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JOURNAL OF PHYSICS: CONDENSED MATTER

J. Phys.: Condens. Matter 21 (2009) 486001 (5pp)

doi:10.1088/0953-8984/21/48/486001

Effect of pressure on lattice distortion,transport and magnetic properties ofPr-substituted La1.2Sr1.8Mn2O7 bilayered
manganite
Y Yamato1 , M Matsukawa1 , S Nimori2 , R Suryanarayanan3,
Y Murano1, Y Nakanishi1 , M Apostu4 , A Revcolevschi3,
K Koyama5 and N Kobayashi51Department of Materials Science and Technology, Iwate University, Morioka 020-8551,
Japan
2
National Institute for Materials Science, Tsukuba 305-0047, Japan
3
Laboratoire de Physico-Chimie de L’Etat Solide, CNRS, UMR 8648, Universite Paris-Sud,
91405 Orsay, France
4
Department of Physical, Theoretical and Materials Chemistry, Faculty of Chemistry,Al I Cuza University, Carol I, 700506 Iasi, Romania
5
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

E-mail: matsukawa@iwate-u.ac.jp

Received 7 September 2009, in final form 9 October 2009
Published 30 October 2009
Online at stacks.iop.org/JPhysCM/21/486001
Abstract
We have studied the effect of pressure on the anomalous lattice striction, both in the ab -plane
and along the c-axis, of (La, Pr)1.2 Sr1.8 Mn2 O7 single crystals over the temperature region wherethe paramagnetic insulator to ferromagnetic metal transition takes place. We have examined thetemperature dependence of the resistivity and the magnetization under applied pressure. The
chemical pressure effect due to Pr-substitution at the La site suppresses the transition
temperature of the parent crystal, while the application of external pressure on Pr-substituted
crystals enhances the double exchange driven metallic state, resulting in a stable rise of Tc .
(Some figures in this article are in colour only in the electronic version)that the La1.2 Sr1.8 Mn2 O7 bilayered manganite exhibits aparamagnetic insulator (PMI) to ferromagnetic metal (FMM)
transition around Tc ∼ 120 K and an associated CMR
effect [5]. The Pr(Nd)-substitution on the La site leadingto {La1−z , Pr(Nd)z }1.2 Sr1.8 Mn2 O7 causes an expansion along
the c-axis but a shrinkage along the a(b)-axis, resulting in achange of the eg electron occupation from the dx 2 −y 2 to thed3z2 −r 2 orbital [6, 7]. These findings accompany not only asuppression of the PMI to FMM transition temperature, Tc ,but also a variation of the easy axis of magnetization from theab-plane to the c-axis. Application of physical pressure is a
powerful tool to investigate the lattice effect on the magnetic
and electronic properties of doped manganites as well as
the chemical pressure effect due to the other rare-earth ion
substitution on the La site [8]. Many studies have been carried

1. Introduction
The observation of a phenomenon of a colossal magnetoresistance (CMR) effect has renewed the interest for doped
manganites with a perovskite structure [1]. Although theinsulator to metal (IM) transition and its associated CMR
are well explained on the basis of the double exchange(DE) model, it was pointed out that the dynamic Jahn–Teller(JT) effect, due to the strong electron–phonon interaction,
plays a significant role in the appearance of CMR as well
as of the DE interaction [2, 3]. Furthermore, Dagotto
et al proposed a phase separation model where ferromagnetic(FM) metallic and antiferromagnetic (AFM) insulating clusterscoexist, which strongly supports recent experimental studies ofthe physics of manganites [4]. Moritomo et al have reported
0953-8984/09/486001+05$30.00

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J. Phys.: Condens. Matter 21 (2009) 486001

