PHYSICAL REVIEW B 78, 132411 共2008兲

Effect of pressure on the steplike magnetostriction of single crystalline bilayered manganite(La0.4Pr0.6)1.2Sr1.8Mn2O7
Y. Yamato,1 M. Matsukawa,1,* T. Kumagai,1 R. Suryanarayanan,2 S. Nimori,3 M. Apostu,4 A. Revcolevschi,2 K. Koyama,5and N. Kobayashi5
1Department of Materials Science and Engineering, Iwate University, Morioka 020-8551, Japan
2Laboratoire de Physico-Chimie de L’Etat Solide, CNRS, UMR8182, Universite Paris-Sud, 91405 Orsay, France
3National Institute for Materials Science, Tsukuba 305-0047, Japan
Department of Physical, Theoretical and Materials Chemistry, Faculty of Chemistry, Al. I. Cuza University,Carol I, 700506 Iasi, Romania
5Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
共Received 2 July 2008; revised manuscript received 4 September 2008; published 29 October 2008兲

4We report the effect of pressure on the steplike magnetostriction of single crystalline bilayered manganite
共La0.4Pr0.6兲1.2Sr1.8Mn2O7, for our understanding of the ultrasharp nature of the field-induced first-order transition from a paramagnetic insulator to a ferromagnetic metal phase. The application of pressure suppresses asteplike transformation and causes a broad change in the magnetostriction. The injection of an electric current
to the crystal also weakens the steplike variation in both the magnetostriction and magnetoresistance. The
stabilization of ferromagnetic interaction or the delocalization of charge carriers is promoted with the appliedpressure or applied current, resulting in the suppressed steplike behavior. Our findings suggest that the step
phenomenon is closely related to the existence of localized carriers such as the short-range charge-orbitalordered clusters.
DOI: 10.1103/PhysRevB.78.132411

PACS number共s兲: 75.80.⫹qI. INTRODUCTION

Perovskite manganites show a great variety of fascinating
properties such as colossal magnetoresistance 共CMR兲 effectand charge-ordered insulating phase.1 The most interesting
one is the existence of a phase-separated state, the coexistence of antiferromagnetic charge-orbital-ordered 共COO兲 insulating and ferromagnetic 共FM兲 metal regions.2 Several recent studies on metamagnetic transition in phase-separated
manganites have revealed that ultrasharp steps in magnetization curves appear at low temperatures.3–8 To account for
this, a martensitic model due to local strain fields stored in
the lattice between competing COO and FM phases with
their different unit cells has been proposed though questions
have been raised against this model.
For our understanding of the dynamics of a steplike firstorder transition from a paramagnetic insulating 共PMI兲 to aferromagnetic metal 共FMM兲 phase in CMR manganites, we
examine the pressure effect on c-axis magnetostriction ofsingle crystalline 共La0.4Pr0.6兲1.2Sr1.8Mn2O7. For the Prsubstituted 共La0.4Pr0.6兲1.2Sr1.8Mn2O7 crystal, a spontaneous
ferromagnetic metal phase 共originally present with no Pr
substitution兲 disappears at the ground state but a fieldinduced PMI to FMM transformation is observed over awide range of temperatures.9,10 A magnetic 共H , T兲 phase diagram, established from magnetic measurements, is separated
into three regions labeled as PMI, FMM, and bistable states,
as shown in Fig. 1 of Ref. 11. The proximity of free energies
between the PMI and FMM states is of importance to realize
the bistable state where the FMM and PMI states coexist. Astronger field is needed to induce the PMI to FMM transition
at low temperatures since the thermal energy is reduced upon
lowering temperature.
II. EXPERIMENT

Single crystals of 共La1−z , Prz兲1.2Sr1.8Mn2O7 共z = 0.6兲 weregrown by the floating-zone method using a mirror furnace.
1098-0121/2008/78共13兲/132411共4兲

