pr L3edge.13 Similar evidence was presented also for thePr0.7Ca0.3CoO3 systems doped by Y,14–16 Sm,17 and Tb.18
Many studies have been done especially on the (Pr1yYy)0.7
Ca0.3CoO3 system, which displays a MI-SS transition andpartial Pr3þ ! Pr4þ valence shift at critical temperatures
increasing from TMI–SS ¼ 40 K for y ¼ 0.0625 up to
TMI–SS ¼ 132 K for the yttrium solubility limit y ¼ 0.15. No
valence shift and, therefore, no MI-SS transition have been
observed for y ¼ 0 and 0.05 under ambient pressure.
It is natural to suspect that the external magnetic fieldstabilizes the higher spin states and should thus influence thetransition temperature. Magnetic control of the SS transition
in a spin-crossover complex was attempted for several ferrous compounds. Fe(phen)2(NCS)2 (phen ¼ ortho-phenantroline) showed the spin-crossover between LS and HS statesof ferrous ions at 176 K. The transition temperature
decreased by only 0.11 6 0.04 K as the magnetic field wasincreased from 1.0 to 5.5 T, demonstrating that the variation

115, 233914-1

C 2014 AIP Publishing LLC
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J. Appl. Phys. 115, 233914 (2014)is slight.19 A small decrease in the spin-crossover temperature was found also in [Fe(2-pic)3]Cl2  EtOH (2-pic ¼ 2amino-methyl-pyridine), in which the spin-crossover temperature between LS- and HS-Fe2þ states was suppressed only
by approximately 0.2 K at 7 T from 113 K.20 Similar experiments performed so far on the present (Pr1yYy)0.7
Ca0.3CoO3 (y ¼ 0.075  0.15) system showed a comparable
variation with the magnetic field.21 In particular, TSS ¼ TMI¼ 64:0 K, determined for the y ¼ 0.075 sample from susceptibility measurements at 0.1 T, decreased by 0.35 K in a magnetic field of 7 T.
This paper focuses on the y ¼ 0.0625 compound thatexhibits pronounced thermal hysteresis during the transition,
related most likely to its low critical temperature. The magnetic
and electrical properties for high external fields up to 17 T are
presented. In contrast to yttrium-richer members of the(Pr1yYy)0.7Ca0.3CoO3 series, a significant shift in the criticaltemperature with applied field and a complete suppression of
the MI-SS transition in fields of 9 T and higher, is observed.
II. EXPERIMENTAL PROCEDURE

The (Pr1yYy)0.7Ca0.3CoO3 (y ¼ 0.0625) compound was
prepared by a solid-state reaction method. Raw powders of
Pr6O11, Y2O3, Co3O4, and CaCO3 were weighed with proper
molar ratios and ground. Mixed powders were calcined at1000  C for 24 h in air. The calcined material was pulverized, ground, and pressed into a pellet 20 mm in diameterand 4 mm in thickness. The pellet was sintered at 1200  C
for 24 h under 1 bar of flowing oxygen gas. The powder
X-ray diffraction pattern was taken at room temperature, inwhich a single-perovskite phase of orthorhombic symmetry
with Pbnm space group was confirmed. The measured density of the sample was above 90% from ideal. The rectangular shaped sample was prepared by cutting an as-sinteredpellet. The electrical resistivity, qðTÞ, was measured in
the temperature range 4–80 K by a dc four-probe method witha typical current density of 0.01 A/cm2 under magnetic fields
up to 17 T using the 20-T superconducting magnet (20 T-SM)
at the HFL-SM, Tohoku University. The magnetic properties
were investigated in the temperature range 2–300 K using DC
superconducting quantum interference device (SQUID)
magnetometer (MPMS XL, Quantum Design (QD)). The DC
extraction magnetization up to 12 T was measured using an
AC/DC magnetometer of the physical property measuring system (PPMS with the ACMS option, QD).
III. RESULTS AND DISCUSSION

The temperature dependence of the electrical resistivity,qðTÞ, for the (Pr0.9375Y0.0625)0.7Ca0.3CoO3 sample in various
magnetic fields up to 17 T is presented in Figure 1. The absolute value of qðTÞ is shown for zero magnetic field; the othercurves are shifted appropriately for ease of viewing. To illustrate the thermal hysteresis, several modes of measurements
were used. For zero magnetic field, qðTÞ was measured in a
run cycle commencing with cooling and subsequent heating.
The results show that qðTÞ moderately increases withdecreasing temperature, undergoes an abrupt jump of about
one order of magnitude at TMI-SS ¼ 40 K, and increases

FIG. 1. Temperature dependence of the electrical resistivity, qðTÞ, of(Pr0.9375Y0.0625)0.7Ca0.3CoO3 sample in various magnetic fields up tol0 H ¼ 17 T. Measurements in each successive field are offset by an order ofmagnitude for clarity.

