Preparation and magneto-caloric effect of La 1x Ag x MnO 3x=0.10–0.30 perovskite compounds Nguyen The Hiena,b,*, Nguyen Phu Thuya,b a Cryogenic Laboratory, College of Natural Science, Fa
Trang 1Preparation and magneto-caloric effect of La 1x Ag x MnO 3
(x=0.10–0.30) perovskite compounds Nguyen The Hiena,b,*, Nguyen Phu Thuya,b
a
Cryogenic Laboratory, College of Natural Science, Faculty of Physics, Vietnam National University, 334 Nguyen Trai Road,
Thanh Xuan, Thoung Dinh, Hanoi, Viet Nam
b
International Training Institute for Materials Science (ITIMS), The ITIMS Building, DHBK, 1 Dai Co Viet Road,
Hanoi, Viet Nam Received 16 February 2002; received in revised form 16 March 2002
Abstract
conventional solid-state reaction and the sol–gel method While all samples with Ag concentrations up to 0.20 consist of single-phase perovskites with rhombohedral structure, unreacted Ag was found in the samples with Ag concentrations
of 0.22 and higher Magnetic properties of the as-prepared materials have been investigated The magneto-caloric effect
in these compounds has been found to be considerably large and higher than that in other perovskite compounds in which La is substituted by divalent alkali-earth elements r 2002 Elsevier Science B.V All rights reserved
Keywords: Magneto-caloric effect; La 1x Ag x MnO 3 ; Perovskite compounds
1 Introduction
The lanthanum-based manganite and cobaltate
perovskite compounds, such as La1xAxMnO3,
La1xAxCoO3with A=Ca, Sr and Ba, etc have
shown a variety of interesting electrical, electronic
and magnetic properties that have great potentials
for application Due to their colossal
magneto-resistance (CMR) effect, these materials have been
considered as promising candidates for magnetic
sensor, magneto-resistive memory and recording
applications, etc They have attracted, therefore,
much research work in the last few years [1–3]
Recent investigations have revealed that replacing
the divalent alkali-earth metals by monovalent elements, such as Na, K and Li, also leads to similar phenomena in these compounds [4–6] Since the success in the fabrication of a continuously working demonstration magnetic re-frigerator [7] and the discovery of the giant mag-neto-caloric effect (MCE) in the Gd5(Ge1xSix)4
compounds (with 0pxp0.5) [8], there is a growing interest concerning the MCE and mag-netic refrigeration Investigations are now focussed
on new materials with high MCE at high (close to room) temperatures to be used as magnetic refrigerants [9] It has been shown that the perovskite compounds with lanthanum and diva-lent alkali-earth elements also exhibit a large MCE and, therefore, they can be considered as potential candidates for application as refrigerants in
*Corresponding author Fax: +84-4-858-4438.
E-mail address: thehien@cryolab-hu.edu.vn (N.T Hien).
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 1 1 8 - 3
Trang 2magnetic refrigeration, especially with respect to
the material costs [10–12] Recent magneto-caloric
investigations have also revealed considerable
MCE in the lanthanum manganites where
mono-valent elements instead of dimono-valent alkali-earth
metals are substituted for La [4,5,13] In the
present paper, we report on the solid-state
reac-tion and sol–gel preparareac-tion, and the MCE in
La1xAgxMnO3 perovskite compounds with x ¼
0:1020:30:
2 Experimental
Samples of La1xAgxMnO3 were prepared by
both the conventional solid-state reaction (with
x ¼ 0:10; 0.13, 0.15, 0.17, 0.20, 0.22, 0.25, 0.27
and 0.30) and the sol–gel method (with x ¼ 0:10;
0.15, 0.20, 0.25 and 0.30) For the conventional
solid-state reaction samples, powders of La2O3of
3N purity, AgNO3 or Ag2O (4N) and MnO2
(4N) were two times manually ground, mixed,
pel-letised and fired at 8501C for 10–20 h Finally, the
prefired pellets were reground, pressed again
and sintered at 9501C for 48 h For the sol–
gel samples, solutions of lanthanum nitrate
hy-drate La(NO3)2 6H2O (4N), AgNO3 (4N),
Mn(CH3COO)2 (4N), C6H8O7 2H2O (4N),
