PHYSICAL REVIEW B 88, 224412 (2013)

Spin-state crossover and low-temperature magnetic state in yttrium-doped Pr0.7 Ca0.3 CoO3
K. Knı́žek, J. Hejtmánek, M. Maryško, P. Novák, E. Šantavá, and Z. JirákInstitute of Physics ASCR, Cukrovarnická 10, 162 00 Prague 6, Czech Republic

T. Naito and H. Fujishiro
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan

Clarina de la Cruz
Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA(Received 17 September 2013; published 11 December 2013)
The structural and magnetic properties of two mixed-valence cobaltites with a formal population of 0.30
Co4+ ions per f.u., (Pr1−y Yy )0.7 Ca0.3 CoO3 (y = 0 and 0.15), have been studied down to very low temperaturesby means of high-resolution neutron diffraction, SQUID magnetometry, and heat-capacity measurements. The
results are interpreted within the scenario of the spin-state crossover from a room-temperature mixture of theintermediate-spin Co3+ and low-spin Co4+ (IS/LS) to the LS/LS mixture in the sample ground states. In contrast
to the yttrium-free y = 0 that retains the metallic-like character and exhibits ferromagnetic (FM) ordering below
55 K, the doped system y = 0.15 undergoes a first-order metal-insulator transition at 132 K, during which not
only the crossover to low-spin states but also a partial electron transfer from Pr3+ 4f to cobalt 3d states takes
place simultaneously. Taking into account the nonmagnetic character of LS Co3+ , such a valence shift electronictransition causes a magnetic dilution, formally to 0.12 LS Co4+ or 0.12 t2g hole spins per f.u., which is the reason
for an insulating, highly nonuniform magnetic ground state without long-range order. Nevertheless, even in thatcase there exists a relatively strong molecular field distributed over all the crystal lattice. It is argued that the
spontaneous FM order in y = 0 and the existence of strong FM correlations in y = 0.15 apparently contradict
the single t2g band character of LS/LS phase. The explanation we suggest relies on a model of the defect-induced,
itinerant hole-mediated magnetism, where the defects are identified with the magnetic high-spin Co3+ species
stabilized near oxygen vacancies.
DOI: 10.1103/PhysRevB.88.224412

PACS number(s): 71.30.+h, 65.40.Ba

I. INTRODUCTION

states,9 but with certain simplification the metallic nature ofthese FM phases can be related to electron transfer between
neighbors of the IS Co3+ /LS Co4+ or HS Co3+ /HS Co4+
kinds, and eventually also LS Co3+ /LS Co4+ .10 At least in
the compositional region just above xc , the La1−x Srx CoO3
systems seem to be dominated by a dynamic mixture of IS
Co3+ and LS Co4+ , which is described for illustration as
the IS/LS phase. This conclusion finds support among othersby the observed values of ordered ferromagnetic moments,
making in particular 1.70μB per f.u. for x = 0.30.
When large cations in La0.7 Sr0.3 CoO3 are substituted byrare-earth or calcium ions of a smaller size, the FM ordering
still exists but the spontaneous moments actually observed
are much suppressed. Although this effect was originallyrelated to a phase separation into FM and non-FM, thefinal state at the lowest temperature is now proved, based
mainly on the uniformity of a molecular field acting on Nd3+
moments in Nd0.7 Ca0.3 CoO3 ,11 to be essentially of single
FM phase. The observed spontaneous moments, approachingin Nd0.7 Ca0.3 CoO3 and Pr0.7 Ca0.3 CoO3 a limiting low valueof 0.30μB per f.u., can thus be ascribed to a stabilizationof the LS/LS phase, i.e., to the alternative ground state of
the hole-doped cobaltites, which consists of a dynamic LS
Co3+ /LS Co4+ mixture.
The present paper deals with Pr0.7 Ca0.3 CoO3 and related
compounds, (Pr1−y Yy )0.7 Ca0.3 CoO3 . These systems, all with
30% doping, exhibit a quasimetallic conductivity at ambientconditions, but in contrast to the formation of a FM state in purePr0.7 Ca0.3 CoO3 , the samples with partial substitution of Pr3+

Perovskite cobaltites display a wide variety of structuraland physical properties depending on the composition andtemperature. Two distinct behaviors can be identified. One
is characteristic for the undoped LaCoO3 and its rare-earth
analogs. Their insulating ground state derives from Co3+ ionsin the diamagnetic low-spin (LS) states. With increasing temperature, two spin-state crossovers take place. First, the paramagnetic high-spin (HS) states are induced by thermal excitation
and are gradually stabilized, which results in a spin-state
disproportionated phase with strong HS/LS nearest-neighborcorrelations or even short-range orderings,1 which can beclassified as a Mott insulator. At elevated temperature thecorrelations melt,2 and a more uniform phase of quasimetallic
character is established. To account for this change and to
explain the paramagnetic properties actually observed, thehigh-temperature phase of LaCoO3 was tentatively describedas consisting of the intermediate spin (IS) of Co3+ species(S = 1, meff = 2.83μB ).3–5 It should be noted, however, thatdynamical mean-field theory (DMFT) calculations suggest for
the high-temperature phase of LaCoO3 a complex global statewith the main weight of LS and HS states with only short visits
to IS configurations.6
Another extreme case are the metallic ground states withbulk ferromagnetic (FM) ordering, known for hole-doped
systems La1−x Srx CoO3 above a critical concentration xc =0.22.7,8 Recent DMFT calculations suggest also for this casea complex distribution of local cobalt valences and spin
1098-0121/2013/88(22)/224412(8)224412-1©2013 American Physical Society

