Journal of the Physical Society of Japan
Vol. 77, No. 8, August, 2008, 084603#2008 The Physical Society of Japan

Thermal Conductivity Anomalies of RECoO3 (RE = La–Nd)
Related to Valency and Spin State of Co Ion
Hiroyuki FUJISHIRO, Tomoyuki N AITO, Yohei K ASHIWADA, and Yosuke F UJINE
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551(Received February 25, 2008; accepted June 10, 2008; published August 11, 2008)

The thermal conductivities ðTÞ of Co- and La-site-substituted LaCoO3 were measured and the originof the ðTÞ peak was investigated. The sharp ðTÞ peak at approximately 30 K is strongly suppressed by
the introduction of a small amount of Jahn–Teller (JT)-active Co4þ ions because of Co-site substitution
by divalent ions such as Ni2þ and Cu2þ , similarly to the La-site substitution by Sr2þ . In contrast, the ðTÞpeak of samples with the Co-site substituted by tetravalent ions such as Sn4þ and Zr4þ , which introduce
Co2þ ions, moderately decreased. The temperature at which a small and broad ðTÞ peak in the Prcontaining RECoO3 [rare-earth (RE) = La1y Pry and Pr1z Ndz ] is also observable shifts from 110 K for
RE = La0:5 Pr0:5 to 180 K for RE = Pr and Pr0:5 Nd0:5 , which corresponds to the spin-state transition
temperature of Co3þ ions.
KEYWORDS: RECoO3 (RE = La–Nd), spin-state transition, substitution effect, thermal conductivity, thermal
expansion, magnetization
DOI: 10.1143/JPSJ.77.084603

1.

Introduction

The perovskite oxide LaCoO3 has been studied intensely
during the last 40 years because of its unique magnetic
behavior and the related insulator–metal transition. The
ground state is known to be nonmagnetic, corresponding toa low-spin (LS) state of Co3þ ions (t2g 6 eg 0 , S ¼ 0). With
increasing temperature, a paramagnetic state appears at
about T ¼ 80 {120 K and another magnetic state takes place
at T ¼ 400 { 600 K with an insulator-to-metal transition. The
low-temperature magnetic transition was originally interpreted in terms of the gradual increase of the populationof the Co3þ high-spin (HS) state (t2g 4 eg 2 , S ¼ 2).1) Anotherinterpretation was proposed in the second half of the 1990s,
in which the low-temperature magnetic transition was
attributed to a transition from an LS state to an intermediate-spin (IS) state (t2g 5 eg 1 , S ¼ 1).2) This interpretation was
supported by descriptions in many papers.3–6) However, no
studies have deﬁnitively shown that the excited state is the
IS state. Recent measurements indicate that it might still be
the HS state,7–10) and the mechanism and even the spin stateof the excited state remain controversial. For the RECoO3
system (RE: rare-earth) with a smaller RE3þ ion such as
Pr3þ or Nd3þ , the LS state of Co3þ is stabilized up to higher
temperatures, as conﬁrmed by the extrinsic pressure eﬀectusing LaCoO3 11) and the chemical pressure eﬀect using(La1y Pry )CoO3 .12)
Yan et al. reported the temperature dependence of thethermal conductivity ðTÞ and the magnetic susceptibilityðTÞ for single crystals of RECoO3 (RE = La, Pr, Nd).13)
The ðTÞ of LaCoO3 has a sharp peak at approximately
T ¼ 20 K and decreases rapidly with increasing temperature.
The sharp ðTÞ peak was shown to result from the spin-statetransition from LS to IS (or HS) in Co3þ ions; i.e., the IS (orHS) Co3þ ions introduced a local lattice distortion in the LS
Co3þ network because of the larger ionic radius than that of
the LS Co3þ ions. Because the IS Co3þ ions are Jahn–Teller(JT)-active ions, phonon scattering might be further enE-mail: fujishiro@iwate-u.ac.jp

