J. Phys. Soc. Jpn. Downloaded from journals.jps.jp by on 06/06/17
Journal of the Physical Society of Japan
Vol. 69, No. 7, July, 2000, pp. 2082-2086

Thermal Transport Anomaly Associatedwith Weak Ferromagnetism in CaMnO3
Hiroyuki Fujishiro, Shuichi Ohshiden and Manabu Ikebe
Faculty of Engineering, Iwate University, Morioka 020-8551(Received December 9, 1999)

The thermal conductivity κ(T ), thermal diﬀusivity α(T ) and magnetization M (T ) havebeen measured for sintered CaMnO3 and La1−x Cax MnO3 in lightly electron-doped region(0.98 ≤ x ≤ 1.0). κ(T ) and α(T ) of CaMnO3 show an anomalous enhancement below the
weak ferromagnetic transition temperature TN . The enhancement is rapidly reduced by introduction of a small amount of Mn3+ ions in La1−x Cax MnO3 . The analyses suggest the possibility
that the enhancement comes from the contribution of the heat transport by magnetic excitations.
KEYWORDS: CaMnO3 , thermal conductivity, thermal diﬀusivity, weak ferromagnetism, magnetic heat transport§1.

necessary.

Introduction

Charge carrier doping into antiferromagnetic insulators with perovskite-related structures provides astage where a variety of attractive phenomena suchas the high Tc superconductivity, anomalous metallic
state, metal-insulator transition, stripe order etc. areplayed. The background of the drama is the strong
correlation between doped carriers, which has become
one of main themes of recent physical concern. Recently, physical properties of perovskite-type manganites RE1−x AEx MnO3 (RE = trivalent rare-earth ion,
AE = divalent alkaline-earth ion) have been widely investigated in the hole-doped region (x < 0.5). In
the electron-doped region (x ≈ 1.0), however, the detailed reports are rather scarce.1-7) Several Ca-based
RE1−x Cax MnO3 compounds have been recently investigated for x ≈ 1.03-8) and a canted spin structure associated with pretty large negative magnetoresistance (MR)
has been observed for x ≈ 0.9. In order to perform
detailed investigation of the electron-doping, we musthave detailed information about the starting material,
CaMnO3 .
In this paper, we investigate the thermal transport
properties of CaMnO3 and La1−x Cax MnO3 (x ≈ 1.0)
polycrystals and report an anomaly observed in CaMnO3
associated with the weak ferromagnetic order. A slightchange of the Mn valency from Mn4+ to Mn3+ caused by
the diﬀerence of oxygen content and/or by the electron
doping drastically alters the thermal transport properties of this system. The anomaly in CaMnO3 is rather
diﬃcult to understand on the basis of the phonon only
heat conduction and the heat transport due to the magnetic collective modes seems to become important below its magnetic ordering temperature TN . The thermal transport due to magnetic modes has been only
scarcely studied. Several recent reports,9-11) however,
have clearly demonstrated the magnetic mode contribution to the heat conduction. Systematic investigations
on the theme of magnetic heat conduction are deﬁnitely§2.

Experimental

CaMnO3 and La1−x Cax MnO3 (x = 0.99, 0.98) samples were prepared from stoichiometric mixtures of
CaCO3 , La2 O3 and Mn3 O4 raw powders. The mixtures
were calcined at 1000◦ C for 24 h in air, pressed into pellets and then sintered at 1500◦ C for 8 h in air. The
sintered CaMnO3 was named as “as-sintered” CaMnO3
sample. The sintered CaMnO3 and La1−x Cax MnO3
samples were heat-treated at 1500◦ C for 24 h in ﬂowing
oxygen to improve the oxygen non-stoichiometry and we
named these samples as “O2 -annealed” samples. All thesamples were conﬁrmed to be in a single phase at room
temperature by means of X-ray diﬀraction. The densityof each sample was about 85% of the ideal one. The thermal conductivity κ(T ) and thermal diﬀusivity α(T ) wereautomatically measured in a Giﬀord-McMahon cycle helium refrigerator as described elsewhere.12, 13) The magnetization M was measured by a SQUID magnetometer
under a magnetic ﬁeld of 0.5 T after zero ﬁeld cooling.
The thermal dilatation dL/L was measured by a strain
gauge method.
§3.