K. KNÍŽEK et al.

PHYSICAL REVIEW B 88, 224412 (2013)by isovalent Y3+ (for y > 0.06) exhibit a first-order transition
to a weakly paramagnetic insulating state. Let us note that this
distinct transition was observed in the 50% doped cobaltite
Pr0.5 Ca0.5 CoO3 upon cooling below a critical point TMI ∼80 K, and the change of electric properties was accompanied byimportant volume, magnetic, and heat-capacity anomalies.12,13
Later on, a similar transition and anomalies were found also
in other praseodymium-based systems in a larger region of
doping. A question has been raised as to why a sharp transitionis encountered solely in praseodymium-containing cobaltites.
As suggested by generalized gradient approximation (GGA)
calculations and some experimental results, including ananalysis of the temperature change of interatomic lengths
and x-ray-absorption near-edge structure (XANES) spectrain Pr0.5 Ca0.5 CoO3 , the reason for the stabilization of theinsulating low-temperature phase is a shift of the mixed
valence Co3+ /Co4+ toward pure Co3+ , enabled by a valencechange of some Pr3+ ions to Pr4+ ones. The decisive factor
is, therefore, the exceptional closeness in energy of the two
praseodymium states. The valence shift upon phase transition
has been further confirmed and determined quantitatively byobservation of the Pr4+ -related Schottky peak in heat-capacity
measurements14 and by x-ray absorption spectroscopy at thePr L3 edge.15,16 In addition to the valence shift, a crossoverfrom the IS or mixed LS/HS Co3+ state to the LS states has
been evidenced by a drop of the paramagnetic susceptibility
and by low values of effective moments below TMI , as well
as by a detailed fit of the Co x-ray absorption and emission
spectra.17
In contrast to the prototypical compound Pr0.5 Ca0.5 CoO3 ,which is difficult to prepare because of problems withoxygen deficiency and phase separation, the less doped
systems, (Pr1−y Yy )0.7 Ca0.3 CoO3 , appeared more suitable forexperimental studies. The presence of the metal-insulator(MI) transition is demonstrated indirectly by peaks in heat
capacity at TMI = 40, 64, 93, and 132 K for y = 0.0625, 0.075,
0.10, and 0.15, respectively (see Fig. 1). The stabilizationof tetravalent praseodymium in the low-temperature phasehas been indicated by the appearance of the low-temperature
Schottky peaks, arising due to Kramers degeneracy of Pr4+
states and Zeeman splitting of a ground doublet by the actionof the molecular and external magnetic fields. The quantitative
analysis determines that the Pr4+ population varies between
140
93 K

Cp (J mol-1K-1)

120

132 K

100
80

64 K

60
40

y=0.15y=0.10y=0.075
y=0.0625

40 K

20
0
0

50

100

150

200

250

300

Temperature (K)FIG. 1. (Color online) Temperature dependence of the specificheat of (Pr1−y Yy )0.7 Ca0.3 CoO3 (y = 0.0625–0.15). The critical temperatures of the MI transitions are marked.

0.11 and 0.18 per f.u., which means that the hole concentration
is decreased upon the transition from the original 30% level to
19% doping in y = 0.0625 and 12% doping in y = 0.15.14,18(No valence change was observed in pure Pr0.7 Ca0.3 CoO3 .)The motivation for the present study is to elucidate thecharacter of the Pr4+ species formed below the MI transition
and to investigate the microscopic origin of the unexpectedly
strong molecular field of about 17 kOe, which acts on Pr4+
pseudospins in the low-temperature insulating phase. The
study includes neutron diffraction and magnetic measurementson two selected (Pr1−y Yy )0.7 Ca0.3 CoO3 samples (y = 0 and0.15) down to 0.25 K.
II. EXPERIMENTS AND CALCULATIONS(Pr1−y Yy )0.7 Ca0.3 CoO3 (y = 0 and 0.15) samples were prepared by a solid-state reaction as described elsewhere.18 The
powder neutron diffraction was performed on the diffractometer Hb2a at Oak Ridge National Laboratory. The scans wererecorded at selected temperatures between room temperatureand 0.25 K. Two crystal monochromators (Ge113 and Ge115)
were used, providing neutron wavelengths λ = 2.408 and1.537 Å, respectively. Data were collected between 8◦ and126◦ of 2θ with steps of 0.08◦ . Structural refinements weredone by Rietveld profile analysis using the program FULLPROF(Version 5.30—Mar2012-ILL JRC).
The magnetic properties were investigated in the temperature range 2–400 K using the Quantum Design dc superconducting quantum interference device (SQUID) MPMS XL
magnetometer. For the dc extraction magnetization measurements up to 140 kOe, the QD PPMS ac/dc magnetometerwith the ac measurement system (ACMS) option was used.
The experiments included the field-cooled (FC) and zerofield-cooled (ZFC) susceptibility scan in the range 2–300 K
using a dc field of 20 Oe, together with the low-temperature
measurements of virgin magnetization curves and complete
hysteresis loops. In addition, the exchange bias experiments
at 2 K were performed on field-cooled samples for bias fields
from 500 Oe to 70 kOe. The thermoremanence was measured
after cooling the sample in high field (typically 70 kOe) down
to 2 K, switching off the field and scanning the magnetization
at zero field on sample warming.
To determine the ground-state properties of(Pr1−y Yy )0.7 Ca0.3 CoO3 cobaltites, the low-temperaturemagnetic contribution of the Pr3+ and Pr4+ ions wastheoretically analyzed. Toward that end, the crystal-field-split
4f n electronic levels were calculated using the “lanthanide”
package.19 This program takes into account the free-ion(atomic) and crystal-field terms, as well as interactions
with the molecular and external magnetic fields. As far
as the spherically symmetrical free-ion Hamiltonian isconcerned, it depends on many parameters, the valuesof which are either known experimentally or may be
calculated; for details, see, e.g., Ref. 20. The crystal-field
Hamiltonian represents a more formidable problem. In
the present case of Cs symmetry of rare-earth sites, thesingle-electron crystal field is described by 15 parameters.
The best choice for Pr3+ in (Pr1−y Yy )0.7 Ca0.3 CoO3 is theset of crystal-field parameters determined very recently for
PrCoO3 using a first-principles method.21 The output of

224412-2

SPIN-STATE CROSSOVER AND LOW-TEMPERATURE . . .