CH3COOH (4N) and NH4OH (4N) as starting
chemicals were mixed in the nominal
composi-tional ratio of the cations The xerogels obtained
from the procedure were dried at 801C and heated
at 650–7001C for about 2–5 h As-prepared
sam-ples were examined by X-ray diffraction (XRD)
and by electron microscopy as well
Magnetisation as a function of the temperature
was measured in a vibrating sample magnetometer
(VSM) for the temperature range from 100 to
350 K in applied fields up to 1 mT From the data,
the Curie temperatures of the paramagnetic to
ferromagnetic phase transition were deduced
Magnetisation curves were measured in applied
fields up to 8 T, at various temperatures around
the Curie point, in the pulsed-field magnetometer
(PFM) at the International Training Institute for
Materials Science (ITIMS) [14] From these
magnetisation curves, the MCE, i.e., the
mag-netic-entropy change –DS due to the change of
the applied fields DB; have been determined for all samples investigated, using the same procedure described previously [15]
3 Results and discussion
In Fig 1, we show the XRD patterns for some
La1xAgxMnO3samples as representatives for the two series of compounds prepared by the sol–gel (a) and by the solid-state reaction method (b) As can be noted in Fig 1a, the patterns for the three sol–gel samples with xp0:20 consist of reflections typical for single-phase La1xAgxMnO3 perovs-kite compounds with rhombohedral structure, in agreement with the results reported by Tang et al [13] for their solid-state reaction (sintered) samples
of similar compositions In the patterns for sintered samples with xX0:22; however, additional peaks occur at 2y ¼ 381; 44.51 and 64.51, which can be identified as due to the presence of metallic silver in the samples As it is clearly seen in Fig 2b,
80
x = 0.10
x = 0.15
x = 0.20
70
x = 0.13
x = 0.20
x = 0.22
x = 0.27
2 (degree)
(a)
(b)
Fig 1 XRD patterns for some La 1x Ag x MnO 3 samples pre-pared by the sol–gel (a) and the solid-state reaction method (b).
Trang 3the intensity of these additional peak increases in
our corresponding sintered samples with
increas-ing Ag concentration We note that the solid-state
reaction samples were sintered at 9501C for 48 h
whereas the sol–gel samples were heated at 7001C
for 5 h only We have found that even in the low
Ag-concentration region, heating the xerogels at
lower temperature and shorter time, and/or
sintering the solid-state reaction samples at
tem-peratures above 9501C for even longer time, both
leads to inclusions of pure Ag in the samples
Scanning and transmission electron
micros-copy (SEM and TEM) experiments (not shown
here) were carried out to check the grain structure
of the as-prepared samples The SEM
photo-graphs showed that the grains in the sintered
samples reach sizes in the order of microns,
whereas TEM experiments on a sol–gel sample of
La0.90Ag0.10MnO3 revealed grain sizes of about
50 nm The presence of small amounts of pure metallic silver in the high Ag-concentration sintered samples is also indicated by another SEM analysis [16]
Using both aforementioned preparation meth-ods, however, we have not succeeded to fabricate a single-phase sample of the compound with x ¼ 0:30: We note that Tang et al [13] have reported the presence of unreacted metallic Ag, other precursor oxides and LaMnO3in sintered samples with xX0:25:
As an example for the compounds prepared by the sol–gel method, Fig 2a shows the magnetisa-tion of the La0.80Ag0.20MnO3sample as a function
of temperature, measured on a VSM in a field
of 2.5 mT for the temperature region from 100
to 360 K, in both cooled and zero field-cooled modes The curves obtained are denoted
by MðTÞFC and MðT ÞZFC; respectively Both MðTÞFC and MðT ÞZFC curves show a sharp phase transition of the sample at about 305 K from the paramagnetic to the ferromagnetic state There is another phase transition at about 160 K obvious, which is probably related to the so-called reentrant magnetic phase transition The distinct separation between the MðT ÞFC and MðTÞZFC curves in the temperature range below the Curie point suggests
