Physica B 307 (2001) 57–63

Thermal conductivity anomalies around antiferromagnetic
order in La0.50Sr0.50MnO3 and Nd0.50Sr0.50MnO3 crystals
Hiroyuki Fujishiro*
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan
Received 5 March 2001; received in revised form 17 May 2001; accepted 11 June 2001

Abstract
The thermal conductivity kðTÞ has been measured for sintered-La0.50Sr0.50MnO3 (S-LSMO) and Nd0.50Sr0.50MnO3fabricated by the ﬂoating zone method (FZ-NSMO) in magnetic ﬁelds up to 5 T. Both crystals commonly show a ﬁrst
order transition from the ferromagnetic (FM) to the antiferromagnetic (AF) phase with decreasing temperature andkðTÞ shows seemingly similar reductions just below the N!eel temperature TN in zero magnetic ﬁeld. The ﬁeld
dependence of kðTÞ is, in contrast, quite diﬀerent. kðTÞ of FZ-NSMO increases over the entire temperature range below
TN in applied ﬁelds because of the electronic-component (ke ) enhancement. For S-LSMO, where the heat conduction ismainly due to phonons, kðTÞ hardly depends on the magnetic ﬁeld except for the change corresponding to the T N shift.
Analyses suggest that the phonon scattering by conduction electrons is very strong in the metallic FM phase of FZNSMO. r 2001 Elsevier Science B.V. All rights reserved.
PACS: 65.40.b; 75.30.Kz; 72.15.Eeb; 66.30.Xj
Keywords: Thermal conductivity; Nd0.50Sr0.50MnO3; La0.50Sr0.50MnO3; Antiferromagnetic order; Magnetic structure; Phonon
scattering

1. Introduction
A series of distorted perovskite manganeseoxides of the formula, RE1XAEXMnO3 (RE:lanthanide earth ions, AE: alkaline earth ions),
exhibit a variety of dramatic physical properties asa result of various competing interactions such as
double exchange, antiferromagnetic (AF) super-*Corresponding author. Tel.: +81-19-621-6363; fax: +8119-621-6373.
E-mail address: fujisiro@iwate-u.ac.jp (H. Fujishiro).

exchange, charge order (CO), orbital order (OO)
and Jahn–Teller eﬀect [1–3]. A typical manganeseoxide such as La1XCaXMnO3 shows a phasetransition from a paramagnetic insulating (PM-I)
state to a ferromagnetic metallic (FM-M) state
upon cooling for 0:20oXo0:45 and exhibits
colossal magnetoresistance [4,5]. For further carrier doping at around X ¼ 0:5 with Mn3+/
Mn4+B1 : 1, the FM-M state becomes unstable against the AF state, which has been
intensively studied for Nd0.50Sr0.50MnO3 [6,7],
Pr0.50Sr0.50MnO3 [6], La0.50Sr0.50MnO3 [8] and soon. Among these systems, the Nd1XSrXMnO30921-4526/01/$ -see front matter r 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 6 2 4 - X

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H. Fujishiro / Physica B 307 (2001) 57–63(NSMO) system is a typical one well investigatedat around X ¼ 0:5; where the magnetic, structuraland electronic structures are rather complicated[6,9]. The X ¼ 0:50 sample of NSMO shows a
FM-M state below Tc E270 K and transforms toan AF insulating state below TN E150 K with the
CE-type magnetic structure with CO. Recently,
the complicated phase segregation between A-type
AF, CE-type AF and FM phases was observed byneutron diﬀraction [10]. In contrast, the magnetic
structure of La1XSrXMnO3 (LSMO) around
XB0:5 is the A-type AF order without CO, which
was conﬁrmed for the La0.46Sr0.54MnO3 single
crystal [11,12] and the La0.50Sr0.50MnO3 polycrystal [13].
Several theoretical and experimental studies
have pointed out that the electron–phonon coupling and the local lattice distortion of MnO6octahedra play important roles in the anomaloustransport properties of doped manganites [14,15].
The thermal conductivity k is one of the valuable
tools to investigate the lattice dynamics near phasetransitions [16–19] as well as in neutron andelectron diﬀraction studies. For the X ¼ 0:50
manganite samples, the thermal conductivitykðTÞ seems to behave quite diﬀerently at low
temperatures for various AF spin structures. kðTÞof the Pr0.50Sr0.50MnO3 crystal with the A-typespin structure drastically suppressed below TN
from the FM to AF state [16,18]. However, kðTÞ ofPr0.50Ca0.50MnO3 and La0.50Ca0.50MnO3 with the
CE-type AF order shows a small anomaly at the
CO transition temperature TCO and/or at TN fromthe paramagnetic (PM) state [18,19]. The relations
between the kðTÞ anomaly around TN andthe magnetic structure and/or the existence ofCO have not been thoroughly investigated. The
application of a magnetic ﬁeld to the manganese oxides systematically alters the electricalresistivity r; Tc and TN : Therefore, the ﬁeld
dependence of the kðTÞ is expected to provide
useful information about the origin of the kðTÞanomaly around TN : We report the temperaturedependence of the thermal conductivity kðTÞfor La0.50Sr0.50MnO3 and Nd0.50Sr0.50MnO3
samples in applied ﬁelds up to 5 T and discuss
the origin of the kðTÞ anomalies below T N for
both systems.

