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Physica B 378–380 (2006) 534–536
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Anomalies in sound velocity and thermal expansion related to chargeorder and ferromagnetic transitions in Pr0.65Ca0.35(Mn1zCoz)O3
Manabu Ikebea,, Hiroyuki Fijishiroa, Shingo Kanoha, Tetsuo FukasebaFaculty of Engineering, Iwate University, Morioka 020-8551, Japan
Institute of Materials Research, Tohoku University, Sendai 980-8577, Japanb

Abstract
The charge order (CO), ferromagnetic (FM) and metal–insulator (M–I) transitions have been studied for Pr0.65Ca0.35(Mn1ZCoZ)O3(Z ¼ 0–0.10) by measuring the sound velocity vs(T), magnetization M(T), electrical resistivity r(T) and thermal expansion L(T)/LR. Thevs(T) hardening has been found to precisely monitor the CO transition, while the L(T)/LR shrink has been found to be sensitive to theonset of the FM-metallic phase. The competition of CO, FM-metallic and FM-insulating phases has been studied from the viewpoint ofthe enhanced electron–phonon interaction through the Jahn–Teller effect.
r 2006 Elsevier B.V. All rights reserved.
PACS: 75.47.Lx; 62.65.+k; 64.40.De
Keywords: Sound velocity; Thermal expansion; Manganites; Charge order; Ferromagnetic order; Jahn–Teller effect

The small ionic radius of Pr3+ enhances the distortion ofMnO6 network in perovskite Pr1XCaXMnO3 (PCMO) andreduces the doped hole hopping between neighboring Mn
ion sites. Then double exchange (DE) mechanism issuppressed, relatively enhancing the role of the competitive
interaction such as the superexchange (SE) or the Jahn–Teller (J–T) coupling, etc. In pristine Pr0.65Ca0.35MnO3, socalled CE-type charge/orbital order (CO) is stabilized, whichis easily replaced by the ferromagnetic (FM) order byapplying a magnetic ﬁeld, or by a small amount of Mn
substitution by other 3d transition metal elements [1–3]. In
this note, we study the lattice anomalies at CO and FM
transitions in Pr0.65Ca0.35(Mn1ZCoZ)O3 by measuring thesound velocity vs and the thermal expansion L(T)/LR.
PCMO polycrystals were prepared by a solid-state
reaction method. The temperature dependence of vs(T)was measured by a pulse superposition method using thelongitudinal waves of 7 MHz between 4.2 and 300 K.
L(T)/LR (LR ¼ room temperature length) was measured bya strain gauge method. The magnetization M(T) wasCorresponding author. Tel.: +81 19 621 6362; fax: +81 19 621 6363.

E-mail address: ikebe@iwate-u.ac.jp (M. Ikebe).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2006.01.243

measured by a SQUID magnetometer and the electricalresistivity r(T) was measured by a four-lead method. We
performed detailed Rietveld analyses for the X-ray
diffraction, from which the structure was determined tobe a single orthorhombic phase and the doped Co spinstate to be in Co3+ low spin (LS) conﬁguration (t62g: S ¼ 0).
Fig. 1 shows M(T) of PCMO(100Z%) in the ﬁeld of
0.5 T. M(T) of PCMO(2%) and PCMO(4%) displays ananomalous step-like increase at the FM transition,suggesting the typical ﬁrst-order transition. The very sharptransition, making a marked contrast to other 3d transition
metal elements, comes from the nonmagnetic nature ofdoped Co3+(LS) [3]. The M(T) of PCMO(6%) andPCMO(10%) shows a usual continuous behavior at theFM transition.
Figs. 2(a)–(d) present vs(T) and r(T). In Fig. 2(a), r(T)shows no anomaly at TCO, where vs(T) exhibits a drastic
increase accompanying a slight vs softening above TCO. In
Fig. 2(b) and (c), the vs hardening at TCO is retained forPCMO(2%) and PCMO(4%), though the CO transition
cannot be identiﬁed from r(T) and M(T). r(T) ofPCMO(2%) and PCMO(4%) exhibits the sharp colossal
insulator–metal (I–M) transition at TFM. In Fig. 2(d), vsARTICLE IN PRESS
M. Ikebe et al. / Physica B 378–380 (2006) 534–536

enhancement takes place at the FM (insulator) transition
temperature TFI. This means that the lattice hardening
similar to that accompanied by CO occurs in PCMO(10%)
concomitant with the transition to the insulating FM
phase. The vs behavior of PCMO(6%) is quite similar
to PCMO(10%), showing the characteristic vs hardening
at TFI.
Fig. 3 displays the thermal expansion L(T)/LR. In
pristine PCMO(0%), L(T)/LR slightly deviates upwards
from the basic line around TCO (volume expansion) withdecreasing temperature, while L(T)/LR displays a sharp

contraction at TFM in PCMO(2%) and PCMO(4%). In themetallic phase, the local Jahn–Teller distortion around theMn3+ ions is released, which results in the shrink of thelattice volume. It is to be noted that the slight volumeexpansion similar to CO onset is noticeable also inPCMO(6%) and PCMO(10%) at TFI.
In summary, by substituting low spin Co3+ for theMn3+ site in charge ordered Pr0.65Ca0.35MnO3, FM-metalstate and FM-insulator phase successively realize withincreasing doping. Spectacular changes of lattice, i.e., the10-30.2

4
Z=0.04535

Z=0TCO

Pr0.65Ca0.35(Mn1-ZCoZ)O3TFM
Z=0.0220.1

Z=0.02

1TCO

L(T)/LRTCOTN

M (µB/Mn-site)

Z=0.10Z=0.04
TFI

Z=0.06

Z=0.10Z=0Pr0.65Ca0.35(Mn1-ZCoZ)O30100

0300

200

0100

200300

T (K)

T (K)

Fig. 1. Temperature dependence of magnetization M(T).

Fig. 3. Thermal expansion L(T)/LR as a function of T.

5.9107107

6.0
Z=0.04

Z=0
5.8

5.9

5.6
5.5

103

5.4

105

5.8

 (mΩcm)

105vs (105 cm/s)

5.7

 (mΩcm)

TNvs (105 cm/s)

5.7

5.5

TCO

(a)

5.3

103

5.6
TCO
TFM

(c)

101

5.4

101
Z=0.02

Z=0.10

5.7

5.5
103

5.4
TCO

5.3(b)
5.2
0

105

5.8

 (mΩcm)105

5.6

vs (105 cm/s)

5.9

 (mΩcm)

0vs (105 cm/s)

M (µB/Mn-site)

3 Z=0.06

5.7
103

5.6
5.5

TFI

TFM

(d)

100

200

101300

5.4

0100

T (K)

200
T (K)

3+Fig. 2. Sound velocity vs(T) and resistivity r(T) for Co–doped PCMO.

101300ARTICLE IN PRESS536

M. Ikebe et al. / Physica B 378–380 (2006) 534–536

vs(T) hardening below TCO and the L(T)/LR shrink below
TFM, have been observed. The lattice hardening similar tothat of pristine Pr0.65Ca0.35MnO3 at TCO also takes place atthe onset of the insulating FM state for PCMO(6%) andPCMO(10%). It must be emphasized that the similar
lattice change occurs at TCO and at TFI in the Co-doped
PCMO system.

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