The calculated lattice parameters of the tetragonal crystal
structure of the crystals used here were shown in a previous
report.10 The dimensions of the z = 0.6 sample were 3.4⫻ 3 mm2 on the ab plane and 1 mm along the c axis. Measurements of magnetostriction along the c axis were done bymeans of a conventional strain gauge method at the Tsukuba
Magnet Laboratory, the National Institute for Materials Science 共NIMS兲 and at the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku
University. Except for an initial cooldown, the sample waszero-field cooled from 200 K down to low temperatures. We
then started measuring the isothermal magnetostriction uponincreasing 共or decreasing兲 the applied fields, parallel to the caxis. Next, warming the sample from low temperatures up to
200 K in the absence of field and keeping the sample fixed at
200 K for 2 h, the sample was then cooled down to the lowT. The typical cooldown time needed was about 2 h. Checking the stability of the sample’s temperature, we restarted
measuring the magnetostriction. For each measurement, werepeated the same procedure. The normal sweep rate was set
to be 0.26 T/min. Hydrostatic pressures in the magnetostriction experiment were applied by a clamp-type cell using
Fluorinert as a pressure-transmitting medium. The pressurewas calibrated by the critical temperature of lead. After themagnetostriction measurement under the pressure, we restarted measuring dLc共H兲 using the same clamp-type cell but
without the applied pressure. Next, we examined the c-axis
magnetostriction as a function of the applied current on theab plane at ambient pressure. Once the sample was removed
from the clamp-type cell, it was set in a conventional cryostat for magnetotransport measurements. On both ends ofthe sample the electrodes for injection of the electric current
were formed using a gold paste and Cu wires were thenattached with silver paste. For magnetoresistance measurements, we carried out the same temperature profile as that in
the magnetostriction experiments.

132411-1©2008 The American Physical Society

PHYSICAL REVIEW B 78, 132411 共2008兲

BRIEF REPORTS

0.00050.0005(a) 4.3K H||cdL(H)/L(0)-0.0005

dL(H)/L(0)0 GPa
0.8 GPa000 GPa
0.8 GPa-0.001-0.0015-0.0005-0.001-0.0015-0.002-0.002-0.0025(a) 20 K H||c-0.0025

00.0005

12345H (T)68

701230.0005(b) 6 K H||c0 GPa
0.8 GPa

0dL(H)/L(0)dL(H)/L(0)0 GPa
0.8 GPa-0.0015(b) 20 K H||ab0-0.0005

4H (T)-0.0005-0.001-0.0015-0.0015-0.002-0.002-0.0025-0.0025012345H (T)6701

2345H (T)

8FIG. 1. 共Color online兲 The c-axis magnetostriction,dLc共H兲 / Lc共0兲, of 共La0.4Pr0.6兲1.2Sr1.8Mn2O7, under both ambientpressure and a hydrostatic pressure of 0.8 GPa at low temperaturesof 共a兲 4 and 共b兲 6 K. The applied field is parallel to the c axis 共H 储 c兲.
III. RESULTS AND DISCUSSION

Figure 1 shows the c-axis magnetostriction,dLc共H兲 / Lc共0兲, of 共La0.4Pr0.6兲1.2Sr1.8Mn2O7, under both ambient pressure and a hydrostatic pressure of 0.8 GPa at lower
temperatures, where the applied field is parallel to the c axis
共H 储 c兲. Here, the value of dLi共H兲 is defined as Li共H兲 − Li共0兲.
First of all, the steplike lattice transformations appear at 4and 6 K at ambient pressure, as shown in Fig. 1.
On the other hand, the application of external pressure on
the crystal substantially suppresses the steplike transition,
causing a broad variation in the magnetostriction. At 4.3 K,
the critical field Hc is increased from 6.1 T at ambient pressure up to 6.6 T at 0.8 GPa. At 6 K, a huge steplike transition
vanishes under the pressure and a continuously smoothvariation is observed at high fields, followed by the appearance of a tiny step around 8 T. However, the characteristic
field signifying the onset of the field-induced metamagnetictransition, which is not the critical field of the step transitions, is rather lowered at the presence of the applied pressure, as the pressure data measured at high T in Fig. 2 show.
When the magnetic field is applied to the ab plane
共H 储 ab兲, we obtain similar results, as shown in Fig. 3. These