hanced by their introductions. The ðTÞ of PrCoO3 resembles that of LaCoO3 with a sharp suppression at temperatures greater than 30 K, and that of NdCoO3 shows a 1=T
temperature dependence, which originates from a phonon–
phonon scattering mechanism, as can be sometimes seen in
ordinary solids. Because the spin-state transition from LS to
IS (or HS) was estimated to be at 180 K for PrCoO3 , the ðTÞpeak of PrCoO3 is not related to a spin-state transition. At
present, the origin of the ðTÞ peak has not been clariﬁed for
PrCoO3 . Furthermore, the inﬂuences of the Co4þ and Co2þions on phonon scattering in perovskite cobaltites such as
RE1X SrX CoO3 and RECoO3 have not been investigatedsystematically. It is necessary to investigate the origin of theðTÞ peak in RECoO3 and the inﬂuence of the JT-active ionon the phonon scattering.
In the present study, we systematically investigate theeﬀect of the valency and/or the spin state of Co ionson the phonon thermal conductivity ðTÞ in perovskite
cobaltites. First, we measure the ðTÞ of LaCoO3 andLa(Co0:98 M0:02 )O3 . The eﬀect of substitution at the Co site
by various valence cations M is examined, including phonon
scattering by the substituted cation M itself, to determine
whether they are JT-active ions or not. Second, we measurethe ðTÞ of RECoO3 with various ionic radii for samples
with Pr contained in the RE site (RE = La1y Pry and
Pr1z Ndz ). The relation between the ðTÞ anomaly and thespin-state transition of the Co3þ ions is systematicallyinvestigated.
2.

Experimental Procedure

All the samples used in this study are polycrystals
prepared by a solid-state reaction. In LaCoO3 and La(Co0:98 M0:02 )O3 (M: Ni2þ , Cu2þ , Ga3þ , Cr3þ , Zr4þ , Sn4þ ),mixtures of raw powders of La2 O3 , Co3 O4 , and the respective oxides containing M ions were calcined at 1000  C for
24 h in air, pressed into pellets, and then sintered at 1400  C
for 8 h in air.14) In addition, RECoO3 (RE = La1y Pry and
Pr1z Ndz ) and La1X SrX CoO3 (0:005  X  0:02) were
fabricated using the same procedure. The measured densitiesof the samples were about 90% of the ideal values. X-ray

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H. FUJISHIRO et al.

diﬀraction analyses at room temperature conﬁrmed thesingle phase in the crystal structure for the samples except
for La0:5 Pr0:5 CoO3 , which mostly comprised the orthorhombic Pnma phase but also contained a small amount of therhombohedral R3 c phase, as suggested in ref. 12. Thermal
conductivity ðTÞ was measured using a steady-state heatﬂow method. To detect the spin-state transition, a linear
thermal expansion, dLðTÞ=Lð20 KÞ ¼ ½LðTÞ  Lð20 KÞ=
Lð20 KÞ, based on the sample length L at 20 K, or a thermal
expansion coeﬃcient ðTÞ, which was deﬁned as thetemperature derivative dLðTÞ=Lð20 KÞ, was determined using
a strain-gauge method (Kyowa KFL-1-120-C1-16). Measurements of ðTÞ and dLðTÞ=Lð20 KÞ were performed using
a GM cycle helium refrigerator. Magnetization MðTÞ was
measured using a superconducting quantum interference
device (SQUID) magnetometer (Quantum Design) under aﬁeld of 0.5 T after zero-ﬁeld cooling.
3.

