Thuy Cryogenics Laboratory, Faculty of Physics, College of Natural Science, Hanoi National University, 334 Nguyen Trai Road, Thanh Xuan, Hanoi, Viet Nam Abstract A La-deficient sample of
Trang 1Some properties of La-deficient La 0.54 Ca 0.32 MnO 3d
N.H Sinh*, N.P Thuy
Cryogenics Laboratory, Faculty of Physics, College of Natural Science, Hanoi National University, 334 Nguyen Trai Road,
Thanh Xuan, Hanoi, Viet Nam
Abstract
A La-deficient sample of La0.54Ca0.32MnO3d was prepared by the solid-state reaction method.The Curie temperature TCequals 300 K, which is significantly higher than those of the La1xCaxMnO3dsystem.The magnetic-entropy change reaches a maximum value of DSMD5.5 J/kg K at the Curie temperature upon a 5 T magnetic field variation.A saturation magnetic moment sS¼ 2:99 mB/f.u at 5 K has been derived from the magnetization data Values
of 0.0230 and 0.441 for the oxygen deficiency d and the ratio of Mn3+/Mn4+, respectively, have been determined.From our study, it is suggested that this compound is a suitable candidate for application as a working substance in magnetic refrigeration
r2003 Elsevier Science B.V All rights reserved
PACS: 75.30.Sg; 75.47.Lx
Keywords: La-deficient La 0.54 Ca 0.32 MnO 3d , Magnetic-entropy change; Oxidation; Ratio Mn 3+ /Mn 4+ ; Saturation moment
1 Introduction
Without doping, LaMnO3is an insulator at all
temperatures.The insulating nature of this parent
compound as well as its anisotropic magnetic
interaction is related to the structure, in particular
to the Jahn–Teller (J–T) distortion around Mn3+
ions.When this insulator is hole-doped, the Mn4+
ions decrease the cooperative J–T distortion.The
structure plays a crucial role in determining the
electron transport and magnetic properties of this
oxide [1].LaMnO3 with a small proportion of
Mn4+ (p0.05) becomes antiferromagnetically
ordered at low temperatures (TNE150 K).When
La3+ in LaMnO3 is progressively replaced by a divalent cation, as in La1xAxMnO3(A=Ca, Sr, Ba), the proportion of Mn4+ increases and the orthorhombic distortion decreases.The material becomes ferromagnetic with a well-defined Curie temperature at a finite x, and metallic below TC The saturation moment is typically 3.8 mB, which is close to the theoretical estimate based on localized spin-only moment.This suggests that the conduc-tion electrons are fully spin-polarized.Recently, attention was focused on the magnetic-refrigera-tion possibilities of La–Ca–Mn–O compounds, because of the large magnetocaloric effect (MCE)
in this system [2–5].Up to now, MCE has been extensively studied in other ferromagnetic sub-stances.Experimentally much attention has been paid to find refrigerants that have large magnetic-entropy change under a magnetic-field change,
*Corresponding author.Tel : 8585281; fax:
+84-4-8584438.
E-mail address: nhsinh@netnam.vn (N.H Sinh).
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V All rights reserved.
doi:10.1016/S0304-8853(03)00085-4
Trang 2specially, to those that can be used at room
temperature
Many studies have been concentrated on
inter-metallic compounds and alloys of rare earth,
which provide a comparatively large
magnetic-entropy change at the Curie temperature.Among
them, the perovskites La0.67Ca0.33MnO3 and
La2/3Ca1/3MnO3 are the most attractive, because
their TCand magnetic-entropy changes are 257 K
and 4.37 J/kg K at 1.5 T and 267 K and 6.4 J/kg K
at 1.5 T, respectively[6,7].However, it is still lower
than room temperature.Xu et al.[8]have found
TCto be 272 K and a magnetic-entropy change of
2.9 J/kg K at a field change of 0.9 T for La0.54
-Ca0.32MnO3
In this work, we report some properties of
La-deficient La0.54Ca0.32MnO3d, which have been
obtained by measurements of X-ray powder
diffraction, magnetization, magnetocaloric effect,
susceptibility, oxygen deficiency (d), ratio of
Mn3+/Mn4+and SEM
2 Sample preparation
La0.54Ca0.32MnO3d sample was prepared by a
conventional solid-state reaction
method.Stoi-chiometric compositions of La2O3, CaCO3 and
MnO were mixed for 1 h.The mixed powders were
dried at 200C for 3 h and pressed into pellets
The pellets were first presintered at 1000C for
20 h and then cooled down to room temperature
by a turning off the furnace.After that, the pellets
were ground and Mastersize Microplus measured
to collect particles smaller than 100 mm.The
powders were pelletized using a cold isostatic
press.A multi-step procedure is applied for the
heat treatment of the sample.First the sample is
heated up to 1100C and sintered for 24 h, then
subsequently heated to 1250C and sintered for
further 15 h at this temperature.The sintering
procedure is stopped by lowering the sample
temperature to 1150C and kept at this level for
