Journal of Magnetism and Magnetic Materials 285 2005 199–203Large magnetic entropy change in Cu-doped manganites Manh-Huong Phana,b, , Hua-Xin Penga, Seong-Cho Yub, Nguyen Duc Thoc, Nguy
Trang 1Journal of Magnetism and Magnetic Materials 285 (2005) 199–203
Large magnetic entropy change in Cu-doped manganites
Manh-Huong Phana,b, , Hua-Xin Penga, Seong-Cho Yub,
Nguyen Duc Thoc, Nguyen Chauc
a Department of Aerospace Engineering, Bristol University, Queen’s Building, University Walk, Bristol, BS8 1TR, UK
b Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea
c Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Vietnam
Received 8 November 2003; received in revised form 20 June 2004
Available online 17 August 2004
Abstract
Magnetic entropy change above 300 K, which is larger than that of gadolinium (Phys Rev B 57 (1998) 3478), has been observed in a Cu-doped manganites of La0.7Sr0.3Mn1xCuxO3(x ¼ 0:05; 0:1) The large magnetic entropy change originated from a sharp magnetization jump is associated with a first-order crystallographic phase transition of the sample near the Curie temperature These results suggest that the present Cu-doped manganites are suitable candidate materials for magnetic refrigerants in the room temperature magnetic-refrigeration technology
r2004 Elsevier B.V All rights reserved
PACS: 75.30.Sg; 75.30.m; 75.50.y
Keywords: Magnetic entropy; Magnetic refrigeration; Cu-doped manganites
Recently, several works have reported a large
magneto-caloric effect (MCE) in polycrystalline
[1–7] and single crystalline[8] manganese
perovs-kite materials The MCE is an intrinsic
thermo-dynamic property of magnetic solids, and
manifests itself as an adiabatic temperature change
closely related to the magnetic entropy change
caused by the application of magnetic field
Materials with large MCE have attracted growing
interest owing to the possible applications for magnetic refrigerants[2,5,7,8] In general, there are two basic requirements for a magnetic material to possess a large MCE One is a large spontaneous magnetization (such as in the case of a heavy rare-earth metal, Gd, for example)[9,10], the other is a sharp drop in magnetization with increasing temperature, which is associated with the ferro-magnetic-paramagnetic transition at the Curie temperature found in perovskite manganites
[1–7] Additionally, considerable coupling between spin and lattice in the magnetic ordering process in perovskite manganites was believed to occur and
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doi:10.1016/j.jmmm.2004.07.041
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E-mail address: M.H.Phan@bristol.ac.uk (M.-H Phan).
Trang 2result in an additional change of magnetic entropy
[2,4–7] Some remarkable anomalies in the vicinity
of the magnetic ordering transition were also
observed in a series of Cu-doped manganese
oxides of La0.825Sr0.175Mn1xCuxO3 (0.20
pxp0.40)[11] The study of MCE in such a
Cu-doped manganese perovskite can be, therefore, of
great interest In the present work, the MCE in the
La0.7Sr0.3Mn1xCuxO3 (x ¼ 0:05; 0.10)
polycrys-talline perovskites is investigated It was found
that the magnetic entropy change above 300 K is
larger in the present Cu-doped manganites than
that in gadolinium [9] The origin of the large
magnetic entropy change is attributed to the
abrupt reduction of magnetization which is
associated with a first-order phase transition near
the Curie temperature
La0.7Sr0.3Mn1xCuxO3 (x ¼ 0:05; 0.10)
poly-crystalline materials were prepared using a
con-ventional powder solid-state reaction method
Stoichiometric mixtures of La2O3, SrCO3, C uO
and MnCO3powders were used The samples were
pre-sintered at 10001Cfor 15 h followed by
grinding into compound powders The compound
powders were then pressed into pellets and sintered
at 12501Cfor 35 h to give the finished samples
X-ray diffraction (Bruker D5005) confirmed the
single-phase rhomboredral perovskite structure for
both the compound powder and the finished
samples The thermal stability associated with
crystallization and melting was determined by
differential scanning calorimetry (DSC) and
ther-mo-gravimetric analysis (TGA) (TA Instruments
Apparatus SDT 2960) with a heating rate of 201C/
min Magnetic measurements were performed
using a Vibrating Sample Magnetometer in
magnetic fields up to 19 kOe
Fig 1shows the temperature dependences of the
magnetization of the La0.7Sr0.3Mn0.9Cu0.1O3
(x=0.10) sample measured in the fields of 100 Oe
and 5 kOe (the insert of Fig 1) The Curie
temperature (TC), defined by the maximum in the