Results and Discussion

3.1

Eﬀect of La-site substitution in LaCoO3 on thermal
conductivity ðTÞ
Figure 1(a) shows the temperature dependence ofthe thermal conductivity ðTÞ of (La1X SrX )CoO3 and(La0:98 M0:02 )CoO3 (M = Pr3þ , Nd3þ ) samples. Because all
the samples measured in this study are insulators, the heat is
carried by acoustic phonons ( ¼ ph ). The ðTÞ of LaCoO3
has a large peak of 80 mW/(cmK) at approximately 30 K,
drops sharply to a low value with increasing temperature,and subsequently increases slightly up to 300 K. The sharpðTÞ peak was strongly suppressed by the substitution at only
0.5% of the La site by Sr2þ and was completely removed by
2% substitution. In general, crystal disorder is introduced ina lattice by element substitution. As a result, the thermal
conductivity is suppressed in the entire temperature range.
However, it is diﬃcult to infer that the reduction of the ðTÞpeak in La0:995 Sr0:005 CoO3 results from the lattice disorder,
because the ðTÞ peak of the samples in which 2% of La sites
were substituted by Pr3þ and Nd3þ was not reduced. Either
LS or IS Co4þ ions are introduced, both of which havediﬀerent ionic radii from that of Co3þ , when the Sr2þ ions
are substituted at the La3þ site depending on the amount of X
in La1X SrX CoO3 . The disappearance of the ðTÞ peaksuggests the existence of a powerful phonon-scattering
center that is only eﬀective at low temperatures. The
scattering center as above must be large, because the phonon
wavelength is also large at low temperatures. Phelan et al.
suggested the existence of Co4þ (LS)–6Co3þ (IS) clusters
from the results of an elastic neutron scattering study on
La1X SrX CoO3 .15) Co4þ (LS) ions with the t2g 5 eg 0 conﬁguration are also JT-active in the t2g level and might beaccompanied by a local lattice distortion. Moreover, thelattice distortion might be strongly enhanced because of thecluster nature. For these reasons, the cluster model is apossible explanation for the powerful phonon scattering.
Figure 1(b) shows the temperature dependence of thethermal expansion coeﬃcient ðTÞ of La1X SrX CoO3 . For
the LaCoO3 sample, an anomalous thermal expansion takes
place at T > 35 K, which results from the spin-state transition of Co3þ ions from LS to IS (or HS) with increasing temperature. Because the spin-state transition involves
electron transfer from a t orbital to an e orbital and causes an

Fig. 1. (Color online) (a) Temperature dependence of the thermal conductivity ðTÞ of La1X SrX CoO3 (0  X  0:02) and La0:98 M0:02 CoO3
(M = Pr3þ , Nd3þ ). (b) Temperature dependence of the thermal expansion
coeﬃcient ðTÞ of La1X SrX CoO3 (0  X  0:02).

ionic size change of the octahedral-site Co3þ ion; the ionicradius of LS Co3þ was reported to be 0.545 Å, whereas
IS and HS Co3þ have larger radii of 0.56 and 0.61 Å,respectively.16) For X ¼ 0:005 and 0.01, a clear enhancement of ðTÞ was also observed at temperatures greater than
35 K because of the LS ! IS (or HS) spin-state transition,and the transition becomes broad for X ¼ 0:02. Asai et al.
measured the temperature dependence of the lattice constantfor La1X SrX CoO3 (X ¼ 0, 0.08).17) For the X ¼ 0 sample,
an abrupt increase in the lattice constant took place above
50 K. Similar results for the values of ðTÞ were also
reported by Zobel et al.18)
3.2

Eﬀect of Co-site substitution eﬀect in LaCoO3 on
thermal conductivity ðTÞThe explanation in the preceding subsection demonstrated
that the ðTÞ peak in LaCoO3 is strongly suppressed by the
introduction of the Co4þ LS (or IS) ions because of theLa-site substitution by Sr2þ . In this subsection, the measured
thermal conductivity of La(Co0:98 M0:02 )O3 with various
valence cations of M is reported and the inﬂuence of
the introduction of various valence Co ions on ðTÞ isdiscussed. Figures 2(a) and 2(b) respectively show thethermal conductivity ðTÞ and the magnetization MðTÞ of
La(Co0:98 M0:02 )O3 (M: Ni2þ , Cu2þ , Ga3þ , Cr3þ , Zr4þ ,Sn4þ ). The ðTÞ peak was maintained upon the Co-site

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H. FUJISHIRO et al.