Y Yamato et alout so far on the effect of pressures on the structural, magneticand transport properties in bilayered manganites [9–13].
We report here the effect of pressure on latticestriction, both in the ab -plane and along the c-axis,
associated with the spontaneous PMI to FMM transition of(La1−z , Prz )1.2 Sr1.8 Mn2 O7 bilayered manganite single crystals.
Ibarra et al have shown that, in Y-substituted La2/3 Ca1/3 MnO3 ,a volume shrinkage of lattice in the low-T metallic state
below Tc is a consequence of carrier delocalization [14].
A detailed study of anisotropic magnetoelastic effects inbilayered manganite crystals La2−2x Sr1+2x Mn2 O7 (0.3 x  0.45) has indicated a systematic variation in theorbital state of the eg electrons with carrier doping [15].
In addition, neutron diffraction experiments on bilayered
manganites La2−2x Sr1+2x Mn2 O7 (0.32  x  0.40) haverevealed that the lattice anomalies commonly observed at Tcin CMR manganites disappear at the optimal composition ofTc , x = 0.36 [16]. Accordingly, the lattice distortions in
CMR materials provide crucial knowledge about the role of thelattice and orbital degrees of freedom in the CMR mechanism
through the Jahn–Teller-type lattice–orbital coupling.

2. ExperimentSingle crystals of (La1−z , Prz )1.2 Sr1.8 Mn2 O7 (z = 0, 0.2 and0.4) were grown by the floating zone method using a mirror
furnace. The calculated lattice parameters of the tetragonal
crystal structure of the crystals used here were shown in aprevious report [7]. The dimensions of the z = 0.2 sample
are 3.4 × 3 mm2 in the ab -plane and 1 mm along the c-axis.
Measurements of thermal expansion, both in the ab -plane andalong the c-axis, were done by means of a conventional strain
gauge method.
Magnetostrictions and magnetoresistances were measured
at the Tsukuba Magnet Laboratory, the National Institute forMaterials Science (NIMS) and at the High Field Laboratory for
Superconducting Materials, Institute for Materials Research,
Tohoku University. Hydrostatic pressures in the latticestriction, resistivity and magnetization experiments wereapplied by a clamp-type cell using Fluorinert as a pressuretransmitting medium. The pressure was calibrated by thecritical temperature of lead. The magnetization measurements
were made with a superconducting quantum interference
device magnetometer, both at Iwate University and NIMS.

Figure 1. (a) Temperature variation of the ab-plane resistivity of thePr-substituted z = 0.2 crystal under applied pressures of 0, 0.5 and
0.8 GPa. For comparison, the resistivity data are given in a magnetic
field of 8 T ( H  ab). (b) ab-plane resistivity of the parent andz = 0.4 crystals as a function of temperature without appliedpressure. Arrows denote the insulator to metal transition
temperatures at ambient pressure, which almost agree with the Curie
temperatures.

denotes the ab -plane resistivity of the parent and z = 0.4
crystals as a function of temperature without applied pressure.
The Pr-substitution for the La site strongly degrades Tc from120 K at z = 0 (parent) down to 60 K at z = 0.4 through 90 K
at z = 0.2.
Next, let us show in figure 2, the effect of pressure on theab-plane magnetization of the z = 0.2 and 0.4 crystals. Forz = 0.2, the value of magnetization at low temperatures under
1 T reaches ∼70 emu g−1 , close to the fully ferromagnetic
moment of 3.6 μB /Mn site. The pressure dependence ofTc at z = 0.2 is estimated to be ∼17 K GPa−1 from themagnetization data, which is in good agreement with the Tcvariation with pressure from the resistivity data. For z = 0.4,
weak ferromagnetism appears below ∼60 K, where the valueof ρ reaches its maximum value. From both figures 1(b)and 2(b), we notice a slope change in the resistivity curve of

