J. Phys. Soc. Jpn. Downloaded from journals.jps.jp by on 06/06/17

Journal of the Physical Society of Japan
Vol. 70, No. 9, September, 2001, pp. 2616{2621

Enhanced Phonon Heat Conduction Correlatedwith Induced Ferromagnetic Metallic Phase in Pr0:65 Ca0:35 (Mn1Z CoZ )O3
Hiroyuki F UJISHIRO, Shingo K ANOH, Hajime YAMAZAKI and Manabu IKEBE
Faculty of Engineering, Iwate University, Morioka 020-8551(Received February 28, 2001)

The thermal conductivity ðT Þ, the diusivity ðT Þ and the thermal dilatation dLðT Þ=L havebeen measured for Pr0:65 Ca0:35 MnO3 and Pr0:65 Ca0:35 (Mn1Z CoZ )O3 (Z ¼ 0:02{0:10) crystals
under the magnetic eld of up to 5 T. By application of high magnetic elds or Co substitution for
the Mn site, the ferromagnetic metallic (FM{M) state are induced and ðT Þ and ðT Þ aredrastically enhanced below the ferromagnetic transition temperature TF . The enhancement isclosely related to the relaxation of the local Jahn{Teller lattice distortion due to the increased holemobility in the FM{M phase.
KEYWORDS: Pr1X CaX MnO3 , thermal conductivity, thermal diusivity, thermal dilatation, charge ordering, charge
localization, Co substitution

x1.

Introduction

A key point to explain the colossal magnetoresistance(CMR) of perovskite type manganites, RE1X AEX MnO3
(RE: rare earth elements, AE: alkaline earth elements),
is the role of lattice dynamics in the charge transport. In
the case of small (RE, AE) ion systems, such as RE¼Pr,
Nd and AE¼Ca, the MnO6 network is structurally
distorted and the transfer of the eg electrons between Mn
ion sites signicantly decreases.1{3) Then the small (RE,AE) site ions suppress the double exchange (DE)interaction and other competitive eects such as thesuperexchange interaction or the enhanced electronphonon coupling through the Jahn{Teller (J{T) eect
may become more important to control the physicalproperties of the system. Pr1X CaX MnO3 (PCMO) is acharacteristic small (RE, AE) ion system and has been
widely investigated.4,5) The detailed electronic andmagnetic phase diagrams have been already reported.6,7)
In this system, the so-called CE-type charge/orbital
ordering is realized over a broad Ca concentration X(0:3  X  0:75) probably because of the small oneelectron bandwidth W . For X ¼ 0:50, the long-range
charge order (CO) of Mn3þ /Mn4þ ions is collapsed byapplication of very high magnetic elds of several score
tesla.8) The CO state becomes unstable as the Caconcentration X departs from X ¼ 0:50. In
Pr0:65 Ca0:35 MnO3 , the CO insulating state can easily betransformed to the ferromagnetic (FM) metallic state by
applying the magnetic eld of only a few tesla. Thistransition into FM metal (FM{M) phase at temperature
TF results in so-called eld-induced CMR eect.9) The
studies of the substitution of the Mn site by varioustransition metal elements such as Co, Cr and Fe haveshown the possibility of drastically modifying themagnetic and transport properties.10{12) By the Cr
substitution for Mn sites, with electron conguration
Cr3þ (t2g 3 eg 0 ), the substituted Cr ion was suggested toact as the localized random eld.11) Although the chargeeect, the ion size eect and the magnetic nature of

substituted PCMO have been investigated, the interrelation between the thermal transport and the collapseof the CO state, or the onset of the FM order, is still open
to question.
In this paper, we investigate the thermal conductivityðT Þ, thermal diusivity ðT Þ, magnetization MðT Þ,electrical resistivity ðT Þ and the thermal dilatationanddLðT Þ=Lof
Pr0:65 Ca0:35 MnO3
Pr0:65 Ca0:35 (Mn1Z CoZ )O3 (Z ¼ 0:02{0:10) crystals anddiscuss the relation between the ðT Þ and ðT Þ
anomalies and the collapse of the CO state.
x2.

