Large magnetic-entropy change above 300 K in CMRmaterials a Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea b Center for Materials Science, National Un
Trang 1Large magnetic-entropy change above 300 K in CMR
materials
a Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea b
Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam c
Donetsk Physico-Technical Institute of National Academy of Sciences, 83114 Donetsk, Ukraine
Abstract
A large magnetic-entropy change DSMassociated withthe ferromagnetic–paramagnetic transition in CMR materials (La0.65Sr0.35MnO3, La0.6Sr0.2Ca0.2MnO3, La0.6Sr0.2Ba0.2MnO3 and La0.7Ca0.06Ba0.24MnO3) has been observed It is shown that the DSM reaches a maximum value of 2.26 J/kg/K for La0.6Sr0.2Ba0.2MnO3 composition at Curie temperature of 354 K, upon 10 kOe applied field variation Due to the large DSM and high Curie temperature, these CMR materials are suggested to use as active magnetic refrigerants for magnetic refrigeration technology above room temperature
r2002 Elsevier Science B.V All rights reserved
Keywords: Entropy; Magnetocaloric effect; Magnetic refrigeration; Perovskite; Double exchange
Magnetic cooling by the magnetocaloric (MC) effect
has a long history First, in 1926 Debye[1]and in 1927
Giauque [2] predicted the theoretical possibility of
adiabatic demagnetization cooling The MC effect is
well known to be a large change of magnetic entropy
closely related to that of the temperature of an
adiabatically isolated system caused by variation of an
external magnetic field In recent years, Pecharsky and
Gschneidner [3] discovered a giant magnetic-entropy
change associated with the transition temperature (TC)
in Gd metal and then in Gd5Si2Ge2 alloy The last
compound exhibits an MC effect about twice as large as
that exhibited by gadolinium, the best known magnetic
refrigerant material for near room temperature
applica-tions However, the purpose in searching a proper
material with the large magnetic-entropy change and its
possibility of various temperature-ranges application is
always required Rare-earthperovskite manganites of the
general formula La1xMxMnO3 (M=Ca, Sr, Ba, etc.)
have attracted much attention because of their higher potential for magnetic sensor applications based on the magnetoresistance effect[4,5] Additionally, these mate-rials are very convenient for the preparation routes, and their Curie temperature can be justified under the various doping conditions Therefore, the new trends have been focusing on studying the MC effect of perovskite manganites [6–8] Due to the large magnetic-entropy change, they have been widely used for magnetic refrigeration applications in different temperature ranges
In this paper, we present a study of the MC effect in CMR materials (La0.65Sr0.35MnO3, La0.6Sr0.2Ca0.2MnO3,
La0.6Sr0.2Ba0.2MnO3and La0.7Ca0.06Ba0.24MnO3)
La0.65Sr0.35MnO3(No 1), La0.6Sr0.2Ca0.2MnO3(No 2), La0.6Sr0.2Ba0.2MnO3(No 3) and La0.7Ca0.06Ba0.24
M-nO3(No 4) samples were prepared by the conventional solid-state reaction technique from a stoichiometric mixture of La2O3, SrCO3, CaCO3, BaCO3 and MnO2
at a pre-sintering temperature of 12501C for 16 h They were sintered at 13501C for 18 hafter regrinding and pressing for pellets The samples were examined by the X-ray diffraction and showed the single-phase rhombohedral perovskite structure The magnetic
*Corresponding author Tel.: +82-431-261-2269;
fax: +82-431-275-6416.
E-mail address: scyu@chungbuk.ac.kr (S.C Yu).
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved.
PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 1 1 5 1 - 4
Trang 2characteristics were performed with a vibrating sample
magnetometer (VSM) in the fields up to 10 kOe
Fig 1 shows the temperature-dependent
magnetiza-tion for a selected sample (No 3), where its Curie
temperature of 354 K was obtained The Curie
tempera-ture TC; defined as the temperature at which the
qM=qT 2T curve reaches a minimum, has been
deter-mined from the M2T curves The TC of the samples
was summarized inTable 1 InFig 2, the magnetic
field-dependent magnetization curves of No 3 show a strong
variation of magnetization around the Curie
tempera-ture It means that a large magnetic-entropy variation
associated withthe ferromagnetic–paramagnetic
transi-tion temperature (TC) can be made to result; it will be
discussed later
According to the thermodynamic theory, the mag-netic-entropy change caused by the variation of the magnetic field from 0 to Hmax is given by[3]
DSM¼
Z H max
0
qM qT
H
Based on expression (1) the magnetic-entropy changes as
a function of temperature for the samples 1–4 at the external magnetic fields of 10 kOe were calculated and plotted in Fig 3 Large magnetic-entropy changes (jDSmax
M j) are reported for all the samples and they are summarized in Table 1 Among the investigated samples, La0.6Sr0.2Ba0.2MnO3(No 3) exhibits a highest
100 150 200 250 300 350 400 450
0.0
0.3
0.6
0.9
1.2
1.5
1.8
H = 50 Oe
La0.6Sr0.2Ba0.2MnO3(No 3) ZFC
FC
T (K)
Fig 1 Temperature-dependent magnetization taken
bothzero-field-cooled (ZFC) and bothzero-field-cooled (FC) at 50 Oe for a selected
sample of La0.6Sr0.2Ba0.2MnO3 (No 3).