‘‘absolute value’’ of dM/dT, has been determined
from the M–T curve and found to be 347 and
349 K at H=100 Oe and 5 kOe, respectively It is
noted that, at H=5 kOe, the shape of the M–T
curve remains almost unchanged, while the TC is
shifted to a higher temperature (349 K) Similar
behavior was observed for the La0.7Sr0.3Mn0.95
Cu0.05O3 (x=0.05) sample As reported in Ref
[12], the MCE material MnAs0.9Sb0.1 exhibited a smooth temperature variation of the magnetiza-tion under high fields whereas the shape of the M–T curve for MnAs was almost unchanged, except the increase of the magnitude of magnetiza-tion and the shift of TC to higher temperature Consequently, MnAs was found to exhibit a larger magneto-caloric effect than MnAs1xSbx[12] For the present Cu-doped manganites, the consider-able increase of magnetization observed in H=5 kOe is consistent with the result that had been reported on other MCE materials, such as
La0.8Ca0.2MnO3polycrystalline perovskite[2]and
La1.4Ca1.6Mn2O7 layered perovskite [13] There-fore, the La0.7Sr0.3Mn1xCuxO3 (x ¼ 0:05; 0.10) materials in the present study are expected to exhibit large MCE near the Curie temperature In order to confirm this, the isothermal magnetiza-tion of both x ¼ 0:05 and 0.10 samples were measured with a field step of 500 Oe in a range of 0–19 kOe and a temperature interval of 5 K in a temperature range of 100–380 K It is reasonable
to consider the magnetization curves to be isothermal due to the sufficiently low sweeping rate of the magnetic field adopted during the experiment To ensure the readability of the figure,
Fig 1 Temperature dependence of the magnezation for the
La 0.7 Sr 0.3 Mn 0.9 Cu 0.1 O 3 sample in the fields of 100 Oe and 5 kOe (in the insert).
Trang 3only twelve of them are presented in Fig 2
including all the results obtained near the TC It
can be seen clearly from Fig 2 that there is a
drastic change of the magnetization around the
TC, indicating a large magnetic entropy change
This coincides with the rapid reduction of
magne-tization at the TC(Fig 1) Another feature to be
noted is that a large proportion of changes of the
magnetization occurs in a relative low-field range
(o19 kOe), which is beneficial for the household
application of MCE materials[14]
In order to evaluate the MCE of the present
materials, we calculated changes of the magnetic
entropy (DSM) caused by the application of
external magnetic fields from the isothermal curves
of magnetization versus the applied field by using
the following expression[1]
DSM
j j ¼X
i
MiMiþ1
Tiþ1Ti
DHi; ð1Þ
where Miand Mi+1are the magnetization values
measured at temperatures Tiand Ti+1in a field H,
respectively In Fig 3, the magnetic entropy
change (DSM) is plotted against temperature (T)
for x ¼ 0:10 composition at DH ¼ 10; 15 and
19 kOe Upon 10 kOe applied field, the highest
value of 3.24 J/kg K for DSM was found at a
temperature of 347 K (TC) For comparison, in
Table 1, we summarize the TC and DSM of
different magnetic materials which could be
potentially used as magnetic refrigerants in mag-netic refrigerators The MCE is clearly larger in the present Cu-doped manganites compared with that in gadolinium[9]and several other manganese oxides [1–7] For the same applied field,
H ¼ 10 kOe, the maximum DSM of the Cu-doped samples is estimated to be 3.05 J/kg K for x ¼ 0:05 composition and 3.24 J/kg K for x ¼ 0:10 composition, while it is only 2.8 J/kg K for Gd metal [9] More interestingly, the large magnetic entropy changes in both samples were observed at
a temperature above 300 K This allows the water
to be used as a heat transfer fluid in the room-temperature magnetic refrigeration regime[15] In addition, compared with gadolinium and its compounds [9,10], the polycrystalline Cu-doped manganese perovskite materials are easier to fabricate and possessing a higher chemical stability
as well as a higher resistivity The high resistivity is beneficial to lowering the eddy current heating All these characteristics make the polycrystalline Cu-doped manganese a competitive material for the room-temperature magnetic-refrigeration applica-tions
In general, the large magnetic entropy change in perovskite manganites mainly results from the considerable variation of magnetization near TC
In addition, the spin-lattice coupling in the magnetic ordering process also plays an important role[2] Due to strong coupling between spin and lattice, significant lattice change accompanying
Fig 2 Magnetic field dependence of magnetization at various
temperature around T for the La Sr Mn Cu O sample.