Fig. 2. (Color online) Temperature dependence of the (a) thermal conductivity ðTÞ and (b) magnetization MðTÞ of La(Co0:98 M0:02 )O3 forvarious M ions (M = Ni2þ , Cu2þ , Ga3þ , Cr3þ , Sn4þ , Zr4þ ) under anapplied ﬁeld of 0 Ha ¼ 0:5 T after zero ﬁeld cooling. For comparison, theðTÞ and MðTÞ of La1X SrX CoO3 (X ¼ 0, 0.005, and 0.02) are also
shown.

substitution by M = Ga3þ and Cr3þ , and MðTÞ showedsimilar behavior to that of LaCoO3 . These results suggestthat the lattice disorder attributable to the 2% Co-site
substitution by the trivalent ions in LaCoO3 does notinﬂuence the behaviors of ðTÞ and MðTÞ and that the Co3þLS state is maintained. Regarding the ðTÞ behavior of
La(Co0:98 Cr0:02 )O3 , it is possible that the Cr ions aresubstituted for Co ions as Cr4þ (t2g 2 eg 0 ; non-JT) and that,as a result, HS-Co2þ ions (t2g 5 eg 2 ; non-JT; S ¼ 3=2) arecreated, which would explain the unchanged ðTÞ peak.
However, the behavior of MðTÞ cannot be explained using
this scenario. Consequently, the Cr ion should be substituted
at the Co site as Cr3þ . For the divalent-ion substitutionof M = Ni2þ and Cu2þ , the magnitude of the ðTÞ peakmarkedly decreases to the level of ðTÞ in La0:995 Sr0:005 CoO3 . In this case, the Co4þ ions might be created by the 2%
divalent M ion substitution, similarly to the La-site substitution by Sr2þ , causing the scattering of the phonons. It isnoteworthy that the ðTÞ peak is more strongly suppressedby Cu2þ than by Ni2þ in the Co-site substituted sample.
In fact, Cu2þ (t2g 6 eg 3 ) is a JT-active ion, and the phonon
scattering might be further enhanced. However, we cannot
quantitatively explain why the eﬀect of 2% substitution by
Ni2þ or Cu2þ is comparable to that of 0.5% substitution by

