Magnetocaloric effect of La0.8Sr0.2MnO3 compound under pressure

JOURNAL OF APPLIED PHYSICS 97, 10M317 共2005兲
Magnetocaloric effect of La0.8Sr0.2MnO3 compound under pressure
Daniel L. Rocco
Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas-UNICAMP, Caixa Postal 6165,
13083-970 Campinas, SP, Brazil
R. Almeida Silva
Departamento de Física e Ciência dos Materiais, Instituto de Física de São Carlos, Universidade de São
Paulo-USP, CP 369, 13560-590 São Carlos, SP, Brazil
A. Magnus G. Carvalho and Adelino A. Coelho
Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas-UNICAMP, Caixa Postal 6165,
13083-970 Campinas, SP, Brazil
José P. Andreeta
Departamento de Física e Ciência dos Materiais, Instituto de Física de São Carlos, Universidade de São
Paulo-USP, CP 369, 13560-590 São Carlos, SP, Brazil
Sergio Gama
Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas-UNICAMP, Caixa Postal 6165,
13083-970 Campinas, SP, Brazil
共Presented on 9 November 2004; published online 16 May 2005兲
The La0.8Sr0.2MnO3 compound presents a ferromagnetic paramagnetic transition around room
temperature to which a reasonably high magnetocaloric effect is associated, turning this material of
interest for application in magnetic refrigeration. We synthesized this compound in fiber single
crystalline form by the Laser Heated Pedestal Growth method. The sample was characterized by
x-ray diffraction and magnetic measurements as a single phase and with the required magnetic
properties. We measured the magnetic properties and the magnetocaloric effect under hydrostatic
pressure for pressures up to 6 kbar as a function of temperature. Our results indicate that the Curie
temperature increases with pressure while the low temperature transition from the orthorhombic to
the rhombhoedral structures decreases as pressure increases. This is in close agreement with the
literature. Measurement of the magnetocaloric effect at the high temperature transition indicates that
the peak of the effect follows the trend of the Curie temperature, but its maximum value remains
almost constant as a function of pressure. © 2005 American Institute of Physics.
关DOI: 10.1063/1.1856891兴
I. INTRODUCTION
The manganites are ceramic compounds with great interest due to the colossal magnetoresistance effect and also because they present a rich set of physical phenomena, such as
spin, charge, and orbital orderings, which impart them interest for applications as well as theoretical studies.1 The system La1−xSrxMnO3 is of great importance because it can
show a high temperature ferromagnetic transition and also
presents a rich electrical and magnetic phase diagram.2 For
low Sr concentration x ⬍ 0.1, the material presents a metallic
antiferromagnetic state, for 0.1⬍ x ⬍ 0.15 it shows an insulating ferromagnetic state and for x ⬎ 0.15 it shows a metallic
ferromagnetic state.2 Besides this, the magnetic transition
temperature varies strongly with x, from 170 K for x = 0.11
up to 310 K for x = 0.2, and this allows one to easily tune the
transition to a desired value.3
Recently, the manganites have been considered as potential candidates for active magnetic regenerators in magnetic
refrigeration systems because they can show considerable
magnetocaloric effects 共MCE兲, are easily fabricated and can
have the Curie temperatures tuned in a wide temperature
range.4,5 The system La1−xSrxMnO3 shows a great potential
in relation to the MCE because of the properties already
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described and because for x = 0.13 and x = 0.16 it shows MCE
comparable to the ones observed for Gd and Ca doped manganites and for metallic alloys.4 Also, its properties under
pressure have been studied for x = 0.10, which corresponds to
the lower edge of the concentration region in which the low
temperature state is a ferromagnetic insulator.6
The compound with x = 0.2 is of particular interest in
relation to the MCE because it presents a TC of ⬃305 K, and
a structural transition from rhombohedral 共R兲 to orthorhombic 共O*兲 structure at 100 K. Here we present our results on
the determination of its MCE under hydrostatic pressure.
II. EXPERIMENT
The compound with nominal composition La0.8Sr0.2
MnO3 has been prepared directly from the stoichiometric
mixture of oxides and carbonates by the Laser Heated Pedestal Growth 共LHPG兲 method.7 The resulting sample in
single crystalline fiber form has been characterized by x-ray
diffraction and Laue back reflection diffraction, metallographic analysis, electron microprobe analysis, and magnetic
measurements. The MCE has been measured through the
magnetic method using the numerically integrated Maxwell
relation
97, 10M317-1
© 2005 American Institute of Physics
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10M317-2
J. Appl. Phys. 97, 10M317 共2005兲
Rocco et al.