Experimental

The Pr0:65 Ca0:35 MnO3 and Pr0:65 Ca0:35 (Mn1Z CoZ )O3(Z ¼ 0:02, 0.04, 0.06, 0.10) crystals were prepared fromstoichiometric mixtures of Pr6 O11 , CaCO3 , Mn3 O4 andCoO powders. The mixtures were calcined twice at
1000 C for 24 h in air, pressed into pellets and thensintered at 1500 C for 8 h in air. The measured densitiesof each sample are high, about 85% of the ideal one. Xray diraction analyses at room temperature conrmedthat the samples were in a single orthorhombic phase.
The crystal grain sizes of specimens observed by ascanning electron microscope (SEM) were 20{30 mwhich were independent of the content of the Co
substitution Z. The rectangular shaped samples wereprepared with the typical size of 2:0  2:0  15:0 mm3 .
The thermal conductivity ðT Þ was measured by asteady-state heat ow method between 5 and 300 K
using an automated measuring apparatus with double
radiation shields.13) The thermal diusivity ðT Þ was
measured simultaneously with  by a non-steady statemethod.14) ðT Þ is given by =C, the thermal conductivity  divided by the specic heat C. In insulators, inwhich the heat conduction is entirely phonons,  isexpressed as  ¼ hvi2 ph =3 with the average phonon
velocity hvi and the average scattering time ph . The
magnetic eld of up to 5 T was applied parallel to thelongest side of the sample using a cryocooler-cooled
superconducting magnet. The chromel-constantan ther-2616

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Enhanced Phonon Heat Conduction in Pr0:65 Ca0:35 (Mn1Z CoZ )O3

2617

mocouples with  76 m in diameter were used asthermometers, which had been calibrated in the magnetic eld. The electrical resistivity ðT Þ was measured
by a standard four-terminal method and the thermal
dilatation dLðT Þ=L was measured by a strain-gauge
method. For these measurements, a Giord McMahon(GM) cycle helium refrigerator was used as a cryostat.
The magnetization MðT Þ was measured using a SQUID
magnetometer under a magnetic eld of up to 5 T in theprocesses of zero eld cooling (ZFC), eld cooling (FC)and eld warming (FW).
x3.

Results and Discussion

3.1 Pr0:65 Ca0:35 MnO3
Figure 1 shows the temperature dependence of themagnetization MðT Þ of Pr0:65 Ca0:35 MnO3 under themagnetic eld of 0.5 T and 5 T. We notice threecharacteristic MðT Þ anomalies under the low magnetic
eld of 0.5 T (ZFC), i.e., the CO transition at TCO ¼
220 K, the antiferromagnetic (AF) transition at TN ¼
160 K and the canted antiferromagnetic (CAF) transition at TCAF  90 K. Under the magnetic eld of 5 T, themagnetization MðT Þ shows a drastic enhancement withlarge hysteresis, which means the rst order phasetransition between the charge-ordered AF (CO(AF))
state and the FM state. The FM transition temperaturesof TFc  60 K and TFw  90 K for the FC and FW scan,
respectively, are consistent with those of the reported
phase diagram in the eld-temperature plane.7)
Figure 2 shows the temperature dependence of theelectrical resistivity ðT Þ of the Pr0:65 Ca0:35 MnO3 crystal
under the magnetic eld of 0 T and 5 T. The zero-eld
resistivity is insulating over the entire temperature range
and shows a distinct increase below the charge ordering
temperature TCO ¼ 220 K. Applying the magnetic eldof 5 T, the CO transition temperature TCO slightly
decreases to TCO ¼ 210 K and the CO state collapses at
TFc  60 K on the FC scan where the resistivity abruptly
decreases by about six orders of magnitude. Below TFc ,Fig. 1. Temperature dependence of the magnetization MðT Þ ofPr0:65 Ca0:35 MnO3 under the magnetic eld of 0 H ¼ 0:5 T and
5 T. The MðT Þ data for 0 H ¼ 0:5 T are magnied by the factor of
ve for a clearer view.

Fig. 2. Temperature dependence of the electrical resistivity ðT Þ ofthe Pr0:65 Ca0:35 MnO3 crystal under the zero and 5 T magnetic eld.