further as temperature decreases. In the subsequent heating,a similar sharp drop in qðTÞ occurs but at the higher temperature of 41 K, a characteristic of hysteretic behavior for a
first-order transition. We classify the qðTÞ behavior above
the transition temperature TMI-SS as metallic, as reported inprevious papers.8,11
In the presence of a magnetic field, qðTÞ was measured
for three different modes using both zero-field-cooled (ZFC)
and field-cooled (FC) regimes as follows. First, after the
sample was cooled to about 4.2 K in zero magnetic field,qðTÞ was measured under an applied magnetic field during
heating (ZFC). Thereafter, qðTÞ was again measured while
cooling (FCC) and subsequent heating (FCH) runs in thesame applied magnetic field. In the ZFC mode, the MI-SS
transition temperature TMI-SS (ZFC) decreases gradually withincreasing magnetic field, l0 H, and the MI-SS transition survives up to 17 T. In contrast, in the FCC mode, the MI-SS
transition occurs at a lower temperature TMI-SS (FCC), which
decreases more rapidly with increasing applied field, thus
enhancing the hysteresis. The transition abruptly disappears
at about 9 T. The subsequent FCH mode, with magnetic
fields up to 7 T, the TMI-SS (FCH) is nearly the same as
TMI-SS (ZFC). However, for magnetic fields above 9 T, theqðTÞ behavior during FCH retraces that of the FCC run. The
magnetic field dependence of TMI-SS is in clear contrast
with the isostatic pressure dependence of TMI-SS in the
Pr1xCaxCoO3 samples,10 where TMI-SS increases withincreasing pressure; sufficiently high pressures may induce atransition in the sample that is absent under ambientpressure.

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Figure 2 presents the results of the magnetic field scansof electrical resistivity, qðHÞ, for (Pr0.9375Y0.0625)0.7Ca0.3CoO3
at several temperatures. In the ZFC mode, as shown in
Fig. 2(a), the qðHÞ at 4.2 K monotonically decreases(increases) with increasing (decreasing) applied magnetic field
and only little irreversibility is observed. A similar behavior isfound at 10 K but with a more pronounced irreversibility. At
20 K, the qðHÞ shows a sudden drop at about 14.5 T in theascending run, and abruptly increases at about 5.3 T in the descending run with a wide hysteresis of about 9 T. Similar hysteretic behaviors are observed at 30 K and 38 K. However,
the width of the hysteresis and the transition temperature
TMI-SS ðHÞ decrease with increasing temperature. At 41.7 K,
no hysteresis can be found again, because this temperature isjust above the TMI–SS at l0 H ¼ 0 T and the system is in a metallic state. In the FC mode, i. e., after cooling the sample at
17 T, the qðHÞ curves shown in Fig. 2(b), closely resemble
those in the ZFC mode in Fig. 2(a), except that the transition
magnetic field is slightly lower. The absolute value of qðHÞ at
4.2 K is about one order of magnitude smaller than that of theZFC mode, which confirms the metallic state and corroborates
the idea that the MI-SS transition is suppressed at low temperatures above 9 T, as shown in Fig. 1.
Figure 3 depicts the temperature dependence of themolar magnetization, M(T), for (Pr0.9375Y0.0625)0.7Ca0.3CoO3
in various magnetic fields, both in the ZFC and FCC modes.
M(T) at l0 H ¼ 0:1 T decreases with increasing temperature,
jumps up at 41 K, and subsequently decreases with further
increase in temperature. In the FCC mode, M(T) nearly

FIG. 2. Magnetic-field dependence of the electrical resistivity, qðHÞ, for(Pr0.9375Y0.0625)0.7Ca0.3CoO3 at various temperatures—during (a) the ZFC
sample and (b) sample after FC at 17 T.

J. Appl. Phys. 115, 233914 (2014)

FIG. 3. Temperature dependence of the molar magnetization, M(T), of(Pr0.9375Y0.0625)0.7Ca0.3CoO3 in various magnetic fields. The ZFC/FCC
bifurcation, seen for the lowest field at 12 K, points to an existence of minor(yttrium poorer) regions that do not undergo the MI transition and develop
ferromagnetic clusters at a lower temperature.