PHYSICAL REVIEW B 88, 224412 (2013)

III. RESULTS
A. Neutron diffraction studyThe
neutrondiffraction
patterntaken
on(Pr1−y Yy )0.7 Ca0.3 CoO3 (y = 0 and 0.15) is exemplified
together with the FULLPROF fit in Figs. 2, 3, and 4. Although
the mixed-valence cobaltites are generally subjected to anoxygen deficiency, the occupation of oxygen sites in thepresent two samples is surely close to the ideal stoichiometry,the refined values being 2.99 ± 0.01 and 3.01 ± 0.01 perf.u., respectively. The values of other structural parameters1500 150

110

002

40

600

37
150

400

103
211
121

38
120

210

100
50

200

60

61

210
120

62

110
002

112
020

111

113

023
221

0
20

30

40

50

60

70

80

2θ (λ = 2.408 Å)
FIG. 3. (Color online) Neutron diffraction of y = 0.15 at low
temperature.

are summarized in Tables I and II, and the unit cell volume
and selected interatomic distances and angles are plotted in
Figs. 5(a) and 5(b). There is little change with temperature
for the y = 0 sample, except for common thermal expansion.
In contrast, the y = 0.15 sample shows a marked volume
compression on cooling below TMI = 132 K, which isaccompanied by an increase in the orthorhombic lattice
distortion. Closer inspection reveals a larger deviation of the
O-Co-O angles from the ideal 180◦ and some drop of the (Pr,Y, Ca)-O bonding distances, while the Co-O distances remain
practically unchanged. All these signatures are manifestationsof the decreased ionic size upon the Pr3+ →Pr4+ valence
shift—see the similar findings for Pr0.5 Ca0.5 CoO3 in Ref. 23.
As far as the magnetic state is concerned, the y = 0
sample shows below TC = 55 K a long-range ferromagnetic(FM) order of cobalt spins that is readily seen in magnetic
measurements (see below) but is barely visible in the neutron diffraction patterns as a very weak increase of somelines, mainly 110 + 002 and 200 + 112 + 020. The value ofthe spontaneous moment determined from the neutron datareaches 0.34 ± 0.10μB at 0.25 K.
The detection of eventual FM ordering in y = 0.15 is still
more difficult since the cobalt subsystem is now magneticallyvery dilute with only 0.12 LS Co4+ ions per f.u. These
moments alone are below the detection limit of the neutron
022
202

220
004

220
004

0
37
200
150
100
50
0

120

Al

38

Intensity

Intensity

220
40 K
004
2K
Al
0.25 K 022 122
202 212

110

1500

50

500

80
60

40 K
022
2K
202
0.25 K

002

100

1000

800

Intensity

the “lanthanide” calculation shows that the 3 H4 multipletof Pr3+ (4f 2 ) splits into nine nonmagnetic singlets and theground-state properties are manifested by anisotropic Van
Vleck susceptibility with components χa = 0.0254 emu
mol−1 Oe−1 , χb = 0.0181 emu mol−1 Oe−1 , and χc = 0.0123
emu mol−1 Oe−1 along the orthorhombic axes of the P bnm
structure. This yields an average value χvV = 0.018 emu
mol−1 Oe−1 for the polycrystal, which is in excellent
agreement with experimental data on a related PrCoO3 system
in Ref. 21, namely with its nearly temperature-independent
susceptibility in the range up to T ∼ 40 K, i.e., before the firstexcited level at 11 meV starts to be populated.
The calculation for Pr4+ (formally the 4f 1 configuration
but with a large 4f 2 L admixture) is subjected to more
uncertainty and, moreover, there are no orthoperovskites with
Pr4+ majority, which would allow an experimental check.
As a first estimate, the values of crystal-field parameters canbe taken from a very detailed optical spectroscopic study of
TbAlO3 ,22 taking into account that this aluminate possesses thesame P bnm orthoperovskite structure and practically identicaloctahedral tilting as the present (Pr1−y Yy )0.7 Ca0.3 CoO3 (y =0.15) sample in the low-temperature phase. The calculationshows that the 2 F5/2 multiplet of Pr4+ splits into three energy
distant Kramers doublets (see the Appendix of Ref. 11).
The magnetic moments associated with the ground doublet(J  = 1/2) are given by the anisotropic g factor, with principal
components gx = 3.757, gy = 0.935, and gz = 0.606. (Here,the local axes x and y are turned with respect to the main axesof the P bnm structure, making for two inequivalent rare-earth
sites a rotation to ±36◦ around the orthorhombic axis c.)

122
212

210

103
211
121
60

61

110
002

210
112 120
020

113

103
211

1000

Al121

110

500 002

170 K

210
120

Al

122
212

113

301
213
023 123 131
130311
221

132 K

023
221

100 K

0

0

20

30

40

50

60

70

80

25

2θ (λ = 2.408 Å)

30

35

40

45

50

55

2θ (λ = 1.537 Å)

FIG. 2. (Color online) Neutron diffraction of y = 0 at low
temperature.

FIG. 4. (Color online) Neutron diffraction of y = 0.15 around thetransition.