ðT Þ continues to decrease with decreasing temperature,
showing the metallic behavior. On the FW scan, ðT Þ
abruptly increases at TFw  80 K and then the CO state
appears again. The transition between the CO phase andthe FM{M phase under applied eld is of typical rstorder, accompanied with large hysteresis.
Figure 3 shows the temperature dependence of thethermal conductivity ðT Þ under the zero and 5 T
magnetic elds. In the zero magnetic eld, ðT Þ
monotonously decreases with decreasing temperature
except for a slight ðT Þ inection around TCO ¼ 220 K,
which is a characteristic feature around the transition
from the paramagnetic (PM) state to the CO(AF)state.15,16) Applying the magnetic eld of 5 T on the
FC scan, ðT Þ is enhanced below TFc  60 K and thenreaches a maximum at 35 K and decreases withdecreasing temperature. On the subsequent FW scan,ðT Þ is drastically reduced at TFw ¼ 75 K and thenmonotonously increases with increasing temperature.
Based on the Wiedemann{Franz law, the electronic
contribution e ðT Þ is estimated to be very small even in
the FM{M state and the ðT Þ enhancement below TFc

Fig. 3. Temperature dependence of the thermal conductivity ðT Þ ofPr0:65 Ca0:35 MnO3 under the zero and 5 T magnetic eld.

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2618

Hiroyuki FUJISHIRO et al.

(or TFw ) can not be explained by that of e ðT Þ.
Accordingly, the conspicuous enhancement and hysteretic behavior of ðT Þ should come from the phononcomponent ph ðT Þ and should be closely related with therst-order transition of the lattice structure accompanied with the transformation from the CE-type CO(AF)state to the FM{M state.17)
Figure 4(a) shows the temperature dependence of thethermal diusivity ðT Þ with and without applied eld.
Applying the magnetic eld of 5 T on the FC scan, ðT Þ
is drastically enhanced below TFc  60 K and on thesubsequent FW scan, ðT Þ is reduced at TFw ¼ 75 K.
With the increase of T up to TCO , ðT Þ continues todecrease monotonously. It should be also noticed thatðT Þ shows a broad local minimum around TCO ¼ 220 K.
This result means that the diusivity ðT Þ is suppressedover a wide temperature range higher than TCO . In theordinary solids, ðT Þ is expected to increase monotonously with decreasing temperature.18) As mentioned
before, the thermal conduction in this compound isalmost entirely due to phonons. In the temperature
region not so low (T  40 K for this study) the specicFig. 4. (a) Temperature dependence of the thermal diusivity ðT Þof Pr0:65 Ca0:35 MnO3 under the zero and 5 T magnetic eld. (b)Temperature dependence of the phonon scattering rate ph 1 for thepresent sample which was estimated from ph 1 ¼ vl 2 =3ph using themeasured longitudinal sound velocity19) and ðT Þ. The contributionof the sound velocity variation to the ðT Þ anomaly at around TCO isalso shown (see text).

(Vol. 70,

heat also comes overwhelmingly from phonons. Then thephonon diusivity ph ðT Þ can be represented as  ph ¼ hvs ilph =3 ¼ hvs i2 ph =3, where lph , ph and hvs i
is the phonon mean free path, the phonon scattering time
and the average phonon velocity. Figure 4(b) shows thetemperature dependence of the phonon scattering rateph 1 for the present sample. Because we have only thelongitudinal sound velocity vl ðT Þ data19) for the sample,
we used the measured vl ðT Þ values (5500 m/s) for theestimation of ph 1 . The contribution of the hvs i2
anomaly to that of ðT Þ is also shown in Fig. 4(b),which is very small and cannot be the main origin for theðT Þ anomaly. It should be noticed that the phonon
scattering rate ph 1 is actually enhanced around TCO .
Because the transverse sound velocity vt ðT Þ is always
Þ (vt  vl =2), hvs i should be smaller
smaller than vl ðTp
than vl (hvs i  vl = 2). Even if we use thus estimated hvs i(4000 m/s), the almost constant diusivity value  0:5 mm2 /s over the temperature range of 140{
200 K in the CO state corresponds to the phonon meanA.
free path as short as lph  4 
Figure 5 shows the thermal dilatation dLðT Þ=L as afunction of T . With decreasing temperature, dL=L showsan anomalous small expansion at TCO , which is acommon feature of the CE-type CO transition in several
perovskite manganite systems.20,21) In zero magnetic
eld, dL=L monotonously decreases from TCO down to20 K. Under the magnetic eld of 5 T on the FC scan,dL=L shows a drastic contraction at around TFc ¼ 60 K.
On the subsequent FW scan, dL=L abruptly increases at
T  60 K and recovers the zero eld values at T  80 K.
As we have mentioned, dL=L shows an expansion at
TCO , where ðT Þ increases with decreasing temperature.
In contrast, dL=L shows a drastic contraction at the FMtransition (TF ) under the applied eld, where ðT Þ
drastically decreases as well. These results indicate aclear correlation between the behaviors of dLðT Þ=L andðT Þ. In the Mn3þ {Mn4þ mixed valence systems such aspresent PCMO, the most direct eect of the charge
carriers on the lattice distortion may be through theJahn{Teller (J{T) eect of the localized Mn3þ spins.