15 h.A subsequent second annealing at 1050C
for 15 h is followed by the third annealing at 650C
for 24 h.After this annealing, the sample was
furnace-cooled by simply switching off the supply
to the furnace.The structure of the sample was
inspected by X-ray powder diffraction (XPD), using Cu Ka radiation at room temperature.The magnetization curves (from 4 to 300 K) were measured with a vibrating sample magnetometer Resistivity versus temperature curves were mea-sured on cooling from 300 to 77 without an external magnetic field by the four-point probe technique.The magnetocaloric effect measurement was performed in a pulse field.SEM measurements were also carried out
3 Results and discussions The XPD pattern shown in Fig.1 reveals that the sample is of a single-phase orthorhombic-perovskite structure without any impurity phase Lattice parameters that have been determined from XPD pattern are a ¼ 5:446 (A, b ¼ 7:709 (A and c ¼ 5:445 (A which is identified with the Pnma structure in comparison with the crystal structure of the parent compound LaMnO3
(with a ¼ 5:532 (A, b ¼ 5:742 (A, c ¼ 7:728 (A).So
it is found that the crystal structure of
La0.54Ca0.32MnO3d has been distorted by the La-deficiency
Fig.2(a–c) shows the temperature dependence
of the magnetization measured in fields of H ¼
100 1000 and 10000 Oe, respectively, obtained under zero-field (ZFC) and field cooled (FC) conditions
It is found that the magnetic moments of the sample are almost the same in the ZFC and FC curves at 1000 and 10000 Oe.At 100 Oe, it shows only a very slight difference.This suggests that the spin order does not strongly depend on external magnetic fields.Furthermore, a clear anomaly at
50 K is seen.This may be related to a crystal structure phase transition, which must be further elaborated.The Curie temperature TC is deter-mined as 300 K, being the temperature of the maximum dM/dT.This value is much higher than that of La0.67Ca0.33MnO3and La2/3Ca1/3MnO3(by
30 K).In the La1xCaxMnO3 system, with a surplus of Mn, both the anionic and cationic vacancies arise in the actual structure of the oxides
as a result of an oxidation–reduction process created via the heating and cooling procedure in
Trang 3the sample preparation.It is closely related to
changes of manganese valency from Mn4+ to
Mn3+ on heating and from Mn3+ to Mn4+ on
cooling.Thus, the real structure contains Mn3+
and Mn4+ions as well as the anionic and cationic
vacancies.Therefore, the increase of the Curie
temperature of the La0.54Ca0.32MnO3d sample
may originate from this structure.The result of
Chen et al [9] showed that TC increases to its
highest value of 314.5 K in the La-deficient system
La1xMnO3d at x ¼ 0:30: This result also
in-dicates that decreasing the La-content causes a
marked increase of the Curie temperature
Magnetization as a function of applied magnetic
field up to 5 T, at 5 K and 77 K, is shown inFig.3
From these curves, the saturation magnetic
mo-ments have been calculated as sS¼ 2:99 mB/f.u in
La0.54Ca0.32MnO2.977.It is in good agreement with
the magnetic moment value of Mn3+ in this
compound
Magnetization in the dependence on applied
fields up to 5 T was measured at various
tempera-tures, ranging from 200 to 300 K
From the MðHÞ curves with various
tempera-ture intervals, the magnetic-entropy change DSmag
can be approximately calculated by the following
expression:
DSmagðTi; HmaxÞ ¼X Mð i Mi¼1Þ
Ti Ti¼1
Here, Miand Miþ1 are experimental magnetiza-tion values obtained at temperature Ti and Tiþ1; respectively, in a magnetic field Hi: The tempera-ture change DT of the sample is related to the total entropy change by
DT ¼ T DSmag
CP;H
Here, CP;H is the (field dependent) heat capacity
of the sample depending on the applied magnetic field
The obtained magnetic-entropy change DSmagis shown inFig.4as a function of temperature The maximum magnetic-entropy change of
La0.54Ca0.32MnO3d is reached at its Curie tem-perature, where the change of the magnetization with temperature is the fastest.The maximum entropy change, corresponding to a magnetic-field change of 1, 3 and 5 T, is 1.81, 3.92 and 5.50 J/
kg K, respectively.It is clear that the large magnetic-entropy change in this compound origi-nates from the considerable change of the magne-tization near TC
The obtained entropy change shows that these values are interesting with both increasing mag-netic field and doping concentration.It is a possible reason that at higher magnetic fields, the magnetic moments are orientated better than at lower magnetic fields.On the other hand, an amount of Ca2+ substituted for La3+ induces a
Fig.1 XPD result of La 0.54 Ca 0.32 MnO 2.977 The pattern was obtained from powder of a sintered pellet type sample and measured at room temperature.
