Physica B 316–317 (2002) 331–334

Anomalous phonon transport enhancement at ﬁrst-orderferromagnetic transition in (Gd,Sm,Nd)0.55Sr0.45MnO3Hiroyuki Fujishiro*, Shiro Sugawara, Manabu Ikebe
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, JapanAbstract
The thermal conductivity kðTÞ; thermal dilatation dLðTÞ=L and the electrical resistivity rðTÞ were measured for(Gd1YSmY)0.55Sr0.45MnO3 (GSSMO) and (Nd1ZSmZ)0.55Sr0.45MnO3 (NSSMO) systems. For 0:50pY p1:0 ofGSSMO and 0:75pZp1:0 of NSSMO, the ferromagnetic (FM) transition is of the ﬁrst order, while it is of the second
order for 0pZp0:50 of NSSMO. In the samples which exhibit the ﬁrst-order FM transition, kðTÞ is enhanced anddLðTÞ=L contracts abruptly just below the FM transition temperature Tc : These behaviors can be interpreted asoriginating from the reduction of the local lattice distortion below Tc : r 2002 Elsevier Science B.V. All rights reserved.
Keywords: Thermal conductivity; Ferromagnetic transition; Manganese oxides; First-order transition

Carrier-doped perovskite manganese oxides,(RE1XAEX)MnO3 (RE: rare-earth ion, AE: alkaline earth ion), undergo a variety of dramatic
phenomena such as the colossal magnetoresistance(CMR), the insulator–metal (I–M) transition andthe charge/orbital (CO/OO) ordering. In addition
to the double-exchange mechanism, the CMR
effect is believed to involve the strong electron–
phonon interaction, which may originate from theJahn–Teller (J–T) effect due to Mn3+ ions. At
around X B0:50; the ferromagnetic (FM) metallic
state (FM–M) becomes unstable against thecharge-ordered antiferromagnetic (AFM) state inthese systems. In Sm1XSrXMnO3, for example,
the ﬁrst-order FM transition takes place at aroundX ¼ 0:45 [1], where the FM state competes with
the CO/OO state. This ﬁrst-order FM transition
has been theoretically predicted as being caused by*Corresponding author. Tel.: +81-19-621-6363; fax: +8119621-6373.
E-mail address: fujisiro@iwate-u.ac.jp (H. Fujishiro).

a ﬂuctuation-induced mechanism [2], and is
expected to be closely related with the average
ionic radius of (RE,AE) site through its effect on
the one electron bandwidth W : The effect of thedifference in the ionic radius can be systematicallyinterpreted in terms of the tolerance factor f[1], which takes the value of 1 for the cubic
perovskite structure without distortion. In thispaper, we investigate the thermal conductivity kðTÞ and thermal dilatation dLðTÞ=Lof (Gd1YSmY)0.55Sr0.45MnO3 (GSSMO) and(Nd1ZSmZ)0.55Sr0.45MnO3 (NSSMO) systems
mainly at a ﬁxed nominal hole concentration ofX ¼ 0:45 and discuss the relation between the ﬁrstorder FM transition and the average ionic radiusof the (RE,Sr) site (or the tolerance factor f ).
(Gd1YSmY)0.55Sr0.45MnO3 (0pY p1:0) and(Nd1ZSmZ)0.55Sr0.45MnO3 (0pZp1:0) samples
were prepared by a conventional solid-state reaction method. (Nd1ZSmZ)1XSrXMnO3 (Z ¼ 0and 1.0; X ¼ 0:3020:50) samples were also fabricated. The mixtures of raw powders were calcined

0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
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twice at 10001C for 24 h in air, pressed into pellets
and sintered at 15001C for 8 h in air. kðTÞ wasautomatically measured by a steady-state heatﬂow method [3] and dLðTÞ=L was monitored by
the change in the resistivity of the strain gauge.
The electrical resistivity rðTÞ was measured by astandard four-terminal method. The average ionicradius of (RE,Sr)pﬃﬃﬃ site, rA ; and the tolerance factor
f ¼ ðrA þ rO Þ= 2ðrB þ rO Þ (rB : average Mn ionicradius and rO : O ionic radius), was calculated fromthe tabulated radii of (RE,Sr) site cations with 12( Sm3+=1.24 A,(fold coordination (Gd3+=1.21 A,3+2+((Nd =1.27 A, Sr =1.44 A) [4].
Fig. 1 shows the temperature dependence of theresistivity rðTÞ: rðTÞ shows a sharp decrease justbelow the FM transition temperature Tc andbehaves metallic below Tc except for the Y ¼ 0and 0.25 samples. From the magnetization measurement, the Y ¼ 0 and 0:25 samples did notshow the FM transition. For the 0:50pY p1:0 ofGSSMO and 0:75pZp1:0 of NSSMO samples(with relatively small rA ð¼ 1:321–1:334 A),ananomalous reduction and the hysteretic behaviorof rðTÞ characteristic of the ﬁrst-order transition
was observed below Tc : On the other hand, theFM transition in the samples for 0pZp0:50( is of the typical second order. Tc(rA X1:338 A)
increases and the r values decrease with increasingionic radius of RE site (rGd orSm orNd ) probably
because of the reduction in the structural depar-

