ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 272–276 (2004) e283–e284

Magnetism, structure and transport of Y1xCaxCoO3 andLa1xBaxCoO3$
J. Hejtma! neka,*, Z. Jira! ka, K. Kn!ımeka, M. Marys$koa, M. Veverkaa, H. Fujishirobba
Institute of Physics of ASCR, Na Slovance 2, 182 21 Praha 8, Czech Republic
Department of Materials Science and Technology, Faculty of Engineering, Iwate University, Morioka 020-8551, Japan

Abstract
We compare the magnetic, structural and transport properties of La1x Bax CoO3 and Y1x Cax CoO3 perovskites. Theexperimental data supported by ab initio calculations reveal that: (i) Y-based perovskites are up to room temperature
represented by essentially insulating LS state ðTo300 KÞ followed by a gradual increase to IS state ðSB1Þ withenhanced conductivity above 500 K; (ii) the magnetic interactions in La-based perovskites are antiferromagnetic with
SB1:5 ðT > 700 KÞ and (iii) the Ba doping induces the ferromagnetic correlations and leads to a highly conducting
state above 600 K:
r 2004 Elsevier B.V. All rights reserved.
PACS: 71.30.þh; 71.70.dKeywords: Cobaltite; Spin state

Since more than 40 years the spin-state of cobalt ionsand the nature of two broad magnetic–electronictransitions in LaCoO3 have been investigated [1]. The
low-temperature transition at TC110 K is unambiguously identiﬁed as the spin-state transition from lowspin ðS ¼ 0; t62g Þ to intermediate-spin ðS ¼ 1; t52g e1g Þ Co3þ
conﬁguration. On the other hand the origin of transition
at TC500 K is not clearly established, although possible
orbital ordering is proposed [2]. As both the averageradius of interpolated large cation and the mean cobalt
valency determine critically the magnetic, structural andtransport properties, to identify the nature of spin andmetal–insulator transitions in perovskite cobaltites wecompare the high symmetry La1x Bax CoO3 (R3% c spacegroup, Co–O–Co bond angle B164 ) and distorted
Y1x Cax CoO3 (Pbnm, B148 ). Moreover, the chemical$
Supported by programme KONTAKT ME 572 of theMinistry of Education of the Czech Republic.
*Corresponding author. Institute of Physics of ASCR,Cukrovarnicka 10, CZ Praha 6, Praha 16253, Czech Republic.
Tel.: +420-2-203-18-419; fax: +420-2-333-43-184.
E-mail address: hejtman@fzu.cz (J. Hejtm!anek).

composition was chosen with respect to rule out theeventual role of RE-magnetism.
As the classical ceramic route, used for the synthesisof La1x Bax CoO3 ceramics, did not provid the single
phase Y1x Cax CoO3 samples, these were prepared by
the nitrate decomposition method. Magnetic and transport properties of the prepared La–Ba and Y–Ca
perovskites were investigated over a broad temperature
region up to 1000 K: The temperature dependence ofmagnetic susceptibility is in Fig. 1 compared with lines
marking the characteristic spin states. The Ba doping
converts the diamagnetic ground state of LaCoO3 to theparamagnetic one ðSC1:5Þ which is accompanied by a
low-temperature glass-like cusp at TSG C25 K: Contrary
to that, the Ca doping in Y–Ca system does not induce
similar change of magnetic behavior and both samples
preserve the low-spin ground state at low temperature.
The LS–IS transition is gradual and shifted to higher
temperature—from 400 to 800 K:
Electronic structure calculation [3] performed forvarious simulated structures of LnCoO3 showed, thatthe Co–O–Co bond angle is the most decisive forstabilization either LS or IS state. The critical value
being B163:5 is in good agreement with experiment for

0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2003.12.679

ARTICLE IN PRESS!
J. Hejtmanek
et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e283–e284
710S=2(HS)LaCoO3

1

Y0.9Ca0.1CoO3

S=1(IS)
S~0.15(LS)YCoO30.10200

400600800610510410x=0.0310x=0.121011001010000Fig. 1. The temperature dependence of the DC magnetic
susceptibility (measured at 10 kOe) for La1x Bax CoO3 (opensymbols) and Y1x Cax CoO3 (full symbols).

1.4200

LaCoO3 : However, for more distorted YCoO3 ourcalculation propose LS state for the whole temperature
range and cannot explain the gradual LS–IS transition.
The Ca- and Ba-doping have a similar impact on theelectrical resistivity compared with the parent perovskites, see upper panel of Fig. 2. The temperaturedependence of electrical resistivity further demonstrates
that the charge carrier transport is substantiallyhindered in distorted Y–Ca (Pbnm) system comparedwith La–Ba ðR3% cÞ:
The character of the charge carrier conduction
accompanying the phase transitions is evidenced by thethermopower data presented in lower panel of Fig. 2.
Here, the metal–insulator transition (exempliﬁed in theinset) is accompanied by a decrease of the thermopowerfrom a large positive values, characterizing the activatedhopping conduction at lower temperatures, to smaller
values generally associated with degenerate chargecarriers. The values of the thermopower undergoes alsoa distinct decrease upon Ba(Ca)doping when thedecrease with decreasing temperature, in context withthe temperature activated electrical resistivity, mirrors
the variable range hopping of the charge carriertransport at low temperatures.

-1

Thermoelectric power (mVK )

T (K)

400
T (K)
1.2
0.8

1.2
x=0.0

0.4

1.00.8

600

800

1000

Y-Ca-110Eact (∂lnρ)/(∂T )
(eV)

-1-1

La0.9Ba0.1CoO3Electrical resistivity (mΩcm)-3

Susceptibility (emu mol Oe 10 )

e284

La-Ba

200 400 600 800

x=0.00.60.4

x=0.10.2
x=0.1

0.00200

400T (K)

600

800

1000

Fig. 2. The temperature dependence of the electrical resistivity(upper panel) and thermoelectric power (lower panel) for
La1x Bax CoO3 (open symbols) and Y1x Cax CoO3 (full symbols). In the inset the M–I transition is evidenced via theactivation energy of the electrical resistivity.

References[1] W.C. Koehler, E.O. Wollan, J. Phys. Chem. Solids 2 (1957)
100.
[2] M.A. Korotin, et al., Phys. Rev. B 54 (1996) 5309.
[3] P. Blaha, et al., WIEN2k, APW+lo Program for Calculating Crystal Properties, K.-H. Schwarz, TU Wien, Austria,
2001, ISBN 3-9501031-1-2.