ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 272–276 (2004) 104–105

Magnetism and thermoelectricity in La0.8Sr0.2Co1zMzO3(M:3d transition metal)
Yosuke Fujine*, Hiroyuki Fujishiro, Kazuyoshi Suzuki, Yohei Kashiwada,
Manabu Ikebe
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan

Abstract
Magnetic and thermoelectric properties of La0.8Sr0.2Co1zMzO3 system (z ¼ 0:05; M: Ti, Cr, Mn, Fe, Ni, Cu, Zn)
have been investigated in search of a new thermoelectric material. Co-site substitution by 3d transition metal brought
about the large Seebeck coefﬁcient S: As a result, the ﬁgure of merit Z increased. The Seebeck coefﬁcient was found tobe strongly correlated with the behavior of magnetization.
r 2003 Elsevier B.V. All rights reserved.
PACS: 72.15.Jf; 75.50.yKeywords: La0.8Sr0.2Co1zMzO3; Electrical resistivity; Magnetization; Seebeck coefﬁcient; Figure of merit

1. Introduction
Recently, Co-based oxides have attracted muchattention as a possible thermoelectric material with ahigh ﬁgure of merit value (Z ¼ S 2 =rk; S the Seebeck
coefﬁcient, r the electrical resistivity, k the thermal
conductivity). The high degeneracy of Co-spin conﬁguration related to the low-, intermediate- and high-spinstate has been pointed out as an origin of the large Svalue [1]. Terasaki et al. [2,3] discovered NaCo2O4
to have the large S value and the low r value. They also
found that the Co-site substitution by Cu brought
about even higher S value, resulting in larger Z: In thispaper, we report the effect of Co-site substitutionon the magnetic and the thermoelectric properties ofLa1xSrxCo0.95M0.05O3 and also discuss the correlationof the Seebeck coefﬁcient with the electrical resistivity
and the magnetization. We concentrate our attention on
the carrier concentration x ¼ 0:2; around which the S*Corresponding author. Tel.: +81-19-621-6363; fax: +8119-621-6373.
E-mail address: t5102006@iwate-u.ac.jp (Y. Fujine).

value and other physical properties are very sensitive tothe Co-site substitution.

2. Experimental
Polycrystalline samples of La1xSrxCo1zMzO3 wereprepared by a solid-state reaction method. The electricalresistivity was measured by a standard four probe
method. The Seebeck coefﬁcient and the thermal
conductivity were measured by a steady heat ﬂow
method. The magnetization was measured using aSQUID magnetometer.

3. Results and discussion
Fig. 1(a) shows the temperature dependence of theelectrical resistivity rðTÞ: rðTÞ of the pristine sample
exhibits a metallic behavior, while substituted samples
exhibit semiconducting or metallic-semiconducting behavior. It is interesting to note that rðTÞ of this systemdepends on the species of the substituting 3d-transition

0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2003.11.045

ARTICLE IN PRESS
Y. Fujine et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 104–105

Fig. 1. (a) The electrical resistivity, (b) the magnetization, (c)the Seebeck coefﬁcient, and (d) the ﬁgure of merit of
La0.8Sr0.2Co0.95M0.05O3 as a function of temperature. Thesymbols are the same in all ﬁgures. The inset of (a) represents
the electrical resistivity near the room temperature.

Fig. 1(b) shows the temperature dependence of themagnetization MðTÞ: MðTÞ of this system exhibits theferromagnetic behavior for the pristine, and for by Fe,Ni, Cu, and Zn substituted samples. In contrast, by Ti,
Cr, and Mn substituted samples do not show apparent
ferromagnetic behavior. For the substitution by thelarger atomic number element than Co, both themagnitude of MðTÞ and the ferromagnetic transition
temperature Tc decrease with increasing atomic number.
On the other hand, for the substituting element withatomic number smaller than that of Co, the magnitudeof MðTÞ and Tc decrease with decreasing atomicnumber.
Fig. 1(c) shows the temperature dependence of theSeebeck coefﬁcient SðTÞ: As can be seen in Fig. 1(b) and(c), the Seebeck coefﬁcient is correlated with themagnetization. SðTÞ increases with decreasing magnitude of MðTÞ: i.e., the S value decreases with the onsetof the ferromagnetism as the freedom of spin direction isquenched and the magnetic entropy is decreased. This
result reﬂects the fact that the Seebeck coefﬁcient
represents ‘‘entropy per carrier’’. In the conventional
theory, the Seebeck coefﬁcient increases with increasing
the r value. We can ﬁnd in Fig. 1(c), that the Seebeck
coefﬁcient begins to decrease near the ferromagnetictransition temperature, while rðTÞ continues to increase
with decreasing temperature. This result implies that theSeebeck coefﬁcient is more strongly correlated with themagnetization than the electrical resistivity in thepresent cobaltite.
Fig. 1(d) shows the temperature dependence of theﬁgure of merit. To determine the Z value, the thermal
conductivity kðTÞ was measured for respective samples.
By substituting Co-site by 3d-transiton elements, Z
value increases and Cr substituted sample marks about
150  106 K1 near 150 K, which is about 15 times
larger than that of the non-substituted sample.
In summary, we measured the electrical resistivity, themagnetization, the thermal conductivity, and the Seebeck coefﬁcient for La0.8Sr0.2Co0.95M0.05O3. The rðTÞvalue near the room temperature decreased withincreasing atomic number of substituting 3d elements.
S was more strongly correlated with MðTÞ than rðTÞ:The ﬁgure of merit increased by 3d-transition metalelement substituting at Co-site, and the largest Z wasobtained for the substitution by 5% Cr. Z seems to takethe optimum value just before the ferromagnetism isestablished in the La0.8Sr0.2Co0.95M0.05O3 compounds.

References
element. Near the room temperature, as shown in theinset, the electrical resistivity decreases with increasing
atomic number.

105[1] W. Koshibae, et al., Phys. Rev. B 62 (2000) 6869.
[2] I. Terasaki, et al., Phys. Rev. B 56 (1997) R12685.
[3] I. Terasaki, et al., Jpn. J. Appl. Phys. 40 (2001) L65.