JOURNAL OF APPLIED PHYSICS 109, 07E127 (2011)

Thermally and field-driven spin-state transitions in (Pr12yYy)0.7Ca0.3CoO3M. Maryško,1,a) Z. Jirák,1 K. Knı́žek,1 P. Novák,1 J. Hejtmánek,1 T. Naito,2 H. Sasaki,2and H. Fujishiro2
1

Institute of Physics, ACSR, Na Slovance 2, 182 21 Praha 8, Czech Republic
Faculty of Engineering, Iwane University, 4-3-5 Ueda, Morioka 020-8551, Japan2(Presented 17 November 2010; received 12 October 2010; accepted 27 November 2010; published
online 1 April 2011)
The first-order character of the transition from the low-to-intermediate spin state in the system(Pr1yYy)0.7Ca0.3CoO3 (y ¼ 0.075–0.15) has been verified by magnetization measurements. For
y ¼ 0.075, the transition manifests itself in v(T) steps at temperatures depending on external fields.
In this case, isothermal measurements show a field-driven transition exhibiting a field hysteresis.
The entropy contribution is evaluated and discussed. Qualitative analysis based on a two-level
model suggests that the observed influence of the magnetic field can be explained only by
C 2011 American Institute of Physics.
assuming the presence of exchange interactions. V[doi:10.1063/1.3559485]

I. INTRODUCTION

The spin-state transitions (SST) in LaCoO3type cobaltites concern the Co3þ ions, which may occur in the low-spin(LS) s ¼ 0, intermediate (IS) s ¼ 1, or high-spin s ¼ 2 states.
A specific case of simultaneous first-order SST and metal–
insulator transition, accompanied by a large volume change,
was reported on the mixed valency cobaltite Pr0.5Ca0.5CoO3(Ref. 1) and related systems of lower calcium doping with Pr
substituted by smaller rare earths (see, e.g., Ref. 2).
Recently, the exclusive role of praseodymium for such atype of transition has been elucidated.3–5 Based on the electronic structure calculations and detailed analysis of thestructural, electrical, magnetic, and specific heat data, it has
been found that the transition is accompanied by a significant
charge transfer between cobalt and praseodymium ions. In
particular for the system (Pr1yYy)0.7Ca0.3CoO3, about onethird of the Pr3þ ions changes their valency to Pe4þ during
the SST on decreasing temperature.4 In the present contribution we follow previous works2,4 and bring a detailed studyof the first-order SST and its dependence on the appliedmagnetic field, taking a special emphasis on the composition y¼0.075. The new result is the observation of a fielddriven SST, which can be explained by the presence ofthe exchange interaction acting on the paramagnetic IS
Co3þ ion.
II. EXPERIMENTIn our experiments we used the ceramic samples of(Pr1yYy)0.7Ca0.3CoO3 (y ¼ 0.075, 0.1, 0.15) described previously in Ref. 4. The magnetic measurements were carriedout by means of the superconducting quantum interference
device magnetometers MPMS XL and MPMS-5S. The temperature dependence of the susceptibility v under an appliedfield 1 kOe was measured between 5 and 300 K using a temperature sweep rate of 2 K/min. In increasing temperaturea)

Electronic mail: marysko@fzu.cz.

