Physica B 329–333 (2003) 922–923

Magnetic, electrical and thermal properties ofLa0:80Sr0:20ðMnyCo1yÞO3Kazuyoshi Suzuki, Hiroyuki Fujishiro*, Yohei Kashiwada, Yosuke Fujine,Manabu Ikebe
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, JapanAbstract
The thermal conductivity k; thermoelectric power S; electrical resistivity r and magnetization M of La0:80 Sr0:20(Mny Co1y ÞO3 has been measured. By Mn substitution for Co, the initial ferromagnetic-metal behavior is promptly
suppressed and rðTÞ becomes semiconductive. In the semiconductive composition range, S is remarkably enhanced andk is reduced. As a result, pretty large values of the thermoelectric ﬁgure of merit Z are realized.
r 2003 Elsevier Science B.V. All rights reserved.
Keywords: La0:80 Sr0:20 ðMny Co1y ÞO3 ; Thermal conductivity; Thermoelectric power; Thermoelectric ﬁgure of merit

1. Introduction

2. Experimental

ðRE1x AEx ÞMO3 (RE: rare-earth, AE: alkaline-earth,
M: 3d transition metal) type oxides with perovskite
structure have attracted renewed interest. The hole
doping into these oxides leads to novel and anomaloustransport phenomena. The manganite, La1x Srx MnO3 ;for example, exhibits the well-known colossal magnetoresistance (CMR) [1], while the cobaltites,
La1x Srx CoO3 ; exhibits complex magnetism and metal-insulator transition depending on the hole doping x[2,3]. Recently, cobaltites such as NaCo2 O4 were
proposed to be hopeful candidates for thermoelectric
application [4]. The undoped LaCoO3 is a nonmagnetic
insulator with the low spin state at low temperatures.
The hole-doped compound, La0:80 Sr0:20 CoO3 (LSCoO),
transforms to a ferromagnet showing barely metallic
electrical conduction. Because unusual physical phenomena tend to appear near the boundaries of competing phases, we examine Mn substitution effect inLa0:80 Sr0:20 ðMny Co1y ÞO3 (LSCoOðyÞ; 0pyp0:2) andalso survey the potentiality of the Mn-substituted
perovskite cobaltate as a thermoelectric material.

The single-phase La0:80 Sr0:20 ðMny Co1y ÞO3 polycrystals were prepared by a solid-state reaction method.
Automatic measurements of the thermal conductivitykðTÞ and the Seebeck coefﬁcient SðTÞ were performed
by a steady-state heat ﬂow method in a GiffordMcMahon (GM) cycle herium refrigerator. The magnetization MðTÞ was measured by a SQUIDmagnetometer.

*Corresponding author. Fax: +81-19-621-6373.
E-mail address: fujishiro@iwate-u.ac.jp (H. Fujishiro).

3. Results and discussion
Fig. 1(a) shows MðTÞ in the ﬁeld of 0:5 T as afunction of temperature T: In LSCoOð0:01; 0:02Þ; theferromagnetic (FM) transition temperature Tc decreases
with increasing y: In LSCoO(0.05), the maximum ofMðTÞ occurs at about 55 K and its value falls below halfof the saturation moment of La0:80 Sr0:20 CoO3 : Withfurther increase of y (X0:10), the FM order iscompletely quenched out. Fig. 1(b) presents the temperature dependence of rðTÞ: The ferromagnetic
LSCoOð0:01; 0:02Þ samples retain the metallic conduction below Tc ; though rðTÞ increases at low temperatures due to possibly the localization effect. In0921-4526/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0921-4526(02)02603-0

K. Suzuki et al. / Physica B 329–333 (2003) 922–923

Fig. 1. (a) MðTÞ and (b) rðTÞ of La0:80 Sr0:20 ðMny Co1y ÞO3 as afunction of T for y ¼ 0; 0.01, 0.02, 0.05 and 0.10. The symbols
are the same in (a) and (b).

LSCoOð0:05; 0:10Þ; the quench of FM order results in asemiconductive behavior of rðTÞ and rðTÞ values
sharply increase with increasing y:
Fig. 2(a) gives kðTÞ vs. T: The electronic component
ke estimated on the basis of the Wiedemann–Franz law
is also contained in the ﬁgure. kðTÞ is remarkably
reduced by only 1% Mn substitution, which originatesfrom the heavy reduction in the phonon component kph :The kðTÞ reduction seems to saturate for yX0:05: It may
be worthwhile to note that kðTÞ of LSCoOð0:05; 0:10Þ
has a shoulder-like kink at around 50 K; where MðTÞtakes the maximum in these samples. Fig. 2(b) presents
the coefﬁcient S as a function of T: SðTÞ increases withincreasing y and LSCoO(0.10) shows the largest value of
B120 mV=K at around 160 K: For applicational purpose, a large value of the thermoelectric ﬁgure of merit Z(¼ S2 =krÞ is of intrinsic importance. The calculated Zvalues are also given in Fig. 2(b). Pretty large Z values
are realized in LSCoO(0.10). These Z values are,however, more than an order smaller than those of
typical semiconductive thermoelectric materials now onpractical use. We found that the Z values ofLSCoO(0.20) were greatly deteriorated because of toolarge r values.
In summary, MðTÞ; rðTÞ; kðTÞ and SðTÞ weremeasuredforMn
substituted
La0:80 Sr0:20923

Fig. 2. (a) The temperature dependence of thermal conductivitykðTÞ: The estimated electronic contribution ke ðTÞ is also shown.
(b) The Seebeck coefﬁcient SðTÞ and the ﬁgure of merit Z(¼ S2 =krÞ of La0:80 Sr0:20 ðMny Co1y ÞO3 for y ¼ 0; 0.05 and0.10.

ðMny Co1y ÞO3 : The FM-metal phase was preserved ininitial narrow y region ðyp0:02Þ: For yX0:05; the FMorder was greatly damaged or destroyed and the rðTÞbehavior became semiconductive. The phonon component of kðTÞ sharply reduced by Mn substitution for Coand the reduction saturated for yX0:05: The Seebeck
coefﬁcient was noticeably enhanced by Mn substitution,
especially in the semiconductive composition region.
However, the values of the ﬁgure of merit Z remained anorder of magnitude smaller compared to the level of thepossible practical use. The other 3d transition metal
substitution effect on SðTÞ adopting other RE ions areunder way.

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