Fig. 5. Temperature dependence of the thermal dilatation dLðT Þ=Lof Pr0:65 Ca0:35 MnO3 .

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2001)

Enhanced Phonon Heat Conduction in Pr0:65 Ca0:35 (Mn1Z CoZ )O3

With increasing mobility of the charge carriers, the local
J{T distortions should be relaxed in the FM{M statebecause the J{T distortions become unstable to follow
the rapid motion of carriers. Accordingly, it is quiteplausible that the conspicuous contraction at TFc and
TFw and the anomalous expansion at TCO directly reect
the processes of the relaxation and enhancement of thelocal J{T distortion.
As we have seen for Pr0:65 Ca0:35 MnO3 , the thermal
conductivity ðT Þ is drastically enhanced in Fig. 3 andthe thermal diusivity ðT Þ in Fig. 4(a) is also greatly
enhanced in the eld-induced FM{M state. These results
suggest that the origin of the strong phonon scattering in
the CO state is the local lattice distortions due to the J{
T eect and the ðT Þ enhancement in the FM{M state
comes from the relaxation of the J{T distortions. The
similar correlation between the ðT Þ enhancement andthe dL=L contraction has already been pointed out for
LCMO (X ¼ 0:25), in which the transition from theparamagnetic insulating (PM-I) state to the FM{M state
occurs via the rst-order-like phase transition.22) The
very strong phonon scattering in the Mn3þ {Mn4þ mixed
valence perovskite systems in the CO and PM-I states
may generally comes from the local J{T distortions. This
picture is also consistent with the phonon scatteringenhancement with decreasing temperature at around the
CO transition shown in Fig. 4(b).

2619

temperature. In contrast, MðT Þ of the Z ¼ 0:06 and 0.10
samples gradually increases below TF , which is similar toordinary ferromagnets.
Figures 7(a) and 7(b) present the electrical resistivityðT Þ of the corresponding samples. ðT Þ of the Z ¼ 0:02
sample shows drastic reductions at TFc  60 K and
TFw  80 K with large hysteresis and ðT Þ behaves
metallic below TFc and TFw . This large hysteresis ofðT Þ conrms that the CO(AF)-FM transition is of therst order, following the general rule for the transitions
between the two-type ordered states. By the 4% Co
substitution shown in Fig. 7(b), the FM transition
temperature increases to TF  100 K and the hysteresisof ðT Þ between TFw and TFc becomes very small. This
small hysteresis of the Z ¼ 0:04 sample suggests that theCO(AF) phase transition occurs only slightly above TF
and the dierence in the free energy of both states is verysmall. By applying the eld of 5 T to the Z ¼ 0:04
sample, TF increases to 135 K and the hysteresis of ðT Þ
completely vanishes. The absence of the hysteresis
indicates that the CO(AF) phase is eliminated and the
FM order sets in directly from the paramagnetic phase.
For Z ¼ 0:06, TF decreases to 80 K and the transition
becomes broader. On the other hand, ðT Þ of the Z ¼
0:10 sample increases monotonously with decreasing

3.2 Pr0:65 Ca0:35 (Mn1Z CoZ )O3
Figure 6 shows the temperature dependence of themagnetization MðT Þ of the Pr0:65 Ca0:35 (Mn1Z CoZ )O3
samples (Z ¼ 0{0:10) under the eld of 0.5 T after ZFC.
A faint local maximum of MðT Þ is discernible for Z ¼
0:02 at around 160 K, which suggests that TCO is reduced
to 160 K by the substitution of 2% Co for the Mn sites.
The 2% Co substitution also make the CO phase
unstable against the FM order below TF  100 K. It isalso to be noticed that the MðT Þ curves of the Z ¼ 0:02
and 0.04 samples behave somewhat anomalously; they
abruptly reach the saturation value with decreasing

Fig. 6. Temperature dependence of the magnetization MðT Þ of thePr0:65 Ca0:35 (Mn1Z CoZ )O3 samples (Z ¼ 0{0:10) under the eld of
0.5 T after ZFC.

