ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 272–276 (2004) 1796–1797

Thermal conductivity and magnetism in ðCa1X SrX ÞMnO3
Hiroyuki Fujishiro*, Yosuke Fujine, Yasuhisa Mita, Manabu Ikebe
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, JapanAbstract
The thermal conductivity kðTÞ and the magnetization MðTÞ have been measured for the ðCa1X SrX ÞMnO3 system
ð0pX p0:30Þ: The dominant phonon scattering by magnons has been found to be essential to explain the characteristickðTÞ enhancement below the weak-ferromagnetic (WF) transition temperature TN : The WF behavior of MðTÞ isweakened with increasing X and the Dzyaloshinski–Moriya interaction is indicated to be the origin of WF.
r 2003 Elsevier B.V. All rights reserved.
PACS: 65.40.b; 75.30.Kz; 75.25.+z
Keywords: Thermal conductivity; ðCa1X SrX ÞMnO3 ; Magnetization; Phonon scattering

1. Introduction
The physical properties of perovskite-type manganites
RE1 X AEX MnO3 ðRE ¼ trivalent rare-earth ion; AE¼ divalent alkaline-earth ionÞ have been widely investigated for the basic and applicational interests. Inthe electron-doped region ðX B1:0Þ; however, thedetailed reports are rather scarce. Several Ca-based
RE1X CaX MnO3 compounds have been investigated for
X B1:0; because a canted spin structure associated withpretty large negative magnetoresistance (MR) has been
observed for X B0:9 [1–3]. We have already investigated
the thermal conductivity kðTÞ; thermal contractionDLðTÞ=L ð300 KÞ and magnetization MðTÞ for the lightly
electron-doped La1X CaX MnO3 ð0:98pX p1:0Þ system[4]. kðTÞ of CaMnO3 showed a peculiar enhancement
below the weak-ferromagnetic transition temperature
TN ð¼ 130 KÞ: The kðTÞ enhancement was rapidly
suppressed by introduction of Mn3þ spins due to theLa3þ doping and/or the introduction of oxygen deﬁciency.
The reduction of the kðTÞ enhancement could not beexplained by the difference in the lattice states asmonitored by DLðTÞ=L and we examined the possibilityof the magnon heat conduction below TN : In order to*Corresponding author. Tel.: +81-19-621-6363; fax: +8119-621-6373.
E-mail address: fujishiro@iwate-u.ac.jp (H. Fujishiro).

perform detailed analyses of the origin of the kðTÞenhancement, we must collect more detailed information
about the related compounds. In this paper, we investigatekðTÞ and the magnetization MðTÞ for the ðCa1X SrX ÞMnO3 system ð0:0pX p0:30Þ: The substitution of theCa2þ -site by the larger Sr2þ ions is expected not to change
the spin state of Mn4þ ions but to reduce the overall
structure deviation from cubic and introduce lattice
randomness.

2. Results and discussion
ðCa1X SrX ÞMnO3 samples were prepared from stoichiometric mixtures of CaCO3 ; SrCO3 ; La2 O3 andMn3 O4 raw powders. The mixtures were calcined twice
at 1000 C for 24 h in air, pressed into pellets and thensintered at 1450 C for 8 h in air. The samples were heattreated at 1100 C for 24 h in ﬂowing oxygen to improve
the oxygen stoichiometry. All the samples were conﬁrmed to be in a single orthorhombic phase at room
temperature by means of X-ray diffraction. The densityof each sample was about 85% of the ideal one. The
thermal conductivity kðTÞ was automatically measured
in a Gifford–McMahon cycle helium refrigerator asdescribed elsewhere [5]. The magnetization MðTÞ wasmeasured by a SQUID magnetometer under a magnetic
ﬁeld of 0:5 T after zero ﬁeld cooling (ZFC).

0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2003.12.1061

ARTICLE IN PRESS
H. Fujishiro et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1796–1797

1797

Fig. 1. Temperature dependence of the magnetization MðTÞfor the ðCa1X SrX ÞMnO3 system.

Fig. 2. Temperature dependence of the thermal conductivitykðTÞ for the ðCa1X SrX ÞMnO3 system.

Fig. 1 shows the magnetization MðTÞ for the samples.
The MðTÞ curve of the X ¼ 0:0 sample shows a sharp
increase at TN ¼ 130 K; which suggests the onset of theweak ferromagnetism (WF) [6]. The magnitude ofthe WF moment is about 1/80 of the Mn4þ momentðB3 mB Þ: TN gradually increases with increasing X ;e.g., TN ¼ 140 K for X ¼ 0:10 and TN ¼ 150 K for
X ¼ 0:30; respectively. In contrast, the WF moment
decreases with increasing X :ðCa1X SrX ÞMnO3 with orthorhombic structure has
no inversion symmetry at the midpoint of the nearest
Mn4þ spins. In this lattice, Dzyaloshinski–Moriya (DM)antisymmetric exchange interaction is possible, which
results in WF with a canted spin conﬁguration in an
antiferromagnet [7]. The substitution of Ca2þ sites by
the larger Sr2þ ions should bring about the decrease oftilting of the MnO6 octahedra and the local symmetry
between nearest Mn4þ spins approaches cubic. Thiseffect weakens DM interaction and may enhance theantiferromagnetic superexchange interaction, which
results in the decrease of the WF saturation value ofMðTÞ and increase of TN :Fig. 2 shows kðTÞ vs. T: With decreasing temperature,kðTÞ of all the samples remains almost constant down toTN ; where kðTÞ abruptly begins to increase. The kðTÞenhancement below TN is far drastic for the X ¼ 0:0 andis weakened with increasing X : With further decrease ofT; kðTÞ takes a sharp maximum at Tmax ðB40 K for
X ¼ 0:0Þ: Tmax slightly decreases with increasing X ; i.e.,with decreasing kðTÞ of each specimen.
Now let us brieﬂy discuss the origin of the kðTÞenhancement. Since the semiconductive electricalresistivity rðTÞ of the present sample is very large
ðr > 1 O cmÞ; the electron component ke can be entirely
neglected. As inferred from MðTÞ in Fig. 1, 10% Ca2þ site substitution by Sr2þ hardly affects the magnetic
order below TN ; while kðTÞ enhancement is drasticallydepressed in the X ¼ 0:10 sample. This result suggeststhat the enhancement is unlikely to be related with themagnon heat transfer. Then the phonon component kph
should be dominant over the entire temperature range.
In insulators, the phonon–phonon scattering is wellknown to result in the kph peak at a low temperature andkph p1=T dependence at higher temperatures [8]. The
1=T dependence, however, does not explain ourobservation, because dkðTÞ=dT discontinuously changes
around TN : The temperature dependence of kðTÞ ofðCa1X SrX ÞMnO3 is consistent with the kph model withdominant phonon scattering by magnons below TN : The
overall reduction of kðTÞ and the slight decrease of Tmax
with increasing X can be understood as to come fromenhanced phonon scattering of point defect-type causedby the Ca2þ -site substitution [9], while the strength ofthe magnon scattering preserving nearly constant(detailed numerical analyses are to be presentedelsewhere).
In summary, we have analyzed the effect of Ca2þ -site
substitution by Sr2þ on the behavior of MðTÞ andkph ðTÞ in ðCa1X SrX ÞMnO3 : The weak ferromagnetism
was weakened with increasing X as a result of thereduction in the orthorhombic lattice distortion fromcubic. The origin of the apparent kðTÞ enhancement
below TN can be attributed to the phonon scattering bymagnons, which rapidly decreases with decreasing
temperature.

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