3. Results and discussion
First of all, in figure 1(a), we present the temperature variationof the ab -plane resistivity of the Pr-substituted z = 0.2 crystal
under applied pressures of 0, 0.5 and 0.8 GPa. For comparison,
the resistivity data for the same crystal are given in a magnetic
field of 8 T ( H  ab ). The applied pressure enhances the PMIto FMM transition temperature Tc from ∼90 K at 0 GPa up to∼105 K at 0.8 GPa, and reduces the resistivity values over thewhole measured temperature range. The value of Tc increases
with pressure as dTc /d P = 18.8 K GPa−1 , which is thesame as the pressure dependence of the transition temperature,dTc /d P = 19 K GPa−1 , for the parent crystal [11]. Figure 1(b)2

J. Phys.: Condens. Matter 21 (2009) 486001

Y Yamato et alFigure 2. Effect of pressure on the zero field cooled (ZFC) ab-planemagnetization Mab . (a) The z = 0.2 crystal under 0, 0.5 and 1.0 GPain an applied field of 10 mT. The inset shows Mab data of the z = 0.2
crystal in 1 T. (b) The z = 0.4 crystal under 0 and 1.0 GPa in 10 mT.
For comparison, the Mab data of z = 0 crystal at ambient pressure
are displayed.

Figure 3. The effect of pressure on the lattice striction, both in theab-plane and along the c-axis, associated with the spontaneous PMI
to FMM transition. (a) z = 0. (b) z = 0.2 at ambient pressure.
(c) z = 0.2 at an applied pressure of 0.8 GPa. Thed L c (T )/L c (200 K) data are also shown at an applied field of 8 T
under ambient pressure and 0.8 GPa. ( H  ab).

the z = 0.4 crystal at around 200 K and its associated magnetic
anomaly in the magnetization curve at the same temperature.
This peculiar behavior probably indicates that FMM clusters
of a pseudocubic phase such as (La0.6 , Pr0.4 )1−x Sr x MnO3
with high Tc are dispersed within the double layered matrix,
although the x-ray diffraction analysis on all samples includingthe z = 0.4 crystal never detected such impurity phase.
Now, we examine the effect of pressure on the latticestriction, both in the ab -plane and along the c-axis,
accompanied by the spontaneous PMI to FMM transition, as
shown in figures 3(b) and (c). We note that d L = L(T ) − L(200 K) at 0 T. For the z = 0.2 crystal, the value ofd L c (T )/L c (200 K) shows a sizable decrease around Tc at bothambient pressure and an external pressure of 0.8 GPa withdecreasing temperature, while the ab -plane lattice striction
exhibits no prominent anomaly near Tc . Here, the variations of
the c-axis lattice parameters associated with the PMI to FMM
transition are estimated to be c/c ∼ −0.04% and −0.07%,
for 0 and 0.8 GPa, respectively, which agree well with the caxis magnetostriction near Tc . The lattice shrinkage due tothe application of magnetic field is closely related to the CMR
effect in doped manganese oxides.
On the other hand, the ambient lattice striction of theparent crystal near Tc expands along the c-axis but shrinks inthe ab -plane (see figure 3(a)). The qualitative difference in theanisotropic lattice anomalies between the z = 0.2 and parent
crystals is reduced to a change of the eg electron occupation
between the dx 2 −y 2 and the d3z2 −r 2 orbital [6, 7]. In the PMI
state above Tc , we assume that the local lattice distortion ofMnO6 octahedron strongly couples with the orbital state of theeg electron through the local JT effect. Upon decreasing T
across Tc , the lattice distortion is relaxed due to the itinerant
state of carriers, resulting in the observed lattice anomalies.
For the parent crystal, we expect from the large decrease ina/a ∼ −0.08% near Tc that the dx 2 −y 2 orbital state isdominant. Contrary to it, for the Pr-substitution crystal, therapid shrinkage of d L c (T )/L c (200 K) gives rise to the d3z2 −r 2
orbital state being elongated along the c-axis in the PMI phase.
Furthermore, figure 4 represents the ab -plane and the caxis thermal expansion data of the z = 0.4 crystal withTc ∼ 60 K (ambient pressure). We notice a qualitativedifference in the ab -plane lattice striction between 0 and
0.8 GPa. The ambient value of d L ab (T )/L ab (200 K) shows
no visible change around Tc , but the high-pressure data exhibita clear upturn in the ab -plane below Tc accompanied by an3