a spin-glass- or cluster-class-like behaviour often observed in this type of compounds [2] For comparison, we show in Fig 2b, the magnetisation
of the La0.83Ag0.17MnO3sample, prepared by the solid-state reaction method, in the temperature range from 100 to 350 K measured on a VSM in
10 mT in both the cooled and the zero field-cooled mode Also here, the compound shows a spin-glass-like behaviour at low temperatures and
a sharp phase transition at about 290 K from the paramagnetic to the ferromagnetic state Such a significant reduction of the magnetisation at low temperature as observed in the sol–gel sample, however, does not appear in the sintered type of samples Moreover, the MðT ÞZFC curve reveals a lower transition temperature than the MðT ÞFC one Further investigations are under way to elaborate the origin of the above-mentioned phenomena
In Figs 3a and b, we show the magnetisation as
a function of the applied field measured in the
Temperature (K) 0.0
0.2
0.4
0.6
0.8
1.0
1.2
2 /kg)
La 0.80 Ag 0.20 MnO 3
La 0.83 Ag 0.17 MnO 3
FC ZFC
(a)
0
0.2
0.4
0.6
0.8
1.0
1.2
FC
ZF C
2 /kg)
Temperature (K) (b)
130
Fig 2 (a) Magnetisation of the La 0.80 Ag 0.20 MnO 3 sol–gel
sam-ple in the temperature range from 100 to 360 K in an applied
field of 2.5 mT (b) Magnetisation of the La 0.83 Ag 0.17 MnO 3
sintered sample in the temperature range from 100 to 350 K in
an applied field of 10 mT.
Trang 4PFM at various temperatures around the Curie
point, on two La0.80Ag0.20MnO3samples prepared
by the two different methods, again as
representa-tives for the series of compounds investigated
While the sintered sample clearly exhibits a
saturated ferromagnetic state just below the Curie
point, which can be observed in the behaviour of
the MðBÞ curves in Fig 3b, the magnetisation
MðBÞ curves for the La0.80Ag0.20MnO3 sol–gel
sample in Fig 3a at temperatures far below the
Curie point do not show any tendency of
satura-tion even in applied fields as high as 8 T This can
be due to the competition between the
antiferro-and the ferromagnetic phases antiferro-and/or a
super-paramagnetic behaviour of the nanosized particles
in this sample Actually, as mentioned above, we
have observed grain sizes in the order of about
50 nm in a La0.90Ag0.10MnO3 sol–gel sample,
which was prepared by the same procedure as
the one used in this measurement
From these magnetisation curves we derived the magnetic-entropy change DSmag caused by the variation of the applied field as the MCE for the samples Results shown in Fig 4 present the magnitude of the MCE for the La0.78Ag0.22MnO3
sintered sample at different field variations (from zero field up to the indicated value DB) It is clearly seen that for DB ¼ 1 T; the magnetic-entropy change at the Curie temperature in this sample reaches a value of about 2.9 J/kg K, and about 7.8 J/kg K for DB ¼ 3 T: In Fig 5, we show the entropy change as a function of temperature, at a field variation of DB ¼ 1 T for three sintered samples with x ¼ 0:17; 0.20 and 0.22 We note
Field (T)
0
10
20
30
40
50
60
La0.80Ag0.20MnO3 Sol-gel sample
230 K
250 K
260 K
270 K
280 K
290 K
300 K
0
10
20
30
40
50
60
70
80
210K 220K 245K 260K 270K 280K 290K 300K 310K
2 /kg)
2 /kg)
La0.80Ag0.20MnO3
Field (T)
(a)
(b)
0.5
Fig 3 (a) Isothermal magnetisation curves for the
La 0.80 Ag 0.20 MnO 3 sol–gel sample measured at different
tem-peratures from 230 to 300 K (b) Isothermal magnetisation
curves for the La 0.80 Ag 0.20 MnO 3 sintered sample measured at
different temperatures from 210 to 310 K.
Fig 4 Entropy change as a function of the temperature at different field variations for the La 0.78 Ag 0.22 MnO 3 sintered sample.
Smag
K)
T (K)
0 0.5
1 1.5
2 2.5
3
La 0.83 Ag 0.17 MnO 3
La 0.80 Ag 0.20 MnO 3
La 0.78 Ag 0.22 MnO 3
Fig 5 Entropy change at a field variation of 1 T, as a function
of temperature, for La Ag MnO sintered samples.