Fig. 7. Temperature dependence of the electrical resistivity ðT Þ ofthe Pr0:65 Ca0:35 (Mn1Z CoZ )O3 samples (Z ¼ 0{0:10) under the zeroand 5 T magnetic eld.

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2620

Hiroyuki FUJISHIRO et al.

temperature and shows no anomaly at the FM transition. By applying the eld of up to 5 T, the phasetransition to the metallic state can not be conrmed andthe negative magnetoresistance eect becomes pretty
smaller in this sample.
Figure 8 shows the temperature dependence of thethermal conductivity ðT Þ for the Co substituted
samples. All the ðT Þ data were taken in the coolingrun in the zero eld and, for the Z ¼ 0:02 and 0.04
samples, ðT Þ was also measured in the eld of 5 T. ðT Þof the Z ¼ 0:02 and 0.04 samples are drasticallyenhanced below TFc and the temperature at whichðT Þ is sharply enhanced increases under the appliedeld of 5 T. For the Z ¼ 0:06 and 0.10 samples, there areno ðT Þ anomalies around the FM transition temperature TF even if the magnetic eld of 5 T was applied tothe sample. In this way, the characteristic ðT Þenhancement was observed only for the FM{M transition similar to that of the Z ¼ 0 sample shown in Fig. 3and those for other manganese oxides.22)
Figure 9 shows the temperature dependence of thethermal dilatation dLðT Þ=L for the Co substituted
samples. dLðT Þ=L for the Z ¼ 0:02 and 0.04 samples
shows a clear contraction below TF with decreasing
temperature. The hysteresis between cooling and warming runs is large for Z ¼ 0:02 and relatively small for
Z ¼ 0:04, which is consistent with the behaviors of ðT Þ.
The application of 5 T eld makes increase the(Vol. 70,

Fig. 9. Temperature dependence of the thermal dilatation dLðT Þ=L
for the Pr0:65 Ca0:35 (Mn1Z CoZ )O3 samples (Z ¼ 0{0:06).

temperature, at which dLðT Þ=L sharply contracts, fromthe zero eld 90 K to 130 K, simultaneously wiping out
the hysteretic behavior. dLðT Þ=L for the Z ¼ 0:06
sample (and for the Z ¼ 0:10 sample which is notshown) shows no anomaly around the FM transition.
As we have seen, ðT Þ, ðT Þ and dLðT Þ=L of
Pr0:65 Ca0:35 (Mn1Z CoZ )O3 all behave consistently. For
Z ¼ 0:02, ðT Þ decreases, ðT Þ increases and dLðT Þ=L
contracts at the FM transition temperature TF with thelarge hysteresis. For Z ¼ 0:04, ðT Þ, ðT Þ and dLðT Þ=Lbehave similarly to Z ¼ 0:02 except for much reduced
hysteresis. For Z ¼ 0:06 and 0.10, ðT Þ and dLðT Þ=Lshow no anomaly around TF . Comparing to unsubstituted Pr0:65 Ca0:35 MnO3 , the eect of the Co substitution
for Z ¼ 0:02 and 0.04 is much similar to that of themagnetic eld application, i.e., both eects result in the
introduction of the FM{M phase in the low temperature
ranges. There is an important dierence, however; thesubstitution of Co for Mn ions rapidly depresses theCO(AF) phase, while the application of the magnetic
eld does not damage the CO phase so much seriously, as
is seen from the behaviors of MðT Þ, ðT Þ and dLðT Þ=L in
Figs. 2, 3 and 5. The realization of the FM phase in the
Co substituted PCMO (X ¼ 0:35) may be closely related
to the depression or quenching of the CO phase in thissystem.
Fig. 8. Temperature dependence of the thermal conductivity ðT Þ forthe Pr0:65 Ca0:35 (Mn1Z CoZ )O3 samples (Z ¼ 0{0:10) under the zeroand 5 T magnetic eld. The origin of the y axis is shifted by 10 mW/
cmK for each curve. All the ðT Þ data were measured in the coolingrun and the ðT Þ for the Z ¼ 0:02 and 0.04 samples were alsomeasured under the application of 5 T.

x4.