Table 1
Curie temperature T C and the maximum magnetic entropy change, jDS max
M j; for different compositions
a Prepared using high-purity Gd ( B99.8 at% pure).
b Prepared using commercial purity Gd (95–98 at% pure).
0 2000 4000 6000 8000 10000 0
2 4 6 8 10 12
14
La0.6Sr0.2Ba0.2MnO3(No 3)
410 K
210 K
H (Oe)
Fig 2 Magnetic field dependence of the magnetization for La0.6Sr0.2Ba0.2MnO3 (No 3) at temperatures from 210 to 410 K (DT ¼ 10 K).
Trang 3value of 2.26 J/kg/K for jDSmax
M j at the Curie temperature
of 354 K These results indicate that the present
investigated samples are very good substances for
magnetic refrigeration applications A large
magnetic-entropy variation in perovskite manganites has been
almost interpreted in terms of the double-exchange
model [9] It has been believed to relate closely to the
mechanism of double-exchange interaction between
Mn3+ and Mn4+ ions arising from the change in the
Mn4+/Mn3+ ratio, under the doping process [7,8,10]
For our circumstance, the partial replacement of La
with Sr or (Ba,Ca) could enhance the double-exchange
interaction due to the increase of the Mn4+/Mn3+ratio,
and thus result in the large magnetic-entropy change
Additionally, Guo et al [11] indicated that the large
magnetic-entropy change in perovskite manganites
could originate from the spin–lattice coupling in the
magnetic ordering process Since the strong coupling
between spin and lattice, the significant lattice change
accompanying magnetic transition in perovskite
man-ganites has been observed [12] The lattice structural
change in the /Mn2OS bond distance as well as
/Mn2O2MnS bond angle would in turn favor the
spin ordering Thus, a more abrupt variation of
magnetization near TC occurred, resulting in a large
magnetic-entropy change as the large MC effect For
comparison, the data of several magnetic materials,
which could be used as active refrigerants, are
summar-ized in Table 1 As follows from the table, though the
values of jDSmax
M j are smaller than the most conspicuous
MC material Gd5(Si2Ge2), these perovskite manganites are easy to fabricate and exhibit higher chemical stability as well as higher resistivity which is favorable for the lowering of eddy current heating Besides, it is possible to adjust the Curie temperature of perovskite manganites by either A- or B-site doping, and conse-quently, a large magnetic-entropy change can be tuned from low temperature to near or above room tempera-ture, which is beneficial for operating magnetic refrig-eration at various temperature ranges
In conclusion, a large MC effect in CMR materials (La0.65Sr0.35MnO3, La0.6Sr0.2Ca0.2MnO3, La0.6Sr
0.2-Ba0.2MnO3 and La0.7Ca0.06Ba0.24MnO3) withCurie temperatures above 300 K has been found La0.6Sr
0.2-Ba0.2MnO3exhibits the highest value of 2.26 J/kg/K for jDSmax
M jat the Curie temperature of 354 K, upon 10 kOe applied field variation The increasing of the Mn4+/
Mn3+ ratio leads to an enhancement in the double-exchange interaction of Mn3+and Mn4+ ions, which results in a large magnetic-entropy variation A combi-nation of both the large magnetic-entropy change and high Curie temperature makes CMR materials appro-priate substances for magnetic refrigeration applications above room temperature
Researchat Korea was supported by the Korean ResearchFoundation Grant (KRF-2001-005-D20010)
References [1] P Debye, Ann Phys 81 (1926) 1154.
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210 240 270 300 330 360 390 420 450
0.0
0.5
1.0
1.5
2.0
2.5
SM
T (K)
No 1
No 2
No 3
No 4
Fig 3 The magnetic-entropy change as a function of
tempera-ture for the samples (La0.65Sr0.35MnO3 (No 1),
La0.6Sr0.2-Ca0.2MnO3 (No 2), La0.6Sr0.2Ba0.2MnO3 (No 3) and
La0.7Ca0.06Ba0.24MnO3 (No 4)) upon a 10 kOe field variation.