Results and Discussion

Figure 1 shows the temperature dependence of thethermal conductivity κ(T ) of O2 -annealed CaMnO3 ,as-sintered CaMnO3 and O2 -annealed La1−x Cax MnO3
(x = 0.99, 0.98) samples. With decreasing temperature, κ(T ) of O2 -annealed CaMnO3 gradually decreases
down to Tm ≈ 130 K and then rapidly increases below
Tm , showing a shallow minimum at Tm . With further
decrease of T , κ(T ) takes a sharp maximum at Tm ≈40 K, and then very steeply decreases. The as-sintered
CaMnO3 sample also shows similar κ(T ) behaviors, but
the κ(T ) enhancement becomes moderate. In contrast,
the maximum and the minimum of κ(T ) are almost
wiped out for the O2 -annealed La1−x Cax MnO3 samples;κ(T ) of La0.02 Ca0.98 MnO3 shows a monotonic decrease
with decreasing temperature, though some remnant of2082

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2000)

Thermal Transport Anomaly Associated with Weak Ferromagnetism in CaMnO3

Fig. 1. Temperature dependence of the thermal conductivity
κ(T ) of the CaMnO3 (O2 -annealed and as-sintered) andLa1−x Cax MnO3 (x = 0.99, 0.98: O2 -annealed) samples. The
solid line shows the phonon conductivity κph of O2 -annealed
CaMnO3 calculated on the basis of eqs. (1) and (2). The ﬁtting
parameters are given in Table I. Note the discrepancy between
the data and the calculated line for T > TN .

the κ enhancement is noticeable for La0.01 Ca0.99 MnO3
below T ∼ 120 K.
Figure 2 presents the temperature dependence of themagnetization M (T ) under an applied ﬁeld of 0.5 T.
The M (T ) curves of the as-sintered and O2 -annealed
CaMnO3 samples show a sharp increase at TN = 130 K,
which suggests the onset of the weak ferromagnetism(WF).14) The magnitude of the WF moment is about
1/80 of the Mn4+ moment (≈3μB ) and is slightly decreased by the O2 annealing. What is to be noticed isthat the κ(T ) minimum temperature Tm agrees with the
WF ordering temperature TN of the CaMnO3 samples.
The WF moment is slightly enhanced and TN slightly
decreases to 120 K by the 2% introduction of Mn3+ ionsin La0.02 Ca0.98 MnO3 .
Figure 3 displays the thermal dilatation dL/L as afunction of T . For all the samples, the contraction
is somewhat enhanced at TN with decreasing temperature. The slight enhancement in the gradient of dL/L
at around TN may suggest a possibility of a slight latticechange associated with the onset of the magnetic order.
In Fig. 3, it is noteworthy that the anomalies of dL/L arealmost of the same magnitude for the present samples,making a marked contrast to the diﬀerent κ behavior in
Fig. 1.
Figure 4 shows the electrical resistivity ρ(T ) of thesesamples. ρ(T ) is pretty large, behaves semiconductive and shows no anomaly at around TN . From the
Wiedemann-Franz law, the electrical contribution κe to
the thermal conductivity is estimated to be negligibly
small in these specimens. Then, in a usual sense, theheat conduction is overwhelmingly due to phonons. The
phonon thermal conductivity κph can be analyzed on thebasis of the following relaxation time approximation.15)
3  ΘD /T
3dnR 2 T
x4 ex
νs
κph =
τph dx, (1)x2πMΘD(e − 1)20

2083

Fig. 2. Temperature dependence of the magnetization M (T ) of
the CaMnO3 and La1−x Cax MnO3 samples under an appliedﬁeld of 0.5 T. The M (T ) curve of the CaMnO3 samples showsa sharp increase at TN = 130 K, which corresponds to the weakferromagnetism onset.

Fig. 3. The thermal dilatation dL/L as a function of T . The contraction of the samples with decreasing temperature is slightly
enhanced at TN in all samples.

Fig. 4. Temperature dependence of the electrical resistivity of the
CaMnO3 (O2 -annealed and as-sintered) and La0.02 Ca0.98 MnO3
(O2 -annealed) samples.

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2084

Hiroyuki Fujishiro, Shuichi Ohshiden and Manabu Ikebe(Vol. 69,

Table I. Summary of the parameters determined in the κph ﬁtting processes in La1−x Cax MnO3 samples on the basis of eqs. (1) and(2). The ﬁrst three lines show the ﬁtting parameters for the solid curves shown in Fig. 6. The last line shows the parameters for thesolid curve shown in Fig. 1 in which the U term becomes dominant.
Sampleτb−1 (s−1 )lb (μm)s(K−2 s−1 )p(K−4 s−1 )U (s−1 )X = 1.0 (O2 -annealed)
X = 1.0 (as-sintered)
X = 0.98 (O2 -annealed)

2.6 × 108

2.6 × 108
7.2 × 108

13.3
13.3
4.9

2.7 × 107

2.5 × 102
3.9 × 102
3.6 × 102000

X = 1.0 (O2 -annealed)

2.9 × 108

13.3

2.2 × 105

2.1 × 103

7.1 × 1011

where d is the mass density, n (=5 for La1−x Cax MnO3 )the number of atoms in the chemical formula, M themolar weight, R the gas constant, νs the sound velocity,
ΘD the Debye temperature (=400 K) and x is the phonon
frequency reduced by T . The phonon relaxation time isgiven by−1τph= τb−1 + sT 2 x2 + pT 4 x4 + U x exp(−