2. Experimental
The sintered-La0.50Sr0.50MnO3 polycrystal (SLSMO) was prepared by a standard solid-state
reaction method. The preparation procedure wasdescribed in detail elsewhere [8]. The size of the SLSMO sample is 3.2  2.5  12.0 mm3 of which themeasured density is 87% of the ideal one. The grain
sizes observed by a scanning electron microscopewere distributed between 20 and 40 mm. The
Nd0.50Sr0.50MnO3 crystal was prepared by a ﬂoatingzone (FZ) method (FZ-NSMO). The raw powdersof Nd2O3, SrCO3 and Mn3O4 were calcined twice at
10001C for 24 h in air and then pressed to a bar
shape of B6 mm + in diameter and sintered twice
at 15001C for 8 h in air. A lamp image FZ furnacewith quadruple ellipsoidal millers was utilized forthe crystal growth. The crystal growth was performed at 8 mm/h in ﬂowing air. The crystal wasgrown nearly parallel to the c-axis of the orthorhombic structure as was determined by X-ray
diﬀraction analysis. The obtained crystal was heattreated at 17001C for 24 h in ﬂowing O2 gas toremove the thermal stress during the growth and toensure the oxygen stoichiometry. The dimension of
the FZ-NSMO crystal is 2.5  2.5  6.0 mm3 inwhich the length of 6 mm is along the growth
direction. The rectangular sample includes several
single crystal grains with a size as large as B1 mm.
The thermal conductivity kðTÞ was measured bya steady-state heat-ﬂow method between 5 and300 K using an automated measuring apparatus
with double radiation shields [20]. The magnetic
ﬁeld was applied parallel to the heat ﬂow. The
chromel–constantan thermocouples with 76 mm +in diameter were used as thermometers, which had
been calibrated in the magnetic ﬁeld. The electricalresistivity rðTÞ was measured by a standard fourterminal method. For kðTÞ and rðTÞ measurements, a Giﬀord McMahon cycle helium refrigerator was used as a cryostat. The magnetic ﬁeld
up to 5 T was applied for the kðTÞ and rðTÞmeasurements, using a cryocooler-cooled superconducting magnet. The magnetization MðTÞ was
measured by use of a superconducting quantum
interference device (SQUID) magnetometer inmagnetic ﬁelds up to 5 T in zero ﬁeld cooling(ZFC) and ﬁeld cooling (FC) scans.

H. Fujishiro / Physica B 307 (2001) 57–63

59

3. Experimental results
3.1. Sintered-La0.50Sr0.50MnO3 (S-LSMO) crystal
Figs. 1 and 2 show the temperature dependenceof the magnetization MðTÞ and the electricalresistivity rðTÞ of the S-LSMO crystal in amagnetic ﬁeld. As can be seen from the MðTÞcurve in a ﬁeld of 0.5 T [19], this specimen shows a
FM transition at Tc ¼ 290 K and transforms to
AF order below TN ¼ 160 K upon cooling. The
phase transition to the AF order shows hysteretic
behavior. The FM transition temperature Tc
increases and the AF transition temperature TN
decreases with increasing applied ﬁeld. It should benoticed that the residual weak-FM moment exists
below TN in higher magnetic ﬁelds. In Fig. 2, thezero-ﬁeld resistivity rðTÞ is slightly inﬂected at
Tc ¼ 290 K and then increases moderately withdecreasing temperature. Below TN E160 K, rðTÞsomewhat sharply increases with hysteresis andthen further increases with decreasing temperature. However, the rðTÞ increase in the vicinity of
TN is only by a factor of about 2 and rather small,which may be characteristic for the A-type AF
order without CO. The rðTÞ value decreases withincreasing magnetic ﬁeld, but the metallic rðTÞbehavior cannot be conﬁrmed below TN : Metallic