J. Phys.: Condens. Matter 21 (2009) 486001

Y Yamato et al

4. Summary
We have studied the effect of pressure on the unusual
lattice striction, both in the ab -plane and along the c-axis,of (La, Pr)1.2 Sr1.8 Mn2 O7 bilayered manganite single crystals.
We have also examined the temperature dependence of theresistivity and the magnetization under applied pressure. The
pressure coefficient, dTc /d P , for the z = 0.2 crystal isestimated to be ∼19 K GPa−1 from the transport and magnetic
properties, which is in good agreement with that of the parent
material. The chemical pressure effect due to Pr-substitution
at the La site suppresses the PMI–FMM transition temperatureof the Pr-free crystal. However, the application of hydrostaticpressure on the Pr-substituted crystal enhances the double
exchange driven FMM state, resulting in a stable rise of Tc .
The pressure effect on lattice distortion near Tc gives rise
to a larger shrinkage of the c-axis by c/c ∼ −0.07%
and −0.11% at z = 0.2 and 0.4, respectively. The c-axis
lattice shrinkage enhanced below Tc indicates that the MnO6octahedron couples with the d3z2 −r 2 orbital state in the PMIstate. Our data are well explained on the basis of both theexternal and chemical pressure effects on the MnO6 octahedronin bilayered manganites [9, 17].

Figure 4. The effect of pressure on the lattice striction, both in theab-plane and along the c-axis, associated with the spontaneous PMI
to FMM transition. (a) z = 0.4 at ambient pressure. (b) z = 0.4 at an
applied pressure of 0.8 GPa.

Acknowledgments
This work was partially supported by a Grant-in-Aid for
Scientific Research from the Japan Society of the Promotionof Science. We thank A Tamura for his technical support.

enhanced lattice shrinkage along the c-axis, c/c ∼ −0.11%(−0.07% at 0 GPa). These findings contrast with a substantial
upturn in d L c (T )/L c (200 K) below Tc ∼ 120 K (ambientpressure) for the z = 0 parent crystal.
We comment now on the pressure effect on the MnO6octahedron in the parent bilayered manganite La1.2 Sr1.8 Mn2 O7 .
Argyriou et al have reported that upon increasing the pressure
up to 0.6 GPa, in the PMI phase, both the Mn–O(1) andMn–O(3) bond lengths shrink but the Mn–O(2) bond length
elongates [9]. Here, the O(1) and O(2) oxygen atoms arelocated at the apical site along the c-axis, where O(1) lies
between two MnO2 layers and O(2) within a rocksalt-typeLa/Sr–O layer. The O(3) oxygen atom is within the MnO2layer. Assuming such a variation of the MnO6 octahedronin the PMI state of Pr-substituted La1.2 Sr1.8 Mn2 O7 , the Mn–
O(1)–Mn interactions along the c-axis and the Mn–O(3)–Mn
interactions in the ab -plane are expected to be strengthened
upon application of pressure. In other words, the double
exchange (DE) interaction between Mn3+ and Mn4+ within
the MnO2 single layer and the DE interaction along the c-axis
within the bilayer are enhanced; this is closely related to a riseof Tc with pressure. On the other hand, for the Pr-substituted
crystal, the Mn–O(1) bond length is elongated upon increasing
Pr, reducing the transfer integral between Mn–O(1)–Mn within
the bilayer [17]. As a result, the FMM phase is suppressedat heavy Pr-substitution. Accordingly, it is concluded fromthe microscopic measurements on both external and chemicalpressure effects that the effect of pressure on Pr-substituted
crystals causes a reduction of the elongated Mn–O(1) bondlength accompanied by an expansion of the Mn–O(2) bondlength, resulting in a stable rise of Tc . These findings are
consistent with our data of lattice distortion.

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