ΘD
).
bT

(2)

Here, τb−1 refers to the phonon scattering strength by
grain boundaries, s by the sheet-like defects and p by
the point like defects. The last term with constant b(usually ≈2) stands for the phonon-phonon umklapp (U)processes.
What is the most puzzling for the heat conductionin the present samples is the κ(T ) minimum at around
TN of the CaMnO3 samples. According to the analyses
based on eqs. (1) and (2), the maximum at T ≈ 40 K
may be explained as to come from the U term, whichbecomes proceedingly dominant at higher temperatures.
An example of κ(T ) ﬁtting with parameter values in Table I are given by a solid line in Fig. 1 for O2 -annealed
CaMnO3 . We had to adopt unusually small value b = 1
in the U term to get a remarkable ﬁtting. As the ﬁtting
line demonstrates, κph should continue to decrease withincreasing temperature in the high temperature region
because the U term with the exponential function has
the strongest temperature dependence among the scattering terms in eq. (2). In case of very dirty crystals
with strong phonon scattering by defects, however, the U
term scattering can often actually be neglected because
the phonon mean free path lph has been already strongly
limited and almost saturated by defect scattering before
the U processes limit lph .
Figure 5 shows the temperature dependence of thethermal diﬀusivity α(T ) (=κ/C = νs lph /3, if theheat conduction is purely of phonon origin, C: speciﬁcheat). Corresponding to the sharp increase of κ below TN (=Tm ), α(T ) of O2 -annealed and as-sintered
CaMnO3 abruptly increases below TN with decreasing temperature. In contrast, α(T ) of O2 -annealed
La0.01 Ca0.99 MnO3 shows a moderate increase at lower
temperatures. With increasing temperature above TN ,however, α(T ) of all the samples shows a very slight decrease. Taking account of the measured νs value of νs ≈7000 m/s16) for La1−x Cax MnO3 (x = 0.95), the value oflph for O2 -annealed CaMnO3 is estimated to be ≈13 Å at
TN . This small value of lph suggests strong phonon scattering even in the O2 -annealed CaMnO3 sample. It is unreasonable that U processes abruptly become ineﬀective

4.2 × 107
6.1 × 107

for T ≥ TN , but the strength of the U process scattering
should change continuously on temperature. The very
steep change of α(T ) of CaMnO3 at TN seems to suggestthat some other heat conduction mechanism is operative
for T < TN . Because the α(T ) and κ(T ) enhancement
begins just at the magnetic order onset with decreasing
T , the additional heat conduction mechanism should be
related to collective magnetic excitations such as spinwaves. Recently, the heat transport by magnetic excitations (κspin ) was clearly demonstrated for CuGeO3 9) andfor Sr14−X AX Cu24 O41 (A=Ca, La).10) The possibilityof heat conduction by spin waves was also suggested fornon-superconductive YBa2 Cu3 O6+δ .11) In the presenceof κspin (=Cspin νspin lspin /3, Cspin : magnon speciﬁc heat,νspin : magnon velocity, lspin : magnon mean free path),the diﬀusivity α = (κph + κspin )/C can abruptly increase
below TN .
In Fig. 6, we make a rough estimation of κspin by extrapolating κph from T > TN on the basis of eqs. (1)and (2). The U term in eq. (2) is neglected assumingthat the lattice defect scattering is strong enough. In
this ﬁgure, the estimated κph is presented by full lines.
The used ﬁtting parameters are summarized in Table I.
The estimated contribution of magnons, κspin = κ − κph ,is presented by closed symbols for each sample. κspin of
annealed and as-sintered CaMnO3 is very large, whilethat of La1−x Cax MnO3 (x = 0.99, 0.98) is greatly diminished. The drastic reduction of κspin caused by asmall amount of electron doping may probably be due toFig. 5. The thermal diﬀusivity α(T ) of the CaMnO3 (O2 annealed and as-sintered) and La0.01 Ca0.99 MnO3 (O2 -annealed)samples as a function of T .