224412-3

K. KNÍŽEK et al.

PHYSICAL REVIEW B 88, 224412 (2013)

TABLE I. Structural parameters for (Pr1−y Yy )0.7 Ca0.3 CoO3 with
y = 0 within the P bnm space group. Refinable atom coordinates:
PrCa 4c(x,y,1/4), Co 4b(1/2,0,0), O1 4c(x,y,1/4), O2 8d(x,y,z).
298

a (Å)b (Å)c (Å)
x,Pr
y,Pr
x,O1
y,O1
x,O2
y,O2
z,O2

5.3468(2)5.3359(2)7.5386(3)0.9952(10)0.0349(7)0.0691(7)0.4920(8)−0.2857(8)0.2820(9)0.0355(3)5.3465(2)5.3353(2)7.5383(2)0.9949(8)0.0320(5)0.0681(5)0.4910(5)−0.2837(4)0.2853(3)0.0360(2)5.3463(4)5.3352(4)7.5388(5)0.9953(20)0.0352(16)0.0707(15)0.4924(17)−0.2879(15)0.2819(18)0.0355(6)5.3637(2)5.3516(2)7.5702(4)0.9934(10)0.0277(4)0.0637(22)0.4901(32)−0.2807(25)0.2807(25)0.0319(22)diffraction, but some chance of detection is offered by very
low temperatures where full alignment of Pr4+ moments (theJ  = 1/2 pseudospins) due to the above-mentioned molecularfield of Hm ∼ 17 kOe could be anticipated and might add tothe cobalt FM order. Nonetheless, even at 0.25 K we failed todetect any observable diffraction intensity that could prove theexistence of long-range ordering in the low-temperature phaseof y = 0.15. Let us note that for prototypical Pr0.5 Ca0.5 CoO3
as well, no long-range FM order was found,24 although themolecular field in that system is as high as Hm ∼ 75 kOe.14
We address this issue below and suggest an explanation.
B. Magnetic measurements

The
basic
magnetic
characterizationofthe(Pr1−y Yy )0.7 Ca0.3 CoO3 system was presented in ourprevious paper.18 New data on ZFC/FC susceptibilities havebeen taken for low dc fields. The results are given in theupper panel of Fig. 6, while the lower panel illustrates the
behavior in high magnetic fields. In particular, it shows themagnetization curves for the y = 0 and 0.15 samples, taken
at 2 K in the field range 0–140 kOe and back.
The susceptibility data evidence a FM transition in the y =0 sample at TC = 55 K, determined by the inflection point on
the fast rise of susceptibility curves, while the y = 0.15 sample
is characterized by a drop of susceptibility that accompanies
the MI transition at TMI = 132 K. It should be noted that preformation of FM clusters at 250–270 K, reported in some recent

214
a
b
c/√2
V

5.30
5.28
5.26

212

3

216

5.32

Cell volume (Å )

40

5.34

210

3.10
Pr-O short
Pr-O long

3.05

158

3.00
Co-O1-Co
Co-O2-Co

2.55
2.50

156

Co - O - Co (deg)2Lattice parameters (Å)

0.25

Pr - O (Å)

T (K)

218

154

2.45
0

100

200

300

Temperature (K)FIG. 5. (Color online) Lattice parameters and bond lengths for
y = 0.15.

studies of similar cobaltites,25,26 is not observed for the present
samples. In particular, neither ZFC/FC bifurcation nor finite remanence extending to such high temperatures can be detected.
The magnetization data for y = 0 in the lower panel ofFig. 6 show a significant paraprocess that is superimposedon a hysteresis loop of relatively large coercivity (see Fig. 7)that reaches 9 kOe at 2 K. A similar paraprocess extending tovery high fields has been observed also for related manganites
Pr0.7 Ca0.3 MnO3 .27 We thus ascribe this linear term to the Van
Vleck susceptibility of Pr3+ and, taking experimental dataon the purely LS Co3+ system PrCoO3 in polycrystalline
form as a standard, χvV = 0.0188 emu mol−1 Oe−1 , or equivalently 0.0034μB /kOe,21 we find for the valence composition
2+
3+
4+ 2−
Pr3+
0.7 Ca0.3 Co0.7 Co0.3 O3 of the y = 0 sample a reduction of theparaprocess by a factor 0.78. This is in reasonable agreementwith the actual content of 0.70 Pr3+ .
After subtraction of the Pr3+ -related paraprocess, themagnetization of the cobalt subsystem in y = 0, presented
in Fig. 7, saturates for 140 kOe at a value of 0.58μB /Co.

TABLE II. Structural parameters for (Pr1−y Yy )0.7 Ca0.3 CoO3 with y = 0.15 within the P bnm space group. Refinable atom coordinates:
PrYCa 4c(x,y,1/4), Co 4b(1/2,0,0), O1 4c(x,y,1/4), O2 8d(x,y,z).
T (K)

0.25

2

10

100

132

170

298

a (Å)b (Å)c (Å)
x,Pr
y,Pr
x,O1
y,O1
x,O2
y,O2
z,O2

5.2828(4)5.3285(4)7.4855(6)0.9924(6)0.0451(3)0.0793(4)0.4873(3)−0.2916(3)0.2918(2)0.0400(2)5.2823(5)5.3273(5)7.4846(7)0.9902(9)0.0428(6)0.0742(7)0.4861(6)−0.2932(4)0.2913(4)0.0414(3)5.2824(5)5.3275(5)7.4847(7)0.9898(9)0.0438(6)0.0746(7)0.4864(6)−0.2929(4)0.2906(4)0.0415(3)5.2865(5)5.3296(5)7.4898(8)0.9933(9)0.0439(5)0.0778(6)0.4872(5)−0.2912(4)0.2918(3)0.0402(3)5.3043(6)5.3349(6)7.5053(9)0.9937(11)0.0411(6)0.0755(7)0.4883(6)−0.2884(4)0.2899(4)0.0382(3)5.3242(4)5.3401(4)7.5275(6)0.9949(10)0.0377(5)0.0724(7)0.4900(5)−0.2868(4)0.2874(4)0.0370(3)5.3405(6)5.3513(6)7.5471(8)0.9966(11)0.0324(2)0.0707(10)0.4928(16)−0.2848(14)0.2848(14)0.0354(10)224412-4

PHYSICAL REVIEW B 88, 224412 (2013)
0.3

10

y=0
FC
y=0
ZFC
y=0.15 FC
y=0.15 ZFC

TC

1

0.2

Moment (μB)

Susceptibility (emu·mol -1·Oe-1)

SPIN-STATE CROSSOVER AND LOW-TEMPERATURE . . .