Fig 3 The magnetic entropy change ðDS M Þ as a function of temperature in various magnetic fields for La 0.7 Sr 0.3 Mn 0.9
Cu 0.1 O 3
Trang 4magnetic transition in perovskite manganites has
been observed[2,16] The lattice structural change
in the hMn2Oi bond distance as well as the
Mn2O2Mn
h i bond angle would, in turn, favor
the spin ordering Thereby, a more abrupt
reduc-tion of magnetizareduc-tion near TCoccurs and results in
a significant magnetic-entropy change[1–7] In this
way, a conclusion might be drawn that a strong
spin-lattice coupling in the magnetic transition
process would lead to additional magnetic entropy
change near TC, and consequently, enhances the
MCE
In the present work, the observation of an
endothermic peak of 348 K on the DSCcurves of
both the samples indicates that there exists a
first-order phase transition in these samples [5]
Furthermore, it should be noted that most of the
MCE materials were found to undergo a
first-order magnetic transition[9,10,12,17] As reported
in Ref [17], the magnetic entropy change (DSM)
around the first-order transition was about three
times larger than that obtained around the
second-order transition in the compound of
Nd0.5Sr0.5MnO3 Similarly, the drop of MCE
related to the change from first-order to
second-order magnetic phase transition was observed in
La2/3(Ca1xSrx)1/3MnO3perovskites with
increas-ing Sr-doped content[18] Consequently, it would
not be too unreasonable to suggest the large
magnetic entropy in the present Cu-doped
man-ganites might be connected with the abrupt
reduction in magnetization[17–20] The additional
entropy change can be attributed to the fact that the magnetic transition greatly enhances the effect
of the applied magnetic field That is also the reason why a sharp magnetic phase transition retains almost unchanged even under high fields It
is therefore proposed that the partial substitution
of Cu for Mn in the La0.7Sr0.3Mn1xCuxO3
perovskites would favor a soft ferromagnetic character (see Fig 4) It is noteworthy that the present Cu-doped samples exhibited a relatively small magnetic hysteresis with coercivity of
40 Oe near TC (T ¼ 340 K), which is beneficial
to the magnetic cooling efficiency[15,20]
In summary, the magnetocaloric effect in the
La Sr Mn Cu O (x ¼ 0:05; 0.10) materials
Table 1
Curie temperature T C and the maximum magnetic entropy change DS max
M
for different materials
Fig 4 The M–H curves obtained for the La 0.7 Sr 0.3 Mn 1x
Cu x O 3 (x ¼ 0:05; 0:1) samples.
Trang 5is studied A larger magnetic entropy change than
that of gadolinium has been observed in the
Cu-doped manganites This, together with the other
ideal MCE behaviors, makes these materials
possible for the room-temperature
magnetic-re-frigeration applications The large magnetic
en-tropy change, in the Cu-doped manganites, caused
by the abrupt reduction of magnetization is
associated with a first-order crystallographic phase
transition near the Curie temperature
Acknowledgements
One of the authors (M.H Phan) would like to
thank Professor F de Boer for helpful discussions
Research at Chungbuk National University was
supported by the Korea Research Foundation
Grant No KRF-2001-005-D20010 Research at
Center for Materials Science was supported by the
Vietnam National Program for Fundamental
Research Grant No 420110
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