Sr2þ ; the amount of Co4þ is four times larger in the former
than in the latter.
For M = Sn and Zr, which take various valences in anionic state, the ðTÞ peak reduction is less pronounced,
possibly because of the formation of M4þ . Upon M4þ cation
substitution, the Co2þ ion is expected to appear because of
the charge neutrality. In our previous study, the sign of theSeebeck coeﬃcient of the samples was reported to be
negative, which strongly indicated that Sn and Zr ions weretetravalent species in La(Co0:98 M0:02 )O3 14) and, as a result,
the Co2þ ion was created. The moderate suppression of theðTÞ peak in La(Co0:98 M4þ 0:02 )O3 might be a characteristicfeature of phonon scattering by Co2þ ions, of which the spinstate is reported to be HS (t2g 5 eg 2 , S ¼ 3=2).19) These results
are the ﬁrst report for the phonon scattering by Co2þ ions,as far as we know. The determination of the reason is inprogress, why the suppression by Co2þ is so moderate incomparison with that by Co4þ .
A qualitative understanding of the phononic heat transport
can be obtained from the relation   Cvl, where C, v, and lrespectively denote the speciﬁc heat, the sound velocity, andthe mean free path of the phonons. The reported speciﬁc heatCðTÞ of the LaCoO3 crystal increases with increasing
temperature and shows no anomaly at 20 –100 K.20) The
low-temperature spin-state transition from LS to the excitedstate is not clearly reﬂected in the heat capacity. The
temperature dependence of the sound velocity vðTÞ of
LaCoO3 was measured by Naing et al.21) vðTÞ decreasedslightly with decreasing temperature, then increased by 15%
below 50 K and saturated at 10 K. From these results of CðTÞand vðTÞ, the large ðTÞ peak in LaCoO3 originates mainly
from the enhancement of the phonon mean free path lðTÞ at
low temperatures because of the disappearance of strong
phonon scattering.
3.3 Thermal conductivity of RECoO3
Figure 3(a) shows the temperature dependence of the thermal conductivity ðTÞ of (La1y Pry )CoO3 and (Pr1z Ndz )CoO3 samples. The ðTÞ peak of the (La1y Pry )CoO3
samples decreases for y ¼ 0:2 without a temperature shiftof the ðTÞ peak. The reduction of the ðTÞ peak results fromthe lattice disorder caused by the substitution of Pr3þ ionswith a diﬀerent ionic radius at the La site. However, for the(La0:5 Pr0:5 )CoO3 sample, the ðTÞ peak increased and thetemperature at which the peak appears decreases. The ðTÞpeak of the y ¼ 1 sample (PrCoO3 ) was greatly enhanced to
200 mW/(cmK) at 15 K. Because the LS ! IS (or HS)
spin-state transition in PrCoO3 was reported to take place at
180 K,13) a diﬀerent mechanism should exist causing the
sharp ðTÞ peak other than the spin-state transition. The ðTÞpeak of (Pr1z Ndz )CoO3 increases with decreasing ionicradius of the element at the RE site, and the temperature atwhich the ðTÞ peak appears increases slightly. For NdCoO3 ,ðTÞ shows 1=T temperature dependence, which is conventional ðTÞ behavior in ordinary solids.
Yan et al. reported a small and broad ðTÞ peak at 180 K
in PrCoO3 , which was closely related to the spin-statetransition of Co3þ ions from LS to IS.13) We measured theðTÞ of the Pr-containing RECoO3 sample, and the relation
between the small and broad ðTÞ peak and the spin-statetransition of the Co3þ ions was investigated. Figure 3(b)084603-3

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ðTÞ peak as shown in Fig. 3(b). It should be noted that all
the samples that have a second peak in ðTÞ contain Pr ions.
The second peak might be a characteristic feature of the Prcontaining RECoO3 samples. The strong energy overlap of
the outer-lying 4 f orbital of Pr3þ with the neighboring
oxygen 2p levels of PrBa2 Cu3 O7 results in a characteristically mixed or ﬂuctuating valence.22) In the RE1X AEX CoO3 system (AE = Ba, Sr, Ca), the insulator–metaltransition, which is related to a spin-state transition, can beobserved only in the Pr1X CaX CoO3 system.23,24) Consequently, novel characteristics in the Pr-containing perovskite
cobaltites are reported. The behaviors of ðTÞ for PrCoO3
and Pr-containing RECoO3 might be related to the characteristic electron conﬁguration of Pr ions.
Figure 3(c) shows the temperature dependence of thelinear thermal expansion dLðTÞ=Lð20 KÞ for RECoO3 withelements of various ionic radii at the RE site. For(La1y Pry )CoO3 , the temperature at which the anomalous
thermal expansion takes place increases and the change indLðTÞ=Lð20 KÞ becomes moderate with increasing y. For
example, T ¼ 110 K for RE = La0:5 Pr0:5 and T ¼ 180 K for
RE = Pr. These results suggest that the LS state of the Co3þion becomes stable at low temperatures with decreasing
eﬀective ionic radius at the RE site and that the transition
becomes more gradual. The characteristic temperatures
shown with arrows in Fig. 3(c) are nearly equal to thevalues of T(2nd) shown in Fig. 3(b). It is noticeable that thesmall and broad second peak of ðTÞ for the Pr-containing
RECoO3 is related to the spin-state transition of Co3þ .
4.