Fig. 1. Temperature dependence of the magnetization MðTÞ ofthe sintered-La0.50Sr0.50MnO3 (S-LSMO) crystal in magnetic
ﬁelds after ZFC. The MðTÞ data for the FC in m0 H ¼ 0:5 T are
also presented.

Fig. 2. Temperature dependence of the electrical resistivityrðTÞ of the S-LSMO crystal in applied ﬁelds.

behavior of rðTÞ was reported for a FZ-LSMO(X ¼ 0:54) single crystal with A-type AF order by
Akimoto et al. [11] The A-type LSMO (X ¼ 0:50)
thin ﬁlm was fabricated by the pulsed laser
deposition (PLD) method on a SrTiO3 substrate,
where the ratio of the lattice constants (c=a) could
be set to be E1 and the AF and metallic rðTÞbehavior was observed below TN [21]. The in-plane
resistivity of the Nd0.45Sr0.55MnO3 single crystal
with the A-type AF structure is metallic below TN
and the out-of-plane resisitivity is semiconductive[22]. The semiconductive rðTÞ behavior of thepresent S-LSMO crystal shown in Fig. 2 may result
from the eﬀect of the c-axis oriented grains or somelocalization eﬀect. Since sintered polycrystals areeasy to realize a precise control of the hole content
X and a uniform hole distribution, the AF
transition from the FM order usually becomes
sharper than that of the FZ-LSMO crystal.
Fig. 3 shows the temperature dependence of thethermal conductivity kðTÞ of the S-LSMO crystal
in magnetic ﬁeld. The kðTÞ curves in the FC runs
are presented. In zero ﬁeld, an anomalous reduction of kðTÞ is observed just below TN ¼ 160 K
and then kðTÞ decreases with decreasing temperature. Below TN ; the application of the magnetic
ﬁelds hardly aﬀects kðTÞ except for the shift of thekðTÞ reduction point corresponding to the decrease of TN : For T > 160 K, kðTÞ very slightly
increases with increasing magnetic ﬁeld.

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H. Fujishiro / Physica B 307 (2001) 57–63

Fig. 3. Temperature dependence of the thermal conductivitykðTÞ of the S-LSMO crystal in applied ﬁelds after the FC. The
electronic thermal conductivity ke ðTÞ at 0 and 5 T is also shown.
For comparison, kðTÞ data of the sintered La0.890Sr0.110MnO3crystal in 0 T is included, in which the FM insulating state isstable over the entire range below Tc :Fig. 4. Temperature dependence of the magnetization MðTÞ ofthe FZ-Nd0.50Sr0.50MnO3 (FZ-NSMO) crystal in applied ﬁelds
after ZFC. The MðTÞ data for the FC run for m0 H ¼ 0:5 T are
also presented.

3.2. FZ-Nd0.50Sr0.50MnO3 (FZ-NSMO) crystal
Figs. 4 and 5 show MðTÞ and rðTÞ of the FZNSMO crystal in magnetic ﬁeld. The crystal showsan FM transition at Tc ¼ 265 K and an AF
transition at TN ¼ 160 K. When the higher magnetic ﬁeld is applied to the sample, Tc increases
and TN decreases gradually, which is the behavior
similar to that of the S-LSMO crystal shown in
Fig. 1. In Fig. 5, rðTÞ in the zero ﬁeld decreases
below Tc ¼ 265 K and abruptly increases below
TN ¼ 150 K about two orders of magnitude withlarge hysteresis. It is to be noticed that thetemperature dependence of rðTÞ of the present
FZ-NSMO crystal are almost equal to that of the
FZ-Nd0.51Sr0.49MnO3 crystal reported by Kajimoto et al. [9]; the common features are the metallic
rðTÞ behavior below TN and the relatively lowrðTÞ value at low temperatures (B101 mO cm),
which make a sharp contrast with the very large
semiconductive rðTÞ of the FZ-Nd0.50Sr0.50MnO3single crystal [6]. In the present FZ-NSMO crystal,
the large enhancement of rðTÞ at TN makes amarked contrast with the small enhancement in SLSMO and likely to correspond to the occurrenceof CO in FZ-NSMO. However, it is also to be