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2000)

Thermal Transport Anomaly Associated with Weak Ferromagnetism in CaMnO3

Fig. 6. The estimated phonon (κph ) and the magnetic (κspin )contributions to the thermal conductivity κ(T ). The phonon
contribution κph calculated on the basis of eqs. (1) and (2), andparameter values in Table I is presented by full lines. The contribution of the magnetic mode κspin = κ − κph is presented byclosed symbols (see text).

magnon scattering by introduced Mn3+ impurity spins,
which may strongly limit the magnon mean free pathlspin .
Assuming the magnon heat conduction, let us makea very rough estimation of the magnon mean free pathlspin of CaMnO3 . We estimate the magnetic speciﬁc heat
Cspin (erg/cm3 ) of Mn4+ spins based on the molecular ﬁeld approximation and measured density d. The
magnon velocity νspin is estimated to be about 7.5 ×
105 cm/s, assuming the antiferromagnetic dispersion (neglecting the weak ferromagnetic component),
2z|J|Saq,(3)h̄
where, J is the exchange constant, z is the number of thenearest Mn spins, S (=3/2) is the spin quantum numberof Mn4+ and a (≈4 Å) is the average distance between
Mn ions. The exchange constant J is estimated from TN
again by use of the molecular ﬁeld approximation,ω = νspin q ≈TN =2zJS(S + 1)
,
3kB

(4)

where kB is the Boltzman constant. Based on theseapproximations, the maximum κspin (T ≈ 40 K)≈110 mW/cmK in Fig. 6 results in lspin ≈ 800 Å or∼200 Mn atomic spacings. By doping 1% Mn3+ in
La0.01 Ca0.99 MnO3 , the average Mn3+ distance is about
36 Å or ∼9 Mn spacings. The magnon mean free pathcan be reduced roughly to this value of the average Mn3+
distance as a result of Mn3+ impurity scattering and it
is reasonable that 1% Mn3+ doping has drastically depressed the magnon heat conduction κspin .
Although we have emphasized the possibility of themagnon heat conduction, another possibility for the enhancement of κ(T ) and α(T ) may be of phonon conduction origin. If we assume that Mn magnetic moment ﬂuctuations strongly scatter phonons above TN and that thephonon scattering by Mn spins is greatly reduced below

2085

TN in the magnetically ordered state, then the phonon
thermal conduction is enhanced below TN . Because spin
ﬂuctuations which we assume to scatter phonons must be
reduced at low temp eratures in the magnetically orderedstate, this scenario provides a reasonable explanation forthe κ(T ) behavior of CaMnO3 . However, the drastic reduction of κ(T ) resulting from only 1% La substitution
for Ca may be diﬃcult to explain by this model; it seems
unlikely that addition of only 1% Mn3+ spins so drastically enhances the spin ﬂuctuations at low temperaturesin the ordered state. It should be noticed again that thethermal dilatation dL/L in Fig. 3 shows almost the same
behavior for CaMnO3 and La1−x Cax MnO3 and there
should be no signiﬁcant lattice state diﬀerence in thesesamples between T < TN and T > TN . The sound velocity measurement for La0.05 Ca0.95 MnO3 showed quitea small change at TN ,16) supporting our conclusion fromdL/L data.
§4.

Summaryi) The thermal conductivity κ(T ) and the thermal
diﬀusivity α(T ) were measured for CaMnO3 andlightly electron-doped La1−x Cax MnO3 (x ≥ 0.98)
polycrystals. κ(T ) of CaMnO3 gradually decreased
with decreasing temperature down to the weakferromagnetic transition temperature TN (=120 K).
Both κ(T ) and α(T ) showed a peculiar enhancementwith further reduction of temperature below TN .
Thus κ(T ) resulted in an unusual minimum at TN .
The κ(T ) and α(T ) enhancement below TN was very
rapidly suppressed by introduction of Mn3+ spins in
La1−x Cax MnO3 .
ii) The thermal dilatation dL/L showed a small
anomaly at TN and the behaviors of dL/L werevery similar for CaMnO3 and La1−x Cax MnO3 . The
diﬀerence of κ(T ) and α(T ) behaviors between
CaMnO3 and La1−x Cax MnO3 cannot be explained
as coming from the diﬀerence in the lattice states.
iii) The thermal diﬀusivity remained almost constant
above TN and the phonon mean free path lph (T ≈TN ) estimated from the α(T ) values was as small as≈13 Å for O2 -annealed CaMnO3 . This fact implies
that a strong phonon scattering mechanism whichalmost saturates lph is at work at least for T > TNeven in this best heat conducting sample.
iv) As possible origins of the heat transport enhancement below TN , two possibilities were discussed.
One is the magnon heat conduction κspin which may
be operative below TN . The other is the phonon conduction strongly limited by the scattering by spin
ﬂuctuations. Although the eﬀect of electron (Mn3+ )doping seems to strongly suggest the importance ofκspin , we cannot arrive at an unambiguous answer at
the present stage. The thermal transport measurements in very strong magnetic ﬁelds may provide animportant clue to distinguish the two scenarios.
Acknowledgment
The authors wish to thank T. Kikuchi of Iwate University for his assistance in sample fabrication and Prof.
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Hiroyuki Fujishiro, Shuichi Ohshiden and Manabu Ikebe

the sound velocity measurements. The present study ispartially supported by REIMEI Research Resources ofJapan Atomic Energy Research Institute.
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