FIG. 2. 共Color online兲 The c-axis magnetostriction,
dLc共H兲 / Lc共0兲, of 共La0.4Pr0.6兲1.2Sr1.8Mn2O7, under both ambientpressure and a hydrostatic pressure of 0.8 GPa at 20 K: 共a兲 H 储 c and共b兲 H 储 ab.

findings are indicative of the occurrence of steplike transformation almost independent of the easy axis of magnetization.
The small differences in the higher critical fields in the caseof H 储 ab probably arise from the easy axis of M lying along
the c axis. Under the application of 0.8 GPa, we obtain bothHc ⬃ 7.6 T at 4.3 K and 8.1 T at 6 K accompanied by adegradation of a discontinuous jump and the disappearanceof any steplike profile in the dLc共H兲 / Lc共0兲 curve at 8 K.
In our previous paper,12 we found out that the applicationof pressure enhances a field-induced ferromagnetic state ofPr-substituted La1.2Sr1.8Mn2O7. Our findings are understoodfrom the viewpoint that the double-exchange interactiondriven FM state is strengthened by the applied pressure. For
the magnetic-field-cooled sample, the remarkable step observed in dLc共H兲 / Lc共0兲 is monotonically decreased upon increasing the cooling field from 1 up to 1.6 T, accompanied bya nonlinear dependence of Hc.13 The steplike behavior vanishes when the cooling field exceeds 1.6 T. For each fieldcooled run, we expect that the FM region is initially formed
within the PMI matrix before the magnetostriction measurements are carried out. A larger volume fraction of the FMphase causes a more suppressed variation in dL. Thus, apressure-induced suppression in the ultrasharp magnetostric-132411-2

PHYSICAL REVIEW B 78, 132411 共2008兲

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0.00050.0005(a) 4.3K H||ab-0.0005

0 GPa
0.8 GPa-0.001-0.0015-0.002-0.0025-0.0005-0.0010.01 mA
1 mA
2 mA-0.0015

I||ab-0.002

0246810H (T)0.0005-0.0025(b) 6 K H||ab-0.001-0.0015-0.00200.0005

268106810(b) 6 K H||c4H (T)680.01 mA
0.4 mA
1 mA
2 mA-0.001-0.0015

10I||ab-0.002-0.0025(c) 8 K H||ab02

4H (T)-0.0005

10 GPa0.8 GPa-0.001

0.1mA(c) 4.3K H||c1mA
2mA-0.0015ab

R (H)/ R (0)dL(H)/L(0)4H (T)-0.0005

02.5mA0.1

ab-0.002-0.0025200 GPa
0.8 GPadL(H)/L(0)dL(H)/L(0)-0.0005

00.0005

0-0.0025(a) 4.3K H||c0

dL(H)/L(0)dL(H)/L(0)00246H (T)810120.01

FIG. 3. 共Color online兲 The c-axis magnetostriction,dLc共H兲 / Lc共0兲, of 共La0.4Pr0.6兲1.2Sr1.8Mn2O7, under both ambientpressure and a hydrostatic pressure of 0.8 GPa at low temperaturesof 共a兲 4, 共b兲 6, and 共c兲 8 K. The applied field is parallel to the abplane 共H 储 ab兲.

tion is a reasonable result since the applied pressure enhances the FM state.
Next, we carry out the magnetostriction measurement under the applied electric current, to examine a relationshipbetween charge transport and steplike lattice deformationthat appeared in CMR manganites. In the case of ambientpressure, the c-axis magnetostriction data at 4.3 K are presented in Fig. 4共a兲 as a function of excited current 共I = 0.01,
1, and 2 mA兲. Here, the electric current is applied on the abplane parallel to the MnO2 double layers. Injection of theelectric current over 2 mA exhibits no discontinuous profile
in dLc共H兲 / Lc共0兲. We note that the bath temperature is very
stable against the applied current up to 2 mA within ⬃0.5%
and Joule heating of the sample affects no contribution to thepresent behavior.

0123

4
H[T]

56

7

8

FIG. 4. 共Color online兲 The c-axis magnetostriction,
dLc共H兲 / Lc共0兲, of 共La0.4Pr0.6兲1.2Sr1.8Mn2O7, under the electric current applied on the ab plane at low temperatures of 共a兲 4 and 共b兲 6K. The applied field is parallel to the c axis. 共c兲 The ab-planemagnetoresistance Rab of the z = 0.6 sample at 4.3 K as a function ofthe applied current from 0.1 up to 2.5 mA. The Rab共H兲 data arenormalized by Rab共0兲, which is the zero-field resistance at the lowest applied current 共0.1 mA兲; 共H 储 c兲.