Fig. 5. Temperature dependence of the electrical resistivityrðTÞ of the FZ-NSMO crystal in applied ﬁelds.

noticed that rðTÞ of FZ-NSMO is rather metallic
at low temperatures and this metallic rðTÞ below
TN might suggest the coexistence of very small FM
phase region with CE-type AF phase. The absolutevalue of rðTÞ below TN decreases with increasing
applied ﬁeld and rðTÞ at 5 T is about two orders ofmagnitude smaller than that in 0 T.
Fig. 6 shows the temperature dependence of thethermal conductivity kðTÞ of FZ-NSMO under themagnetic ﬁelds in the FC runs. At zero ﬁeld, kðTÞshows a shallow dip around Tc ¼ 265 K and an

H. Fujishiro / Physica B 307 (2001) 57–63

61

Fig. 6. Temperature dependence of the thermal conductivitykðTÞ of the FZ-NSMO crystal in applied ﬁelds in the FC runs.

anomalous reduction just below TN ¼ 150 K [23].
Below TN ; kðTÞ gradually increases with increasing
magnetic ﬁeld and above TN kðTÞ also increases
slightly. Hysteretic behavior of kðTÞ below TN was
also observed at m0 H ¼ 0 and 1 T which, however,
was not conﬁrmed at m0 HX3 T (not shown in theﬁgure).

4. Discussion
The heat conduction in magnetic materials canbe represented as the sum of phonon (kph ),electron (ke ) and magnon (km ) contributions. In
Fig. 3, we present kðTÞ of S-LSMO(X ¼ 0:11)
which is a FM insulator. kðTÞ of this sample,
which is very small over the entire temperature
regime, shows almost no anomaly or enhancement
below Tc : A close examination of the data suggeststhat the kðTÞ enhancement due to km is about
B1 mW/cm K in the LSMO compound if it exists.
Because there seems to be no special reason that
km is enhanced in S-LSMO(X ¼ 0:50) and in FZNSMO(X ¼ 0:50), we neglect the km contribution
in the present analyses. A similarly very small km
was given also by Cohn et al. [16] for the
La1XCaXMnO3 system.
By using the Wiedemann–Franz (WF) law(ke ¼ L0 T=r; L0 ¼ 2:45  108 WO/T2: the Lorentz number), we can estimate the electronic

Fig. 7. Temperature dependence of (a) the electronic thermal
conductivity ke ðTÞ and (b) the phonon thermal conductivity
kph ðTÞ of the FZ-NSMO crystal in applied ﬁelds. The
Wiedemann–Franz law was applied for estimation of ke ðTÞ:component ke ðTÞ of these crystals using theelectrical resistivities. The estimated ke ðTÞ can beseen as an upper limit of the electronic part in kðTÞ[24]. Fig. 7(a) shows the electronic thermal conductivity ke ðTÞ for the FZ-NSMO crystal. In therespective magnetic ﬁelds, ke increases withdecreasing T (oTc ) and reaches a maximum at
T N and then suddenly decreases with decreasing
temperature T (oTN ). The electronic component
has a large proportion to the total k (e.g., ke
immediately above TN ð¼ 160 KÞ is B24% of thetotal k at 0 T) and increases with increasing
magnetic ﬁeld. Fig. 7(b) shows the temperaturedependence of the phonon thermal conductivity
kph ðTÞ of the FZ-NSMO crystal for each magnetic

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H. Fujishiro / Physica B 307 (2001) 57–63