0021-8979/2011/109(7)/07E127/3/$30.00

the SST manifests itself in an increase of the susceptibilityby Dv ¼ Dm/H. In Fig. 1(a) the results are shown between 20
and 200 K, where we observe the SST at the temperatures
Ts  64, 93, and 132 K with Dv  0.020, 0.0093, and 0.0058
emu mol1 Oe1 for y ¼ 0.075, 0.1, and 0.15, respectively.
At the same temperatures a pronounced peak of the specificheat has been observed, very sharp for the former two samples and more diffuse for the latter one (see Ref. 4).
As a second step, we carried out the measurements
around Ts employing smaller sweep rates that enable the precise determination of thermal hysteresis. For the y ¼ 0:075
sample, where the SST is very sharp, we found that the reliable results, which are not dependent on the sweep rate, canbe achieved with the rate 0.007 K/min. An example of thermal hysteresis curve (THC) is given in Fig. 1(b), whichshows the vðTÞ plot measured at H ¼ 10 kOe during warming
and subsequent cooling. At the temperatures Tsu and Tsd we
see an almost step increase and decrease of the susceptibility
with the thermal hysteresis DT ¼ TsuTsd and the average
SST temperature Ts ¼ 0.5(TsuþTsd). Here, we obtained
Ts ¼ 64:04 and DT ¼ 0:35 K. The same type of experiment
was then performed for several static magnetic fields, including H ¼ 0, where the THC was evaluated from the real partof the ac susceptibility measured with a driving ac field 3.9Oe (the imaginary part was too small to be detected). In accordance with expectation, Ts decreases with increasing field
H and the dependence can be well approximated by the relation Ts ðHÞ ¼ Ts ð0Þ  AH2 [inset of Fig. 1(b)]. The value ofDT increases with increasing field H from 0.19 Oe (H ¼ 0)
to 0.48 (H ¼ 70 kOe). The data on Ts(0), A, and the averagevalue of DT are listed in Table I.
For y ¼ 0:1 and 0.15 the character of the SST becomes
more continuous as can be seen in Fig. 1(a). Analogously asin the case of y ¼ 0:075, we performed the measurements of
the THC around Ts using the sweep rates of 0.15 K/min(y ¼ 0:1) and 0.2 K/min (y ¼ 0:15). In both cases, DT and Aare very small (Table I). The results for y ¼ 0:15 are exemplified in Fig. 1(c).

109, 07E127-1

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07E127-2

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J. Appl. Phys. 109, 07E127 (2011)

FIG. 2. The hysteresis loops for y ¼ 0:075 measured at several temperaturesnear Ts(0); the hysteresis loop corrected for a paramagnetic background isshown by the dotted curve.
FIG. 1. (a) Temperature dependence of the molar susceptibility measuredunder an applied field 1 kOe; (b) thermal hysteresis for y ¼ 0:075 at H ¼ 10kOe, in the inset the dependence of average Ts on magnetic field H; (c) thesame for y ¼ 0:15 and, in the right inset, the part of the thermal hysteresis
curve displayed on a larger scale.

Now we focus our attention on the isothermal mðHÞdependencies taken at temperatures near Ts ð0Þ. In contrast to
the y ¼ 0:1 and 0.15 samples, we find that for y ¼ 0:075 thehysteresis curves indicating field-driven SST transitions. AtT ¼ 64 K the dependence mðHÞ is linear, corresponding tothe field-independent susceptibility m=H of paramagnetic
ions Co3þ, Co4þ, and Pr3þ [Fig. 2(a)]. With decreasing temperature, a metamagnetic-like transition becomes evident
and, due to hysteresis, the mðHÞ dependence transforms into
a closed loop. The contribution of the SST, obtained by subtracting linear dependence corresponding to the paramagnetic background, is illustrated by a dotted line in Fig. 2(b).
At T ¼ 63:3 K, the average field hysteresis makes DH  13kOe.
III. DISCUSSION

The finite values of DT and DH seen in Figs. 1(b) and2(b) indicate the first-order character of SST. In this case,when increasing temperature a latent heat Dq must be transferred to a system and its entropy increases by DS ¼ Dq=T.
The entropy change DS can be determined using theClausius–Clapeyron equation dTs =dH ¼ TDm=Dq. Aftersetting Dm ¼ HDv and integrating, the Clausius–Clapeyron
equationtransformstoTs ¼ Ts ð0Þ  AH 2 ,whereA ¼ Dv=ð2DS) and Ts ð0Þ is the SST temperature for H ¼ 0.
The determined values of DS are included in Table I.
The important experimental fact is a step-like increaseof v in increasing both temperature and field. To elucidate
this finding, we restrict our study to the Co3þ ion and its transition from the LS to IS state. We start from a thermodynamic two-level model of SST.6–8 Using the notation in Ref.
7 the system can be characterized by two energy parameters
E and e, where E is the distance between the lowest LS singlet level and excited IS level with triple orbital and triple
spin degeneracy. The parameter e represents an energy gain
due a change of the interatomic distances. The model calculation yields population n of the IS Co3þ ions, which should
be proportional to Dv. The essential point is that the excitation is accelerated compared to simple Boltzmann statistics(the LS–IS gap shrinks gradually) and becomes even of firstorder type, because the lattice volume adjusts to increasing
number n. For y ¼ 0.075 the calculation with spin s ¼ 1,g ¼ 2, 2l þ 1 ¼ 3, r ¼ e=E ¼ 0:51, and E (Table I) yields
the function nðTÞ exhibiting a step at T ¼ 64 K, but its dependence on H field is very small (0.16 K for dH ¼ 70 kOe).
A reasonable agreement with experiment can be achieved
only if we take into account a positive molecular field (ferromagnetic exchange interaction) acting on the paramagnetic
IS Co3þ ion. This interaction can be approximately included