Trang 4change of the Mn3+/Mn4+ ratio, increasing the
competition between the double-exchange (DE)
and the superexchange (SE) interaction, where in
this case the SE interaction will be dominated by
the interaction of the Mn3+and Mn4+ ions and
by the increase of Mn4+ions in the compound
By the dichromate method, the oxygen
concen-tration in La0.54Ca0.32MnO3d has been
deter-mined.The obtained value is d ¼ 0:0230: Thus, the
actual composition of the sample is La0.54Ca0.32 -MnO2.977.From the oxygen deficiency d; the ratio
of Mn3+/Mn4+was estimated to be 0.3060/0.6940
= 0.441
Fig.5shows the temperature dependence of the susceptibility.From this curve, a transition tem-perature near 300 K is also revealed for the ferromagnetism to paramagnetism transition The temperature dependence of the resistance
of the sample is shown in Fig.6.The data exhibit a maximum in the electrical resistivity
as the temperature decreases.Indeed, most
0 20 40 60 80 100
La
5 K
77 K
B (T) Fig.3 Magnetization plotted as a function of magnetic field at
5 and 77 K for La 0.54 Ca 0.32 MnO 2.977 sample.From these curves, a saturation magnetic moment of 2.99 m B /f.u has been calculated.
0 2 4 6
SM
T (K)
Fig.4 The entropy change as a function of temperature for
La 0.54 Ca 0.32 MnO 2.977 calculated for field variation 1, 3 and 5 T.
0
10
20
30
40
50
B =1000 Oe
ZFC FC
T (K)
0
10
20
30
40
50
60
70
80
90
B = 10000 Oe
ZFC FC
T (K)
0
2
4
6
8
10
12
14
16
B = 100 Oe
ZFC FC
T (K)
Fig.2 The temperature dependence of the magnetization for
La 0.54 Ca 0.32 MnO 2.977 in zero field cooled (’) and field cooled
(&) regimes in (a) 100 Oe; (b) 1000 Oe and (c) 10 000 Oe.
Trang 5metal (I–M) transition around TC.This (I–M)
transition is associated with a peak in the
resistivity curve at a so-called TIM; generally,
TIM is somewhat lower than TC.In our case we
estimated TIMETC: The nature of the I–M
transition can be understood that, in manganates,
Jahn–Teller distortion due to the Mn3+ions plays
a key role.The creation of Mn4+ions removes the
distortion leading to more cubic
structures.There-fore, across the I–M transition occurring around
TC, the J–T distortion decreases, and the
distor-tion becomes more prominent in the insulating
phase.Increasing the static coherent MnO6
distortion favors the insulating behavior and
decreases TC.The structural parameters, in parti-cular the oxygen thermal parameters, show sig-nificant changes across the I–M phase transition Thus, clearly, Mn4+ plays a crucial role in this material.The surface structure of the sample obtained by SEM measurement is shown in Fig.7.It is found that the size, shape and distribution of the grains on the surface of the sample are homogeneous
Table 1 presents data on the MCE for several compounds for comparison
As can be seen inTable 1, La0.54Ca0.32MnO2.977
is suitable for application in magnetic refrigera-tion.Besides the ease of production and the high chemical stability, its Curie temperature is at room temperature range and the material exhibits a large magnetic-entropy change
0
20
40
60
χac
T (K)
Fig.5 Susceptibility curve of La 0.54 Ca 0.32 MnO 2.977 T C has
been determined by dw/dT.
T (K)
.1.0
00.9
00.8
Fig.6 The resistance curve of La 0.54 Ca 0.32 MnO 2.977 The
maximum value on this curve is corresponding to the
insulator–metal transition at T C
Fig.7 SEM of La 0.54 Ca 0.32 MnO 2.977 showing a homogeneous distribution of grains with the same size and shape over the surface of the sample.
Table 1 Curie-temperature and maximum entropy change (DS mag ) for several typical magnetic refrigeration materials.
(J/kg K)
H max (T)
Ref.
La 0.54 Ca 0.32 MnO 2.977 300 5.5 5 Ours
La 0.54 Ca 0.32 MnO 3d 272 2.9 0.9 [9]
La 2/3 Ca 1/3 MnO 3 267 6.4 3 [8]
La 0.67 Ca 0.33 MnO 3 255 4.47 1.5 [7]
La 0.8 Ca 0.2 MnO 3 230 5.5 1.5 [10]
La 0.6 Ca 0.4 MnO 3 263 5.0 3 [11]
La 0.9 Ca 0.1 MnO 3 255 5.93 3 [12]
La 0.8 Ca 0.2 MnO 3 260 7.75 5 [12]
Trang 6In conclusion, we have studied some properties
of La-deficient La0.54Ca0.32MnO2.977.The
ob-tained results on the oxygen deficiency d and
the ratio of Mn3+ and Mn4+ ions revealed
intrinsic processes in the material.It is found
that the ferromagnetism-paramagnetism and
I–M transitions occur near the same
tempe-rature TC
The Curie temperature TC is as high as
room temperature.Moreover, large
magnetic-entropy changes around TC have been observed
With these advantages, the La0.54Ca0.32MnO2.977
compound can be considered as a suitable
candidate for application as a working substance
in magnetic refrigeration technology at room
temperature
Acknowledgements
The authors would like to thank Ph.D student
Nguyen Phuc Duong for help in
magnetiza-tion measurement.This work was supported
by the National project 421101/2002 of
Vietnam and National University Project
QGTD-00-01
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