FIG. 1. Effect of pressure on the low temperature structural transition of the
La0.8Sr0.2MnO3 compound.
⌬S共T,H兲 =
冕冉 冊
H2
H1
⳵M
⳵T
dH.
共1兲
H
Magnetization measurements were done using a commercial
SQUID magnetometer. For the pressure measurements, we
used a Cu–Be clamp type cell, able to work up to 12 kbar at
300 K. The sample was inserted in a Teflon container filled
with mineral oil. Our pressure scale has been obtained using
a MnAs sample as a manometer, measuring TC’s determined
increasing the temperature, and comparing with data from
Menyuk et al..8 For the MCE determination, M ⫻ H curves
共up to H = 5 T兲 at several fixed pressures were taken with
both field and temperature always increasing.
III. RESULTS AND DISCUSSIONS
The results of x-ray Laue back reflection diffraction,
metallographic and electron microprobe analyses confirmed
the single phase nature of our sample, presenting the expected perovskite structure. The magnetic analysis showed
that the sample presents the low temperature transition at 110
K from the R to O* structure, with thermal hysteresis, and the
ferromagnetic paramagnetic transition at 302 K, of second
order character. Application of pressure displaces the low
temperature transition to even lower temperatures, and has
the effect of decrease its magnetic amplitude, as if the pressure diminishes the difference between the two structures.
This effect is shown in Fig. 1, and the curves shown are the
ones for increasing temperature. This transition shifts to
lower temperatures at a rate of −6.9 K / kbar, or, in terms of
d共ln共TS兲 / dp, −0.72 GPa−1. Pressure increases the TC of the
high temperature transition, as shown in Fig. 2, at a rate of
1.4 K / kbar, or, in terms of d共ln共TC兲 / dp, 0.045 GPa−1. This
last value compares favorably with the same quantity observed for the compound with x = 0.15.9 Figure 3 shows the
variation of the transition temperatures as a function of pressure, as well as the respective linear fittings of the curves.
Under pressure, our measurements at 4 K applying magnetic fields up to 7 T show that the saturation magnetization
FIG. 2. Effect of pressure on the Curie temperature of the La0.8Sr0.2MnO3
compound.
of this compound does not change with pressure up to 6 kbar.
We measured the MCE at the ferroparamagnetic transition
for a field variation of 5 T always increasing field and temperature. Figure 4 shows the results obtained. For ambient
pressure the MCE has a value similar to the one measured
for the compounds with x = 0.13 and x = 0.16.4 Under pressure, the MCE shows a slight decrease in the peak value with
pressure and the peak is displaced to higher temperatures,
following the same behavior as the Curie temperature. This
behavior is in contrast with the behavior of the Gd5Ge2Si2
compound under pressure, whose Curie temperature also increases with pressure, and whose MCE shows a markedly
decrease with pressure.10,11 This behavior is also in great
contrast with the one observed for the MnAs compound, for
which the Curie temperature decreases and the MCE becomes colossal.12
IV. CONCLUSIONS
Under pressure, the compound La0.8Sr0.2MnO3 presents
a displacement of the low temperature structural transition to
FIG. 3. Transition temperatures for the ferromagnetic–paramagnetic and
structural transitions as a function of pressure for the La0.8Sr0.2MnO3
compound.
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10M317-3
J. Appl. Phys. 97, 10M317 共2005兲
Rocco et al.
ACKNOWLEDGMENTS
The authors thank financial support from Fundação de
Amparo à Pesquisa do Estado de S. Paulo—Fapesp, from
Coordenação de Aperfeiçoamento do Pessoal de Nível
Superior—Capes and from Conselho Nacional de Desenvolvimento Científico e Tecnológico—CNPq.
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1
2
FIG. 4. Behavior of the magnetocaloric effect for a field variation of 5 T for
the La0.8Sr0.2MnO3 compound.
lower temperatures, at a rate of −6.9 K / kbar, and at the same
time the transition is becoming less pronounced. The temperature of the ferroparamagnetic phase transition increases
with pressure at a rate of 1.4 K / kbar, or, equivalently,
d共ln共TC兲 / dp = 0.045 GPa−1, comparable with the value observed for the x = 0.15 compound. The saturation magnetization is not altered by the pressure up to 6 kbar. The magnetocaloric effect decreases very slightly with pressure, and the
peak value is also displaced to higher temperatures, following the Curie temperature.
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