0.1

TSS

0.1

T=2K
T=5K
T=10K
T=30K

0.0
-0.1
-0.2
-0.3

0.01

-120 -80

0

50

100

150

200

250

300

-40

0

40

80

120

H (kOe)Moment (μΒ per f. u.)

Temperature (K)0.8

FIG. 8. (Color
online)Thehysteresis
loops
for
(Pr1−y Yy )0.7 Ca0.3 CoO3 (y = 0.15). The theoretical Brillouincurves for S = 1/2 and T = 2, 5 and 10 K are added for comparison,marked by the lines.

y=0

0.6
y = 0.15

0.4
0.2
0.0

0

20

40

60

80

100 120 140

H (kOe)FIG. 6. Upper panel: The ZFC and FC susceptibility on(Pr1−y Yy )0.7 Ca0.3 CoO3 (y = 0 and 0.15) measured in a dc field of
20 Oe. Note the logarithmic scale on the moment axis. Lower panel:
The magnetization in fields up to 140 kOe and back, taken at 2 K.

The spontaneous moment obtained from the extrapolation
to zero field is, however, lower and closely approaches
the value 0.34μB , which was deduced from the neutron
diffraction refinement. In our interpretation, the ground state of
Pr0.7 Ca0.3 CoO3 is based on the so-called LS/LS phase for Co3+
and Co4+ with a theoretical FM moment of 0.30μB /Co. With
increasing external field, the IS/LS phase with a theoretical FM
moment of 1.70μB /Co seems to be partially populated. For the
possibility of such two-phase coexistence, see Refs. 10 and 11.
The magnetization data for the y = 0.15 sample reveal
a highly inhomogeneous phase that can be characterized as

0.6
T=2K
T=5K
T=10K
T=30K

Moment (μB)

0.4
0.2
0.0
-0.2
-0.4
-0.6

-120

-80

-40

0

40

80

120

H (kOe)FIG. 7. (Color online) The hysteresis loops for the Pr0.7 Ca0.3 CoO3
sample after subtraction of the Pr3+ contribution.

cluster glass with complex behaviors both in the medium andhigh magnetic fields. As shown in the lower panel of Fig. 6, thehigh-field paraprocess is reduced compared to Pr0.7 Ca0.3 CoO3
and corresponds now to 0.465 of the experimental value for
PrCoO3 . This finding is an independent confirmation of thePr3+ → Pr4+ valence shift below TMI . In fact, it is in very
good quantitative agreement with the valence composition,
which was deduced from Schottky peak analysis in Ref. 18—
4+
3+
2+
3+
4+
2−
Pr3+
0.415 Pr0.18 Y0.105 Ca0.3 Co0.88 Co0.12 O3 .
The magnetization of y = 0.15 after the subtraction of thePr3+ paraprocess is presented in Fig. 8. It saturates at 140 kOe
on 0.30μB per f.u., of which 0.12μB should be attributed to
the Co4+ contribution in the LS/LS phase and the remaining
0.18μB is evidently the contribution of Pr4+ pseudospins in
the y = 0.15 sample, supposing that the value of the gJ  factor
is close to 2. Let us note that such a value is quite reasonable inview of our theoretical estimates for Pr4+ , gx = 3.757, gy =0.935, and gz = 0.606, which yield the average value gJ   =2.07 when numerical integration over the random orientationof the crystallites is done.
The magnetization curves observed for the y = 0.15 sample
at low temperatures deviate largely from conventional FM
behavior. At first glance, the M versus H dependence isreminiscent of a paramagnet close to saturation, but comparedto the standard Brillouin function the increase in low fields
is steeper, suggesting the presence of large spin entities withnonuniform distribution. We estimate from the observed trendthat superparamagnetic domains up to 100 cobalt ions prevailin the y = 0.15 sample. On the other hand, the increase inhigh fields is slower, suggesting strong AFM coupling between
up-grown FM regions or the presence of surface states withlarge magnetic anisotropy as known for a so-called deadlayer in nanoparticles. Upon closer inspection, one may also
notice a certain opening of magnetization loops at the lowest
temperatures, which is characterized by a coercive field of
about 200 Oe at 2 K. A finite but very weak remanent momentof 0.014μB /Co at 2 K quickly vanishes with increasing
temperature (see Fig. 9). Similar behavior has been observed
also for other (Pr1−y Yy )0.7 Ca0.3 CoO3 samples with a MItransition as well as for Pr0.5 Ca0.5 CoO3 , where the remanent
moment at 2 K is, however, an order of magnitude larger.28

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Thermoremanence (μB)

K. KNÍŽEK et al.

PHYSICAL REVIEW B 88, 224412 (2013)

0.03

y = 0.15
y = 0 (×1/10)
PrCa50 (×1/10)

0.02

0.01

0.00
0

10

20

30

40

50

Temperature (K)FIG. 9. The
thermoremanent
magnetizationof(Pr1−y Yy )0.7 Ca0.3 CoO3 samples. The data for y = 0.15 arecompared with the Pr0.5 Ca0.5 CoO3 sample possessing at least 20×larger thermoremanence due to excessive defects.

