DSpace at VNU: Some properties of La-deficient La0.54Ca0.32MnO3-delta

Journal of Magnetism and Magnetic Materials 262 (2003) 502–507

Some properties of La-deficient La0.54Ca0.32MnO3Àd
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 La0.54Ca0.32MnO3Àd was prepared by the solid-state reaction method. The Curie
temperature TC equals 300 K, which is significantly higher than those of the La1ÀxCaxMnO3Àd system. The magneticentropy change reaches a maximum value of ÀDSMD5.5 J/kg K at the Curie temperature upon a 5 T magnetic field
variation. A saturation magnetic moment sS ¼ 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.
r 2003 Elsevier Science B.V. All rights reserved.
PACS: 75.30.Sg; 75.47.Lx
Keywords: La-deficient La0.54Ca0.32MnO3Àd, Magnetic-entropy change; Oxidation; Ratio Mn3+/Mn4+; Saturation moment

1. Introduction
Without doping, LaMnO3 is 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 (TN E150 K). When

*Corresponding author. Tel.: +84-4-8585281; fax: +84-48584438.
E-mail address: (N.H. Sinh).

La3+ in LaMnO3 is progressively replaced by a
divalent cation, as in La1ÀxAxMnO3 (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 conduction electrons are fully spin-polarized. Recently,
attention was focused on the magnetic-refrigeration 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 substances. Experimentally much attention has been
paid to find refrigerants that have large magneticentropy change under a magnetic-field change,

0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0304-8853(03)00085-4


N.H. Sinh, N.P. Thuy / Journal of Magnetism and Magnetic Materials 262 (2003) 502–507

specially, to those that can be used at room
temperature.
Many studies have been concentrated on intermetallic compounds and alloys of rare earth,
which provide a comparatively large magneticentropy change at the Curie temperature. Among
them, the perovskites La0.67Ca0.33MnO3 and
La2/3Ca1/3MnO3 are the most attractive, because

their TC and 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
TC to be 272 K and a magnetic-entropy change of
2.9 J/kg K at a field change of 0.9 T for La0.54Ca0.32MnO3.
In this work, we report some properties of Ladeficient La0.54Ca0.32MnO3Àd, 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
La0.54Ca0.32MnO3Àd sample was prepared by a
conventional solid-state reaction method. Stoichiometric compositions of La2O3, CaCO3 and
MnO were mixed for 1 h. The mixed powders were
dried at 200 C for 3 h and pressed into pellets.
The pellets were first presintered at 1000 C 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 1100 C and sintered for 24 h, then
subsequently heated to 1250 C and sintered for
further 15 h at this temperature. The sintering
procedure is stopped by lowering the sample
temperature to 1150 C and kept at this level for

15 h. A subsequent second annealing at 1050 C
for 15 h is followed by the third annealing at 650 C
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

503

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 measured 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 orthorhombicperovskite structure without any impurity phase.
Lattice parameters that have been determined
( b ¼ 7:709 A
(
from XPD pattern are a ¼ 5:446 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
( b ¼ 5:742 A,

( c ¼ 7:728 A).
( So
(with a ¼ 5:532 A,
it is found that the crystal structure of
La0.54Ca0.32MnO3Àd 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 determined as 300 K, being the temperature of the
maximum dM/dT. This value is much higher than
that of La0.67Ca0.33MnO3 and La2/3Ca1/3MnO3 (by
30 K). In the La1ÀxCaxMnO3 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


504

N.H. Sinh, N.P. Thuy / Journal of Magnetism and Magnetic Materials 262 (2003) 502–507

Fig. 1. XPD result of La0.54Ca0.32MnO2.977. The pattern was obtained from powder of a sintered pellet type sample and measured at
room temperature.

the 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 La0.54Ca0.32MnO3Àd 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
La1ÀxMnO3Àd at x ¼ 0:30: This result also indicates 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 in Fig. 3.
From these curves, the saturation magnetic moments have been calculated as sS ¼ 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 temperatures, ranging from 200 to 300 K.
From the MðHÞ curves with various temperature intervals, the magnetic-entropy change DSmag
can be approximately calculated by the following
expression:
ÀDSmag ðTi ; Hmax Þ ¼

X ðMi À Mi¼1 Þ
DHi :
ðTi À Ti¼1 Þ

ð1Þ

Here, Mi and Miþ1 are experimental magnetization values obtained at temperature Ti and Tiþ1 ;
respectively, in a magnetic field Hi : The temperature change DT of the sample is related to the total
entropy change by
DT ¼ À

TDSmag
:
CP;H

ð2Þ

Here, CP;H is the (field dependent) heat capacity
of the sample depending on the applied magnetic
field.
The obtained magnetic-entropy change DSmag is
shown in Fig. 4 as a function of temperature.
The maximum magnetic-entropy change of
La0.54Ca0.32MnO3Àd is reached at its Curie temperature, 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 originates from the considerable change of the magnetization near TC.