Trang 5that our sample with x ¼ 0:22 shows the highest
MCE
The Curie temperature TC and the maximum
magnetic-entropy change DSmagat DB ¼ 1 T as a
function of the Ag content is summarised in Fig 6
As we can see, the Curie temperature of the sol–gel
samples increases rapidly from 150 (for x=0.10) to
around 300 K for x ¼ 0:15; and becomes almost
saturated at about 310 K for Ag concentrations of
xX0:20: For the sintered samples, TC increases
more gradually from 250 K for x ¼ 0:10; via 280 K
for x ¼ 0:13; 290 K for x ¼ 0:17; to 300 K for x ¼
0:20 and finally to a saturated value of about
306 K for x > 0:20: These values are compared
with those reported by Tang et al [13] for their
solid-state reaction samples of corresponding
composition, i.e 214, 278, 306 and 306 K for x ¼
0:05; 0.20, 0.25 and 0.30, respectively The values
of TC for the sintered solid-state reaction samples
are somewhat lower than those for the sol–gel
samples of the corresponding compositions This
might be caused by the lower actual Ag
concen-tration, as can be inferred from the XRD
experiments, due to the presence of small amounts
of unreacted metallic Ag in the samples In this
figure, we can also see that the MCE in the sol–gel
samples is somewhat lower than that in the
sintered ones This can be ascribed to the fact
that the sol–gel samples were heated at 7001C for
5 h only Under these conditions, the single-phase
La1xAgxMnO3 compounds have been fully formed, but the grains have not been so far developed as in the sintered samples The MCE in our sintered samples reaches a maximum value of 2.9 J/kg K at x ¼ 0:22: This value is, however, somewhat lower than a maximum value of 3.4 J/
kg K reported by Tang et al [13] for their x ¼ 0:20 sample Even so, this is significantly higher than, for instance, 2.4 J/kg K in the perovskite com-pound of La0.60Ca0.40MnO3[11] It is thus, worth
to note that for the same field variation, the MCE
at the Curie temperature in the La1xAgxMnO3
system is remarkably higher than in the perovskite compounds with lanthanum and divalent alkali-earth metals Hysteresis loop measurements (not shown here) yielded coercivities as low as 1 mT, similar to the values reported by Tang et al [13] This revealed the materials to be of soft ferromag-netic type and, in this respect, also suitable for room temperature magnetic-refrigeration applica-tion
In conclusion, we have prepared samples of
La1xAgxMnO3 perovskite compounds (with x¼ 0:1020:30) by both the sol–gel and solid-state reaction methods While single-phase materials of the rhombohedral perovskite compounds have been obtained for Ag concentrations up to x ¼ 0:20; in the samples with xX0:25 small amounts of metallic Ag are still present The materials as obtained show significant MCE at Curie tempera-tures as high as 310 K, along with other interesting magnetic properties, and can be considered as a promising potential candidate for the application
as magnetic refrigerants in room temperature magnetic refrigeration
Acknowledgements
This paper is dedicated to Prof Dr J.J.M Franse from the University of Amsterdam who will celebrate his 65th anniversary these days The authors are grateful to him who, in his long standing scientific co-operation with Vietnam, has given a lot of stimulation and inspiration on the development of the high-pulsed-field magnet-ometer at ITIMS, and of the research activities
on superconducting cuprates at the Cryogenic
Fig 6 The Curie temperature (right scale) and the entropy
change (left scale) as a function of the silver concentrations in
the La Ag MnO system.
Trang 6Laboratory The work described here is actually a
follow up of such developments
This work is part of the research project
QGTD-00-01 granted by the Vietnam National University
(VNU), Hanoi, and partly supported by the State
Programme in Fundamental Research of Vietnam
Furthermore, the authors would like to express
their sincere thank to their colleagues: Mr Pham
Van Tong from the Cryogenic Laboratory, Dr Le
Van Vu and Mr Phung Quoc Thanh from the
Centre for Materials Science (CMS), Faculty of
Physics, College of Natural Science, VNU Hanoi;
Dr Tran Quang Vinh and Mr Ngo Van Nong
from ITIMS, and Prof Nguyen Hanh from the
Faculty of Chemical Technology, Hanoi
Univer-sity of Technology, for their close co-operation
and fruitful discussions
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