Another signature of the complex magnetic state of they = 0.15 sample is a shift of hysteresis loops, demonstrated
in Fig. 10. The effect is strongly dependent on the magnitudeof the bias field.
IV. DISCUSSION

Moment (μB)

0.08
0.04

Exchange bias (Oe)Cobaltites are highly complex systems. To understand thephysical properties of mixed-valence cobaltites in the 30%
doping range, at least in an illustrative way, the IS/LS scenario
for Co3+ /Co4+ is generally applied. It is worth mentioningthat this scenario implies carriers in two bands of very different
characters and mobilities. The eg quasiparticles are light but
move in a disordered background of nearly localized t2g
charges and spins, and become strongly scattered, while thet2g quasiparticles, although heavy and short-lived, experiencea practically uniform background because of very fast fluctuations of the eg electron subsystem. The decreased scatteringin the latter case may be why the overall conductivity is likely
contributed to by both the eg and t2g channels.9 With decreasing
temperature, the system La0.7 Sr0.3 CoO3 of the broad eg
band maintains the IS/LS phase, whereas the narrowband

120
80
40
0
0

20 40 60
H (kOe)0.00
T = 2K-0.04
FC in 20 kOe
FC in 1000 Oe

-0.08
-3

-2

-1

0

1

2

3

H (kOe)FIG. 10. (Color
online)Thehysteresis
loops
for
(Pr1−y Yy )0.7 Ca0.3 CoO3 (y = 0.15), taken at 2 K after the coolingin bias fields 1 and 20 kOe. The inset shows the bias values (theabscissa shift of the loops) in a broader range of fields.

systems such as Pr0.7 Ca0.3 CoO3 or Nd0.7 Ca0.3 CoO3 exhibita gradual crossover toward lower spin states. In contrast to
the true metallic conductivity of La0.7 Sr0.3 CoO3 , the electricalresistivity observed in Pr0.7 Ca0.3 CoO3 or Nd0.7 Ca0.3 CoO3
shows a steady increase with decreasing temperature but
extrapolates to a finite value instead of diverging. This suggeststhat also in this latter case the LS/LS phase of 30% t2g hole
doping should be considered as intrinsically metallic.11
The spin-state crossover in the yttrium-containing samples(Pr1−y Yy )0.7 Ca0.3 CoO3 is much more abrupt since it isaccelerated by the electron transfer from Pr3+ to Co4+ at the MI
transition. In the LS/LS ground state, the carrier concentrationin the Co-O subsystem is thus reduced, in particular for they = 0.15 sample down to 12% t2g hole doping. Following theobserved resistivity dependence down to 6 K, the actual regimeof conduction is variable range hopping.14,18 This means thatthe charge transport is realized via localized states close to
the Fermi level, or, in other words, the doping level in they = 0.15 sample is below the mobility edge for the t2g bandof the LS/LS phase.
The difference between the y = 0 and 0.15 samples ismanifested also in their magnetic state. The y = 0 sample
behaves at the lowest temperatures like a standard FM systemwith broad hysteresis loops that evidence the existence ofa domain structure. The long-range magnetic order andspontaneous moments of about 0.30μB /Co are found byboth neutron diffraction and magnetization measurements.
We should mention, however, that the transition from theparamagnetic state is unconventional. First, no heat-capacity
anomaly is observed at TC = 55 K. Another feature reported
in some previous studies of Pr0.7 Ca0.3 CoO3 is the observationof nanosized FM objects at temperatures as high as ∼250 K.
These have been detected by both small-angle neutron scattering and ZFC/FC bifurcation of the susceptibility.25 The lackof the latter anomaly in the present study suggests that if suchpreformed FM objects are intrinsic to Pr0.7 Ca0.3 CoO3 , theirsize and number are likely sample-dependent.
The long-range FM order is absent in our y = 0.15
sample as well as in Pr0.5 Ca0.5 CoO3 , which is known as
the prototype of the MI transition in Pr-based cobaltites.
Three magnetic contributions are effective in the insulatingphase of these systems: spins of LS Co4+ , pseudospins of
the Pr4+ ground doublet and singlet states of Pr3+ withextremely large Van Vleck susceptibility χvV = 0.0188 emu
mol−1 Oe−1 , or equivalently 0.0034μB /kOe (χvV for the spinand orbital singlet LS Co3+ is two orders of magnitude smaller,
0.0002 emu mol−1 Oe−1 ,29,30 and has thus a negligible effecton the total magnetization).
It has been shown above that the paraprocess due to thePr3+ contribution is simply separable, and consistent data withrespect to the Pr3+ → Pr4+ valence shift are obtained. The
resolution of the LS Co4+ and Pr4+ contributions appeared
impossible, but their overall magnetization in Fig. 8 suggests
a magnetically nonuniform phase that can be classified as thecluster glass or glassy ferromagnetism.8 We may conclude,
based on the initial slope of magnetization curves and thepresence of finite thermoremanence in the y = 0.15 sample,
that the prevailing superparamagnetic domains with easy
saturation in low fields (with an average size of about 100
atoms) coexist with minor FM regions. In addition, there is224412-6

SPIN-STATE CROSSOVER AND LOW-TEMPERATURE . . .