retraces the curve of the ZFC mode, but the transition during
the cooling run occurs at a lower temperature of 39 K, thus
producing hysteresis. The behavior in the M(T) curves atmagnetic fields below l0 H ¼ 5 T is quite similar to that atl0 H ¼ 0:1 T, but the transition temperature decreases withincreasing magnetic field. However, the transition in the
FCC mode broadens at 7 T, and widens out at magnetic fields
above 9 T. The magnetic response is consistent with the qðTÞcurves shown in Fig. 1.
Similar to the previously studied compounds with higher
yttrium content (y ¼ 0:075  0:15, TMI–SS ¼ 64  132 K),15
the transition in y ¼ 0.0625 is associated with a crossover ofcobalt ions to a low-spin ground state that is promoted by asignificant shift in the mixed-valence Co3þ/Co4þ toward thepure Co3þ. This process is compensated by the valencechange of some Pr3þ ions to Pr4þ ions;13,15 we determined thefinal amount to be 0.11 Pr4þ per f. u. based on the Schottky
peak in heat capacity under ZFC (experiment to be publishedelsewhere). Surprisingly, we detected a certain reduced
amount of Pr4þ even after a cooling run in a 9-T field, when
the MI-SS transition was suppressed and the y ¼ 0.0625 sample remained in the metallic state. We note that the jumps inelectrical resistivity and magnetization actually observed do
not represent the whole crossover as the phase evolution is notconfined solely to temperatures near TMI–SS ; see, e. g., thesmooth temperature dependence of the x-ray absorption nearedge structure (XANES) data at the Pr- L3 edge for both thePr0.5Ca0.5CoO3 and (Pr1yYy)0.7Ca0.3CoO3.12,15 To explain
this behaviour, we may refer to a model of the LS/HS crossover in a mean field approximation, presented in Ref. 22. It isshown that the crossover generally proceeds in a gradual way
as local excitations, but in the presence of strong intercorrelations it may acquire the characteristics of a phase[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:
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J. Appl. Phys. 115, 233914 (2014)transition as found for the present cobaltite. In these model
simulations, three stages have been identified. First, HS states
are locally excited and their equilibrium population slowly
increases with increasing temperature. Then, the HS concentration undergoes a sudden jump. After this, there is again asmooth increase up to a high-temperature saturation of the
HS/LS ratio.
The transition temperature TMI–SS as a function of themagnetic field for the (Pr0.09375Y0.0625)0.7Ca0.3CoO3 sample
is shown in Figure 4, which was plotted based on the resistivity and magnetization data in Figs. 1 and 3. The TMI–SS
values determined from M(T) almost coincide with thosedetermined from qðTÞ, which suggests that the MI and SS
transitions remain coupled for all applied fields, and has thus
a common microscopic origin. This is obviously resulting
from the stabilization of the IS-Co3þ state by the magnetic
field because of its larger spin, which affects the thermally
driven transition from the low-temperature LS-Co3þ dominant phase to the metallic phase of mixed IS-Co3þ/ LS-Co4þ
character. The formula for the shift in the transition temperature with magnetic field can be deduced from general thermodynamic relations (see, e. g., Refs. 21 and 22),
TMI–SS ðl0 HÞ ¼ TMI–SS ð0Þ  Aðl0 HÞ2 ;(1)

where TMI–SS ð0Þ is the transition temperature at zero fieldand A ¼ Dv/ð2DSÞ is a parameter that is physically related
to the change in magnetic susceptibility Dv and the change
in entropy DS during the first-order transition. Using thisequation and A as a fitting parameter, the experimentallyobtained results are well reproduced. The fits for the ZFC
and FCC runs are shown by the broken lines in Fig. 4, corresponding to A ¼ 0.09 K T2 and 0.2 K T2, respectively.
The results suggest that the MI-SS transition in the ZFC
mode disappears approximately at 21 T, i.e., not far above
our experimental range. However, the MI-SS transition in
the FCC mode is observed to be abruptly suppressed in a 9T magnetic field, although the MI-SS transition line

FIG. 4. Phase diagram of the (Pr0.9375Y0.0625)0.7Ca0.3CoO3 sample in the (H, T)
plane. Dashed lines represent fitted curves for the transition temperature,
TMISS ðl0 HÞ, using Eq. (1).

extrapolates to zero at 14 T. Based on the above mentioned
value A ¼ 0.09 K T2 and the susceptibility change deduced
for the y ¼ 0.0625 sample from Fig. 3, Dv ¼ 0:02 emu
mol1 Oe1, we estimate the entropy change during the thermally induced transition to be DS ¼ 1:1 J mol1 K1. Using
the same approach, DS ¼ 3:2 J mol1 K1 was obtained previously for a sample with higher yttrium content,
y ¼ 0.075.21 These DS values are significantly larger than
the total entropy change obtained from heat capacity
measurements, 0.28 J mol1 K1 and 2.2 J mol1 K1 fory ¼ 0.0625 and 0.075, respectively. This discrepancy suggests that the magnetic part of the entropy change will be
partially compensated by other terms.
In summary, the (Pr1yYy)0.7Ca0.3CoO3 compound(y ¼ 0.0625, TMI–SS ¼ 40 K), situated at the lower limit forthe occurrence of the first-order MI and SS transitions, hasbeen studied using electrical resistivity and magnetization
measurements in magnetic fields up to 17 T. The experimental data suggest that the energy difference between the LS
and IS states of Co3þ and Co4þ ions is fairly small andbecomes comparable with the energy scale of commonly accessible magnetic fields. Thus, the MI-SS transition can be
driven both thermally and magnetically, resulting in a complex phase diagram in the (H, T) plane with important hysteretic behaviors. The detailed mechanism for the MI-SS
transition in the system, as well as the role of Pr3þ ! Pr4þvalence shift still needs to be clarified.
ACKNOWLEDGMENTS

This work was partly supported by JSPS KAKENHI
Grant No. 24540355. The experiments in Institute of Physics
Prague were supported by Project No. 13-03708S of theGrant Agency of the Czech Republic.
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