TABLE I. Parameters Ts(0), A, Dv, DS, and DT deduced from experiment and E/kB used in the model calculation.
yTs(0) (K)A  104 (K kOe2)

Dv (emu mol1 Oe1)

DS (J mol1 K11)

DT (K)

E/kB (K)

0.075
0.1
0.15

64.04
93.13
132.32

3.10
1.43
0.51

0.02
0.00930.0058

3.23
3.25
5.68

0.35
0.35
0.23

280.3
365.4
538.4

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07E127-3

Maryško et al.

FIG. 3. Results of the calculations according to the two-level model: thetemperature (a) and field (b) dependence of the occupation number n.

by changing the factor lB H=kT [see expression (35) in Ref.
7] by replacing T by T  #, where # is a temperature
characterizing the interaction. Using # ¼ 42 K we obtainthe functions nðTÞ, for which Ts(0) ¼ 64 K andTs(70 kOe) ¼ 62.5 K [Fig. 3(a)]. In Fig. 3(b) we show thatthe calculated functions nðHÞ for T ¼ 63:3 and 63.95 K exhibit step changes at 49.5 and 5.5 kOe, respectively. Fory ¼ 0:1 and 0.15 the calculated nðTÞ dependencies were in a
relatively good agreement with experiment for parameters
for r ¼ 0:44 and 0.46 and # ¼ 51 and 48.6 K, respectively.
The final comment concerns the DS values presented inTable I. They are significantly lower than the two-level
model of local excitations predicts for (Pr1yYy)0.7Ca0.3
CoO3—DS ¼ 0:7R ln 9 ¼12.78 J mol1 K1. This discrepancy can be understood considering the metallic character ofphase above Ts . We note that the excited IS states do notexist as bare Co3þ species. They provide eg carriers that aredelocalized over surrounding Co4þ sites, forming a dropletof the metallic phase of t52g r character. In that case theJ. Appl. Phys. 109, 07E127 (2011)

relevant entropy retains only the spin degeneracy part andinstead of common configuration term, S ¼ Rð0:7 ln 0:7
þ0:3 ln 0:3Þ, it contains a term that derives from density ofstates of the final phase, S ¼ cT with c of about 0.040 J
mol1 K2 (see, e.g., Ref. 9). For y ¼ 0:075 with Ts  64 K
one may, therefore, estimate DS ¼ cT þ 0:7R ln 3  R
ð0:7 ln 0:7 þ 0:3 ln 0:3Þ ¼ 2:56 þ 6:40  5:08 ¼ 3.88 J mol1K1, in reasonable agreement with the experimental value of
3.23 J mol1 K1.
In summary, the spin-state transitions in (Pr1yYy)0.7
Ca0.3CoO3 with y ¼ 0.075–0.15 exhibit a finite small hysteresis and the transition temperature depends on the magnetic
field. The latter dependence enables the determination of theentropy contribution which is compared with the theoreticalvalue. For y ¼ 0:075 a field-driven transition is observed forcobaltites. The simple two-level model modified for the presence of the exchange interaction is used to qualitatively
describe a step-like dependencies of the susceptibility ontemperature and magnetic field.
ACKNOWLEDGMENTS

The work was supported under Project No. 202/09/0421
of the Grant Agency of the Czech Republic and GA ASCR
Grant No. 400720619.
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