E (K)Cp / T (J mol-1 K-2)

0.4
0.3
0.2

-40

This issue has been largely discussed in relation to an opposite
effect in Nd-based cobaltites.11

0 kOe
10 kOe30 kOe50 kOe90 kOe

E||
E⊥
〈E〉

40
30
20
10
0

PHYSICAL REVIEW B 88, 224412 (2013)

040

V. CONCLUSIONS

80

H (kOe)

0.1
0.0

0

5

10

15

Temperature (K)FIG. 11. (Color online) The heat capacity divided by temperaturefor (Pr1−y Yy )0.7 Ca0.3 CoO3 (y = 0.15), after subtraction of lattice andnuclear terms. The full lines present the theoretical fit based on the
broadening due to anisotropic Zeeman splitting, supposing the axial
symmetry of the g factor. The actual values of Zeeman energy areplotted in the inset.

surely a certain population of smaller magnetic clusters oreven individual spins that are manifested by the frequencydependent ac susceptibility below 10 K (not shown here).
The pending question is the origin of the relatively strong
molecular field that acts on Pr4+ pseudospins in the systems
with the occurrence of the MI transition, in spite of the highly
diluted and therefore inhomogeneously distributed Co4+ spins.
To illustrate the problem, we revisit here the low-temperature
Schottky data measured on the polycrystalline sample y =0.15 in our previous paper.18 The Schottky peaks are broadenedwith respect to the ideal Schottky form, but as seen in Fig. 11,
an excellent fit can be obtained for the g factor of axial
anisotropy, g = 5.86, g⊥ = 1.25, irrespective of the strengthof the applied field. Similar quasiaxial anisotropy of the gfactor has been found in recent analysis of the Pr4+ -related
Schottky peaks in Pr0.5 Ca0.5 CoO3 .14 Important implications
can be drawn from the analysis. First, the ratio g /g⊥ ∼= 4.7
agrees very well with the pseudoaxial theoretical ratio for
Pr4+ , 2gx /(gy + gz ) = 4.9, suggesting that there is little orno additional inhomogeneous broadening. Hence, the Pr4+
pseudospins experience a molecular field of uniform strength,
Hm = 17 kOe, although they are distributed randomly in themagnetically nonuniform phase. The origin of the molecularfield should be, therefore, more complicated than in the case of
Nd3+ -containing perovskites with a FM ground state. Second,
the Schottky peak profile retains the same g-anisotropy-related
broadening even at zero field, which means that the molecularfield present spontaneously in the single crystal grains is
not oriented along a particular crystallographic axis but
behaves as in a random anisotropy system. Third, the shift of
Schottky peaks with increasing external field is strictly linear,suggesting an extreme softness, which is again in variancewith the behavior in FM systems with a domain structure. It
is also of interest that the absolute g values deduced fromthe shift in applied field, in particular the average valuegJ   = 3.22, are much larger than the theory, and also thesaturated magnetization gives (gJ   ∼ 2). This discrepancy
suggests that there is a spin-polarizable medium in the Pr4+
surroundings that acts as an enhancement of the external field.

Neutron diffraction and magnetic studies have been
performed on two cobaltite systems (Pr1−y Yy )0.7 Ca0.3 CoO3
(y = 0 and 0.15) with practically ideal oxygen stoichiometry.
Both samples are presumably in a dynamic mixture of IS
Co3+ /LS Co4+ states at room temperature and exhibit withdecreasing temperature a spin-state crossover. In both cases,
the ground state is identified, based on observed magnitudes
of magnetic moments and robustness up to high fields, with amixture of LS Co3+ /LS Co4+ states.
The Pr0.7 Ca0.3 CoO3 sample with a formal concentrationof 0.30 LS Co4+ in the diamagnetic background of LS Co3+
shows characteristics of a common FM phase (TC = 55 K)
with prevailing long-range order. Its electronic properties
evidence the intrinsic metallicity despite the granularity of
the ceramic sample.
The behavior of yttrium-substituted samples is much more
complex because of a partial Pr3+ → Pr4+ and Co4+ → Co3+
valence shift, which diminishes the carrier doping in the Co-O
subsystem and facilitates also the spin-state crossover to the
LS/LS phase. This transition, accompanied with a drop ofelectrical conductivity, occurs for y = 0.15 at TMI = 132 K.
The transport data published earlier show that an effective
conduction mechanism at the low-temperature phase is thevariable range hopping, which anticipates a tunneling oft2g carriers between more distant Co sites close in energy.
As far as the magnetism is concerned, the present neutron
diffraction, performed down to 0.25 K, does not show any
long-range ordering of Co4+ and/or Pr4+ moments, and also
the magnetization loops display no or a negligible opening
and are characteristic rather of a glassy state. The main
finding is the observation of a uniform internal field acting on
Pr4+ pseudospins, which is surprising for the low-temperaturephase of y = 0.15 with reduced concentration of cobalt spins(formally 0.12 LS Co4+ per f.u.).
The issue deserving the most attention is the origin of FM
interactions that are responsible for the long-range orderingin the 30% hole-doped Pr0.7 Ca0.3 CoO3 or Nd0.7 Ca0.3 CoO3 , aswell as for the strong molecular field acting on rare-earth ionsin the (Pr1−y Yy )0.7 Ca0.3 CoO3 systems with severely reducedhole dopings. Regardless of the character of the LS Co3+ /LS
Co4+ mixture, either localized or forming a very narrowt2g band, the FM interactions in a pure phase are unlike.
Namely, the superexchange interactions between LS Co4+
5
(the t2g
configuration) should be of AFM type according to
the Goodenough-Kanamori rules. In addition, when collective
t2g states and the single-band Hubbard model are considered,no spontaneous FM ordering can be foreseen. Neither ofthe double exchange interactions that are effective in theroom-temperature IS/LS phase is possible. In our opinion,a possible explanation stems from magnetic defects that arise
due to minor oxygen vacancies inherently present in doped
cobaltites. This results in a reduced coordination of nearby
cobalt ions, likely the pyramidal one, which stabilizes local
HS Co3+ states. Their concentration is estimated based on
the near oxygen stoichiometry in our samples to be ∼ 2% or

224412-7

K. KNÍŽEK et al.

PHYSICAL REVIEW B 88, 224412 (2013)less. We suggest that these very dilute defects, clearly below
the percolation limit, interact ferromagnetically via itinerantt2g holes. The spin polarization of the narrow t2g band thenmediates the nearly homogeneous molecular field over the
crystal lattice.
To summarize: The sole existence of FM order in the
LS/LS cobaltite phases is a general problem. Our explanation
relies on a scenario of magnetic defects, which represent a
nonhomogeneity but polarize t2g carriers at the top of oxygen

π hybridized band. Irrespective of the nature of these defects
and their actual location, the spin density is thus distributed
over the whole sample.