ﬁeld, which was estimated by subtracting theelectronic contribution ke from the measured k:What is to be noticed is that the phononic
component kph is sharply enhanced just below
TN ; while it is nearly independent of the appliedmagnetic ﬁeld in the temperature ranges above TN
ð> 150 KÞ and at low temperature (To100 K).
These results indicate that the observed magnetic
ﬁeld dependence of k of FZ-NSMO can beinterpreted as to come mainly from ke : The
application of the WF law with the standard L0
value is justiﬁable in FZ-NSMO because kph
hardly depends on the applied ﬁeld. In general,kph ðTÞ in condensed matter is determined by thestrength of various phonon scattering mechanisms
such as grain boundaries, various lattice defects,
conduction electrons and so on [24]. In ordinary
metals, the conduction electrons behave as mainheat carriers and, at the same time, act as powerful
scatterers to phonons. In this sense, the FZNSMO crystal is a pretty good metal in the
ferromagnetic state between TN oToTc ; where
the phonon scattering by conduction electrons isvery eﬀective. The phonon scattering by conduction electrons suddenly disappears in the chargeordered insulating phase below TN ; which is themain origin for the kph enhancement below TN :Because of the large resistivity of the S-LSMO
crystal, ke is very small. For example, thetemperature dependence of ke ðTÞ is shown in
Fig. 3. ke is about 5% of the total k at 300 K anddecreases with decreasing temperature. Therefore,
the kðTÞ reductions below TN of the measuredkðTÞ shown in Fig. 3 mainly result from those ofkph ðTÞ; which should originate from the enhancements in the phonon scattering related with thephase transition from FM to AF order.
There may be some problems about the applicability of the WF law to ke in polycrystals becauseof the diﬀerence in the electrical and thermaltransport through the grain boundaries. It isconceivable that heat transport between grains
can be easier than electric transport, thus making
the eﬀective Lorentz number much larger. Actually, several sintered LSMO samples showedanomalously large rðTÞ values caused by inappropriate sintering or annealing processes. We
carefully omitted such samples. In Fig. 3, kðTÞ of

S-LSMO does not show any clear dependence on
the applied ﬁeld for T > TN ð0TÞ and T > TN ð5TÞ:
Because rðTÞ of S-LSMO is larger in spite of aclear ﬁeld dependence, this fact indirectly guarantees that ke is really small in S-LSMO.
As we have seen, the ﬁeld dependence of thephonon thermal conductivities kph ðTÞ of the FZNSMO and S-LSMO crystals shows quite diﬀerent
behavior below TN : The ﬁeld dependence of kðTÞbelow TN for the FZ-NSMO crystal shown in
Fig. 6 is caused by the enhancement of ke as aresult of the colossal magnetoresistance. The
phonon scattering by conduction electrons, which
is dominant at TN oToTc ; diminishes below TN :It is possible that, actually, the reduction of thephonon scattering by conduction electrons screens
the enhancement of the phonon scattering due toother mechanisms which appears at the ﬁrst ordermagnetic transition in FZ-NSMO. It is also
worthwhile to note that the phonon scattering byconduction electrons is almost absent in the FM
semiconductive state of S-LSMO, making amarked contrast to FZ-NSMO.

5. Conclusion
The thermal conductivity kðTÞ has been measured for the sintered-La0.50Sr0.50MnO3 (S-LSMO)
and FZ-Nd0.50Sr0.50MnO3 (FZ-NSMO) crystals inmagnetic ﬁelds up to 5 T. The origin of the kðTÞanomaly around the Ne! el temperature TN was
discussed. Important experimental results and thenoticeable points obtained in this study aresummarized as follows:(1) In these crystals, kðTÞ shows seemingly
similar reductions just below TN in zero magnetic
ﬁeld. However, the ﬁeld dependence of kðTÞ isquite diﬀerent. kðTÞ of FZ-NSMO increases below
TN with increasing magnetic ﬁeld, which can beinterpreted as caused by the enhancement of theelectronic thermal conductivity ke ðTÞ: On theother hand, kðTÞ of the S-LSMO crystal does
not depend on the magnetic ﬁeld except for theshift of TN because of the smaller ke : The
reduction of kðTÞ below TN in S-LSMO comesH. Fujishiro / Physica B 307 (2001) 57–63

from the reduction in the phonon component kph
caused by the phonon scattering enhancement.
(2) The phonon thermal conductivity kphðTÞ isenhanced below TN in FZ-NSMO as shown in
Fig. 7(b). The origin of the kph enhancement is theelimination of the strong phonon scattering by the
conductive charge carriers below TN : It is plausiblethat kph ðTÞ would be reduced also in FZ-NSMO,
if it is not for the phonon scattering by the charge
carriers in the FM metallic phase.

Acknowledgements
The author wishes to thank S. Sugawara and
T. Mikami of Iwate University for their assistancein the sample preparation and in the thermal
conductivity measurements. The author also
wishes to thank Prof. K. Noto of Iwate University
for the collaboration in using the cryocooled
superconducting magnet. Useful discussion with
Prof. M. Ikebe is greatly acknowledged.

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