Summary

The thermal conductivity ðT Þ, thermal diusivityðT Þ, thermal dilatation dLðT Þ=L, electrical resistivityðT Þ and magnetization MðT Þ were measured for

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2001)

Enhanced Phonon Heat Conduction in Pr0:65 Ca0:35 (Mn1Z CoZ )O3

Pr0:65 Ca0:35 MnO3
and
Pr0:65 Ca0:35 (Mn1Z CoZ )O3(Z ¼ 0:02{0:10) crystals under the magnetic eld of up
to 5 T. Important results and conclusions obtained inthis study are summarized as follows;(1) The application of 5 T magnetic eld toPr0:65 Ca0:35 MnO3 induces the ferromagnetic metal(FM{M) state below TFc (on cooling scan) and TFw(on warming scan) with large hysteresis, whichmeans that the charge-ordered antiferromagnetic(CO(AF)) phase gives place to the FM{M phase
below TF (¼ TFc or TFw ) through the rst ordertransition. The thermal conductivity ðT Þ is signicantly enhanced in the FM{M state, but theenhancement is far too large to attribute to that of
the electronic contribution e ðT Þ. Accordingly, theenhancement should be attributed to the reductionin the phonon scattering, which brings about theincrease of the phonon contribution ph ðT Þ.
(2) The thermal diusivity ðT Þ of Pr0:65 Ca0:35 MnO3
decreases with decreasing temperature down to thecharge ordering temperature TCO ¼ 220 K and
exhibits a local minimum around TCO . This factindicates that the phonon scattering is intensied as
the CO phase is approached from the higher
temperature side.
(3) The thermal dilatation dLðT Þ=L shows anomalous
gradual expansion at around TCO and sharp
contraction in the applied eld at the induced FM
transition temperature TF . The expansion at TCO
and the contraction at TF are correlated with theincrease and the decrease in the electrical resistivityðT Þ. The dLðT Þ=L anomalies reect the accumulation and relaxation processes of the local Jahn{
Teller (J{T) distortion caused by the localizationand delocalization of the charge carriers in therespective phases.
(4) Taking account of the results of dLðT Þ=L, the sharpenhancement in ðT Þ and ðT Þ in the induced FM{
M state is considered to originate from theenhanced phonon component caused by the relaxation of the local J{T distortion. The local J{Tdistortions around the Mn3þ spins seem to be verystrong phonon scatterers which limits the phononmean free path lph to very small values in the COphase and in the disordered phase above TCO . The
observed minimum of ðT Þ at around TCO can also
be understood by this picture.
(5) By the Co substitution for the Mn sites, the FM{Mphase is induced in Pr0:65 Ca0:35 (Mn1Z CoZ )O3
under zero magnetic eld. ðT Þ and ðT Þ isenhanced and dL=L sharply contracts below TF ,which is very similar to the behaviors of
Pr0:65 Ca0:35 MnO3 under applied elds. These results
again demonstrate the strong phonon scattering due
to the local lattice distortions in the insulating

2621

phases and their relaxation in the metallic phase.
(6) There is an important dierence, however, between
the eects of the magnetic eld application and the
Co substitution on Pr0:65 Ca0:35 MnO3 . The Co
substitution rapidly quenches the charge order,
drastically lowering TCO , while the application ofmagnetic elds does not alter TCO so seriously. The
appearance of the FM phase is closely related to thedepression or quenching of the charge order phase in
Pr0:65 Ca0:35 (Mn1Z CoZ )O3 .
Acknowledgments
The authors wish to thank Prof. T. Fukase of Tohoku
University for the collaboration in the sound velocity
measurements. Prof. K. Noto of Iwate University is
acknowledged the usage of the cryocooled superconducting magnet.
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