1

17

J. B. Goodenough, J. Phys. Chem. Solids 6, 287 (1958).
R. A. Bari and J. Sivardiére, Phys. Rev. B 5, 4466 (1972).
3
T. Kyômen, Y. Asaka, and M. Itoh, Phys. Rev. B 71, 024418 (2005).
4
Z. Jirák, J. Hejtmánek, K. Knı́žek, and M. Veverka, Phys. Rev. B
78, 014432 (2008).
5
K. Knı́žek, Z. Jirák, J. Hejtmánek, P. Novák, and W. Ku, Phys. Rev.
B 79, 014430 (2009).
6
V. Křápek, P. Novák, J. Kuneš, D. Novoselov, D. M. Korotin, andV. I. Anisimov, Phys. Rev. B 86, 195104 (2012).
7
R. Caciuffo, D. Rinaldi, G. Barucca, J. Mira, J. Rivas, M. A. Señarı́sRodrı́guez, P. G. Radaelli, D. Fiorani, and J. B. Goodenough, Phys.
Rev. B 59, 1068 (1999).
8
J. Wu and C. Leighton, Phys. Rev. B 67, 174408 (2003).
9
P. Augustinský, V. Křápek, and J. Kuneš, Phys. Rev. Lett. 110,267204 (2013).
10
A. O. Sboychakov, K. I. Kugel, A. L. Rakhmanov, and D. I.
Khomskii, Phys. Rev. B 80, 024423 (2009).
11
Z. Jirák, J. Hejtmánek, K. Knı́žek, M. Maryško,
P. Novák, E. Šantavá, T. Naito, and H. Fujishiro, J. Phys.-Condens.
Matter 25, 216006 (2013).
12
S. Tsubouchi, T. Kyômen, M. Itoh, P. Ganguly, M. Oguni,
Y. Shimojo, Y. Morii, and Y. Ishii, Phys. Rev. B 66, 052418(2002).
13
S. Tsubouchi, T. Kyômen, M. Itoh, and M. Oguni, Phys. Rev. B 69,
144406 (2004).
14
J. Hejtmánek, Z. Jirák, O. Kaman, K. Knı́žek, E. Šantavá, K. Nitta,
T. Naito, and H. Fujishiro, Eur. Phys. J. B 86, 305 (2013).
15
J. L. Garcı́a-Muñoz, C. Frontera, A. J. Barón-González, S. Valencia,
J. Blasco, R. Feyerherm, E. Dudzik, R. Abrudan, and F. Radu, Phys.
Rev. B 84, 045104 (2011).
16
H. Fujishiro, T. Naito, S. Ogawa, N. Yoshida, K. Nitta, J. Hejtmánek,
K. Knı́žek, and Z. Jirák, J. Phys. Soc. Jpn. 81, 064709 (2012).
2

ACKNOWLEDGMENTS

We thank J. Kuneš and C. Leighton for stimulating comments. This work was supported by Project No. 204/11/0713
of the Grant Agency of the Czech Republic.

J. Herrero-Martin, J. L. Garcı́a-Muñoz, K. Kvashnina, E. Gallo,
G. Subias, J. A. Alonso, and A. J. Barón-González, Phys. Rev. B
86, 125106 (2012).
18
J. Hejtmánek, E. Šantavá, K. Knı́žek, M. Maryško, Z. Jirák, T.
Naito, H. Sasaki, and H. Fujishiro, Phys. Rev. B 82, 165107(2010).
19
S. Edvardsson and D. Aberg, Comput. Phys. Commun. 133, 396(2001).
20
S. Hufner, Optical Spectra of Transparent Rare Earth Compounds(Academic, New York, 1978).
21P. Novák, K. Knı́žek, M. Maryško, Z. Jirák, and J. Kuneš, J. Phys.Condens. Matter 25, 446001 (2013).
22
J. B. Gruber, K. L. Nash, R. M. Yow, D. K. Sardar, U. V. Valiev,
A. A. Uzokov, and G. W. Burdick, J. Lumin. 128, 1271
(2008).
23
A. J. Barón-González, C. Frontera, J. L. Garcı́a-Muñoz, J. Blasco,and C. Ritter, Phys. Rev. B 81, 054427 (2010).
24
A. J. Barón-González, C. Frontera, J. L. Garcı́a-Muñoz, J. Blasco,
C. Ritter, S. Valencia, R. Feyerherm, and E. Dudzik, Phys. Proc. 8,
73 (2010).
25
S. El-Khatib, S. Bose, C. He, J. Kuplic, M. Laver, J. A. Borchers,
Q. Huang, J. W. Lynn, J. F. Mitchell, and C. Leighton, Phys. Rev.
B 82, 100411 (2010).
26
D. Phelan, Y. Suzuki, S. Wang, A. Huq, and C. Leighton, Phys. Rev.
B 88, 075119 (2013).
27
R. M. Thomas, V. Skumryev, J. M. D. Coey, and S. Wirth, J. Appl.
Phys. 85, 5384 (1999).
28
M. Maryško, Z. Jirák, J. Hejtmánek, and K. Knı́žek, J. Appl. Phys.
111, 07E110 (2012).
29
J. S. Griffith and L. E. Orgel, Trans. Faraday Soc. 53, 601(1957).
30
H. Kamimura, J. Phys. Soc. Jpn. 21, 484 (1966).

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