Fig. 3. (Color online) (a) Temperature dependence of the thermal conductivity ðTÞ of (La1y Pry )CoO3 and (Pr1z Ndz )CoO3 . (b) Magniﬁcationof ðTÞ around the second peak in RECoO3 with RE ions of various ionicradii (see text). (c) Temperature dependence of the linear thermal expansion dLðTÞ=Lð20 KÞ for RECoO3 with RE ions of various ionic radii.

shows ðTÞ around the small peak in RECoO3 with the Prcontaining RE site. Hereafter, we refer to the small ðTÞpeak as the second peak. This peak appears for RE =La0:5 Pr0:5 at Tð2ndÞ ¼ 110 K, and T(2nd) approaches 180 K
for the RE = Pr and RE = Pr0:5 Nd0:5 samples. The second
peak of the RE = Pr0:5 Nd0:5 sample is more clearlyobservable and ðTÞ is clearly suppressed at an intermediate
temperature range of 20 –180 K. In addition, a further
reduction in ðTÞ also occurs at T > 180 K because of thespin-state transition. For a smaller RE ion such as RE = Nd,
the second peak was obscured by the large phonon–phonon

Summary

The thermal conductivities ðTÞ of LaCoO3 , and Laand Co-site-substituted LaCoO3 and RECoO3 samples (RE:La1y Pry , Pr1z Ndz ) have been measured and the originof the ðTÞ peak in LaCoO3 has been investigated bycomparison with the valence and spin state of Co ions. The
important experimental results and conclusions are summarized as follows.
1) A sharp ðTÞ peak of LaCoO3 at approximately 30 K isstrongly suppressed by the introduction of a small
amount of Co4þ ions because of Co-site substitution by
divalent cations such as Cu2þ and Ni2þ , in addition
to La-site substitution by Sr2þ . A large-scale lattice
distortion such as a Co4þ (LS)–6Co3þ (IS) cluster with
JT-active ions is a possible candidate for the strong
phonon-scattering centers. The ðTÞ peak of LaCoO3
was not inﬂuenced by the 2% substitution of RE3þ atthe La site or by the substitution of M3þ at the Co-site,which suggests that the ionic radius mismatch does notinﬂuence the phonon scattering.
2) The ðTÞ peak is moderately suppressed in the
La(Co0:98 M4þ 0:02 )O3 sample (M = Sn4þ , Zr4þ ). The
ðTÞ suppression might be a characteristic feature ofphonon scattering by Co2þ ions, which is ﬁrst reported
in this study. A detailed study to understand the reason
why the suppression by Co2þ is so moderate in comparison with that by Co4þ is in progress.
3) In Pr-containing RECoO3 , a small and broad ðTÞpeak appears in (La0:5 Pr0:5 )CoO3 at T ¼ 110 K. The
temperature at which the ðTÞ peak appears increases
and approaches 180 K in PrCoO3 and Pr0:5 Nd0:5 CoO3 .

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These temperatures correspond to those at which theanomaly of the linear thermal expansion dLðTÞ=Lð20 KÞ is detected. The ðTÞ anomaly is closely
related to the spin-state transition in Co3þ ions, which
is obscured by the large phonon–phonon peak for the
NdCoO3 system with a smaller RE ion and by the largeðTÞ peak at 30 K for the LaCoO3 system with a larger
RE ion.

11)
12)
13)
14)
15)