ture from the ideal cubic. In the present GSSMOand NSSMO systems, the tolerance factor fapproaches 1, the ideal cubic value, with increasing rA :Fig. 2 shows the temperature dependence of thethermal conductivity kðTÞ of the typical samples.
kðTÞ is almost entirely contributed by the phononcomponent in the present systems. Corresponding to the ﬁrst-order FM transition for respective samples (0:50pY p1:0 of GSSMO and
0:75pZp1:0 of NSSMO), a step-like kðTÞ enhancement can be seen just below Tc : For thesecond-order FM transition (0pZp0:50), kðTÞshows a gentle increase below Tc : In our previous investigations for La1XCaXMnO3 andLa1XSrXMnO3 systems, the kðTÞ anomalies
around Tc is closely correlated with those of thethermal dilatation dLðTÞ=L [5,6]. Fig. 3 showsdLðTÞ=L for the NSSMO system. The sharp andhysteretic reduction of dLðTÞ=L was observed justbelow Tc only for the Z ¼ 1:0 and 0.75 samples. In
these samples, the decrease of the temperature
derivative of dLðTÞ=L is also noticeable above Tc ;which means the gradual accumulation of thelattice distortion on approaching Tc from highertemperatures. In contrast, for the Zp0:50 samples, the anomaly of dLðTÞ=L around Tc is gentle
and become weaker with decreasing Z: The latticeFig. 1. The temperature dependence of the resistivity rðTÞfor the (Gd1YSmY)0.55Sr0.45MnO3 (0pY p1:0) and(Nd1ZSmZ)0.55Sr0.45MnO3 (0pZp1:0) samples.

Fig. 2. The temperature dependence of the thermal conductivity kðTÞ of the typical samples. The closed and open arrows
indicate the ﬁrst- and second-order FM transition.

H. Fujishiro et al. / Physica B 316–317 (2002) 331–334

333Fig. 3. The temperature dependence of the thermal dilatationdLðTÞ=L for the NSSMO system. The closed and open arrows
represent the ﬁrst- and second-order FM transition.

contraction at and below Tc exempliﬁed indLðTÞ=L may be at least partly attributable tothe reduction of the local J–T distortion below Tc
as a result of the increased itinerancy of the charge
carriers. For T > Tc ; the random local J–T
distortions strongly scatter phonons, limitingkðTÞ to very small values, while the local J–T
distortions are relaxed for ToTc and the phonon
scattering is diminished in the FM–M phase.
Fig. 4 shows the FM transition temperature Tc
versus the average ionic radius rA : The data for
La1XSrXMnO3 system by us [7] and for the(Nd1ySmy)1/2Sr1/2MnO3 system by Kuwahara
et al. [8] are also included. The upper horizontal
axis shows the tolerance factor f for the X ¼ 0:45system. The closed circles, open circles and open
squares represent the FM transition temperatures
Tc of the ﬁrst-order accompanied with metallicconduction, the second-order transition withmetallic conduction and the second-order transition with insulating behavior (FM–I), respectively.
It should be noted that there is a peculiar ﬁrstorder transition region into the FM-metallic
phases in the Tc  rA plane. This region is situatedbetween the charge-ordered AFM insulating andthe FM insulating phases. Millis et al. theoretically
predicted that the ﬁrst-order transition may bepossible between paramagnetic insulator and FMFig. 4. The FM transition temperature Tc versus the average
ionic radius rA and the tolerance factor f (scale on the upperabscissa) for RE1XSrXMnO3 system (RE=Gd, Sm, Nd, La).
~ shows the ﬁrst-order FM transition temperature by
Kuwahara et al. [8].

metallic phases for an intermediately strong
electron–phonon coupling [9]. These ﬁrst-order
FM–M region shown in Fig. 4 is similar to the caseof (La1yPry)1X(Ca1ZSrZ)XMnO3 (X B0:25)[10].
In summary, the thermal conductivity
kðTÞ; thermal dilatation dLðTÞ=L and theelectrical resistivity rðTÞ were measuredfor (Gd1YSmY)0.55Sr0.45MnO3 (GSSMO) and(Nd1ZSmZ)0.55Sr0.45MnO3 (NSSMO) systems.
The thermal conductivity was very small above
the FM transition temperature Tc probably
because of strong phonon scattering by the local
and spatially random J–T lattice distortions. ForToTc ; the local distortion was partially relaxed
and kðTÞ was enhanced. The degree of thedistortion relaxation seems to be more substantial
for increasing charge carrier mobility, resulting inhigher phonon thermal conduction. The relation
between the ﬁrst-order FM transition and theaverage ionic radius of the (RE,Sr) site, rA wasdiscussed. There is a peculiar region of the ﬁrstorder FM-metallic phase in the Tc rA plane,

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H. Fujishiro et al. / Physica B 316–317 (2002) 331–334

which is situated between the charge-ordered
AFM insulating and the FM insulating phases.

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