For comparison, let us now display in Fig. 4共c兲 theab-plane magnetoresistance Rab of the z = 0.6 sample as afunction of the applied current from 0.1 up to 2.5 mA. Firstof all, the application of higher current gives rise to a degradation of the magnetoresistive steps observed at lower currents, which is almost consistent with the magnetostriction
data shown in Fig. 4共a兲. Next, the zero-field resistance mea-132411-3

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sured at 2 mA shows a substantial drop of ⬃80%, in comparison to Rab共0兲 at the lowest current 共0.1 mA兲. This result
indicates the current-induced destabilization of the PMI matrix, although the physical role of the application on an electric field is not clear. Similar results are reported in a systematic study on nonlinear transport of a non-charge-orderedmanganite Pr0.8Ca0.2MnO3, which is not so far from the COO
region.14 In a lightly Pr-substituted crystal of
共La0.8Pr0.2兲1.2Sr1.8Mn2O7 with Tc ⬃ 90 K, we also have obtained a current-induced remarkable drop in the ab-plane resistance at selected temperatures just above Tc 共not shownhere兲. The formation of short-range charge ordering in theparamagnetic phase of an optimally doped parent crystal
La1.2Sr1.8Mn2O7 has been observed in a previous study.15 Accordingly, we expect that the application of the electric current removes the localized carriers, such as the COO insulating clusters, from the PMI phase of the Pr-substituted crystal,causing a substantial reduction in the zero-field resistance athigher currents.
Furthermore, neutron-scattering measurements on a bilayered manganite La2−2xSr1+2xMn2O7 共x = 0.38兲 revealed that
the CE-type 共zig-zag FM chains with antiferromagnetic interchain coupling兲 COO clusters freeze upon decreasing temperature from T ⬃ 310 K, preventing the formation of along-range COO state.16 The authors also argued that thePMI state of the crystal without Pr substitution is caused byan orbital frustration and stabilized down to the FMM transition temperature Tc ⬃ 114 K. For the present Pr-substitutedsample, one believes that the short-range COO clusters accompanied by the orbital frustration are present within thePMI matrix. As discussed in Ref. 13, we suppose that theorbital frustration has some relationship with the critical instability of the metastable state of the free energy, resulting
in the steplike phase transition. The application of hydro-*matsukawa@iwate-u.ac.jp
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static pressure stabilizes the ferromagnetic interaction within
the paramagnetic matrix and the applied current reinforces anitinerant state of charge carriers within the insulating matrix.
In other words, these external variables probably lead to acollapse of the CE-type antiferromagnetic charge-orderedclusters if clusters of this type are distributed within thesample. In addition, Mahendiran et al.3 claimed that thecharge-orbital-ordereddomainsintheCo-doped
Pr0.5Ca0.5MnO3 are smaller than those in the undoped
Pr0.7Ca0.3MnO3, suggesting that the onset temperature of thestep transitions in the Co-doped compound is higher thanthat in the undoped one. We emphasize that the distributionof short-range COO clusters embedded in the matrix provides a significant clue for our understanding of the stepliketransformations in CMR manganites. The ultrasharp PMIFMM transition induced by the magnetic field at low temperatures is in its origin quite different from the standard
insulator-metal transition associated with the metamagnetictransition at higher T.
In summary, we have demonstrated the influence of pressure on the steplike lattice deformation of single crystalline
bilayered manganite 共La0.4Pr0.6兲1.2Sr1.8Mn2O7. The external
perturbations such as the application of pressure and electric
current give a substantial suppression on the steplike transition, resulting in a broad change in the magnetostriction. It isa future problem to resolve what role the COO clusters
within the PMI phase play in the occurrence of the steplike
phenomenon observed.
ACKNOWLEDGMENTS

This work was supported by a Grant-in-Aid for Scientific
Research from the Japan Society of the Promotion ofScience.

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