1) P. M. Raccah and J. B. Goodenough: Phys. Rev. 155 (1967) 932.
2) M. A. Korotin, S. Yu. Ezhov, I. V. Solovyev, V. I. Anisimov, D. I.
Khomskii, and G. A. Sawatzky: Phys. Rev. B 54 (1996) 5309.
3) S. Yamaguchi, Y. Okimoto, and Y. Tokura: Phys. Rev. B 55 (1997)
R8666.
4) D. Louca, J. L. Sarrao, J. D. Thompson, H. Röder, and G. H. Kwei:
Phys. Rev. B 60 (1999) 10378.
5) T. Vogt, J. A. Hriljac, N. C. Hyatt, and P. Woodward: Phys. Rev. B 67
(2003) 140401.
6) R. F. Klie, J. C. Zheng, Y. Zhu, M. Varela, J. Wu, and C. Leighton:
Phys. Rev. Lett. 99 (2007) 047203.
7) T. Kyômen, Y. Asaka, and M. Itoh: Phys. Rev. B 67 (2003) 144424.
8) Z. Ropka and R. J. Radwanski: Phys. Rev. B 67 (2003) 172401.
9) A. Podlesnyak, S. Streule, J. Mesot, M. Medarde, E. Pomjakushina, K.
Conder, A. Tanaka, M. W. Haverkort, and D. I. Khomskii: Phys. Rev.
Lett. 97 (2006) 247208.
10) M. W. Haverkort, Z. Hu, J. C. Cezar, T. Burnus, H. Hartmann, M.
Reuther, C. Zobel, T. Lorenz, A. Tanaka, N. B. Brookes, H. H. Hsieh,

16)
17)

18)
19)

20)
21)
22)
23)
24)

084603-5

H.-J. Lin, C. T. Chen, and L. H. Tjeng: Phys. Rev. Lett. 97 (2006)
176405.
K. Asai, O. Yokokura, M. Suzuki, T. Naka, T. Matsumoto, H.
Takahashi, N. Môri, and K. Kohn: J. Phys. Soc. Jpn. 66 (1997) 967.
Y. Kobayashi, T. Mogi, and K. Asai: J. Phys. Soc. Jpn. 75 (2006)
104703.
J.-Q. Yan, J.-S. Zhou, and J. B. Goodenough: Phys. Rev. B 69 (2004)
134409.
Y. Fujine, H. Fujishiro, Y. Kashiwada, J. Hejtmanek, and M. Ikebe:
Physica B 359 – 361 (2005) 1360.
D. Phelan, D. Louca, K. Kamazawa, S.-H. Lee, S. N. Ancona, S.
Rosenkranz, Y. Motome, M. F. Hundley, J. F. Mitchell, and Y.
Moritomo: Phys. Rev. Lett. 97 (2006) 235501.
P. G. Radaelli and S.-W. Cheong: Phys. Rev. B 66 (2002) 094408.
K. Asai, O. Yokokura, N. Nishimori, H. Chou, J. M. Tranquada, G.
Shirane, S. Higuchi, Y. Okajima, and K. Kohn: Phys. Rev. B 50 (1994)
3025.
C. Zobel, M. Kriener, D. Bruns, J. Baier, M. Grüninger, T. Lorenz, P.
Reutler, and A. Revcolevschi: Phys. Rev. B 66 (2002) 020402.
A. V. Chichev, M. Dlouhá, S. Vratislav, K. Knı́žek, J. Hejtmánek, M.
Maryško, M. Veverka, Z. Jirák, N. O. Golosova, D. P. Kozlenko, andB. N. Savenko: Phys. Rev. B 74 (2006) 134414.
S. Stølen, F. Grønvold, H. Brinks, T. Atake, and H. Mori: Phys. Rev. B
55 (1997) 14103.
T. S. Naing, T. Kobayashi, Y. Kobayashi, M. Suzuki, and K. Asai:
J. Phys. Soc. Jpn. 75 (2006) 084601.
R. Fehrenbacher and T. M. Rice: Phys. Rev. Lett. 70 (1993) 3471.
S. Tsubouchi, T. Kyômen, M. Itoh, P. Ganguly, M. Oguni, Y. Shimojo,
Y. Morii, and Y. Ishii: Phys. Rev. B 66 (2002) 052418.
T. Fujita, S. Kawabata, M. Sato, N. Kurita, M. Hedo, and Y. Uwatoko:
J. Phys. Soc. Jpn. 74 (2005) 2294.