The obtained entropy change shows that these
values are interesting with both increasing magnetic 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


N.H. Sinh, N.P. Thuy / Journal of Magnetism and Magnetic Materials 262 (2003) 502–507

505

100

50
80

30
B =1000 Oe

M (emu/g)

M (emu/g)

40

20
ZFC
FC

10
0
0

60

La0.54Ca0.32MnO3-δ
40

5K
77 K
20

50

100

150 200
T (K)

250

300

M (emu/g)

0

90
80

70
60
50
40
30
20
10
0

0

2

3

4

5

B (T)
Fig. 3. Magnetization plotted as a function of magnetic field at
5 and 77 K for La0.54Ca0.32MnO2.977 sample. From these
curves, a saturation magnetic moment of 2.99 mB/f.u. has been
calculated.
B = 10000 Oe

ZFC
FC
0

50

100

6
150

200

250

300

∆SM (J/kg.K)

T (K)

16
14
12
M (emu/g)

1

4

2

10
8

6

B = 100 Oe

4

ZFC
FC

2
0
0

50

100

0
200

240

280

320

T (K)
150

200

250

300

Fig. 4. The entropy change as a function of temperature for
La0.54Ca0.32MnO2.977 calculated for field variation 1, 3 and 5 T.

T (K)

Fig. 2. The temperature dependence of the magnetization for
La0.54Ca0.32MnO2.977 in zero field cooled (’) and field cooled
(&) regimes in (a) 100 Oe; (b) 1000 Oe and (c) 10 000 Oe.

change 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 concentration in La0.54Ca0.32MnO3Àd has been determined. The obtained value is d ¼ 0:0230: Thus, the

actual composition of the sample is La0.54Ca0.32MnO2.977. From the oxygen deficiency d; the ratio
of Mn3+/Mn4+ was estimated to be 0.3060/0.6940
= 0.441.
Fig. 5 shows the temperature dependence of the
susceptibility. From this curve, a transition temperature 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
La1ÀxAxMnO3 compounds show an insulator–


N.H. Sinh, N.P. Thuy / Journal of Magnetism and Magnetic Materials 262 (2003) 502–507

506

χac (arb.u)

60

40

20

0
100

150

200

250

300

350

T (K)
Fig. 5. Susceptibility curve of La0.54Ca0.32MnO2.977. TC has
been determined by dw/dT.

decreases TC. The structural parameters, in particular the oxygen thermal parameters, show significant 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 in Table 1, La0.54Ca0.32MnO2.977
is suitable for application in magnetic refrigeration. 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.

R (T)/R(300 K)

. 1.0

0 0.9

0 0.8

100

150

200

250

300

350

T (K)
Fig. 6. The resistance curve of La0.54Ca0.32MnO2.977. The
maximum value on this curve is corresponding to the
insulator–metal transition at TC.

metal (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 TIM ETC : 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. Therefore, across the I–M transition occurring around
TC, the J–T distortion decreases, and the distortion becomes more prominent in the insulating
phase. Increasing the static coherent MnO6
distortion favors the insulating behavior and

Fig. 7. SEM of La0.54Ca0.32MnO2.977 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 (ÀDSmag) for
several typical magnetic refrigeration materials.
Sample

TC (K)

ÀDSM
(J/kg K)

Hmax
(T)

Ref.

La0.54Ca0.32MnO2.977
La0.54Ca0.32MnO3Àd
La2/3Ca1/3MnO3
La0.67Ca0.33MnO3
La0.8Ca0.2MnO3
La0.6Ca0.4MnO3
La0.9Ca0.1MnO3
La0.8Ca0.2MnO3

300
272
267
255
230

263
255
260

5.5
2.9
6.4
4.47
5.5
5.0
5.93
7.75

5
0.9
3
1.5
1.5
3
3
5

Ours
[9]
[8]
[7]
[10]
[11]
[12]
[12]

N.H. Sinh, N.P. Thuy / Journal of Magnetism and Magnetic Materials 262 (2003) 502–507

In conclusion, we have studied some properties
of La-deficient La0.54Ca0.32MnO2.977. The obtained 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 temperature TC.
The Curie temperature TC is as high as
room temperature. Moreover, large magneticentropy 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 magnetization measurement. This work was supported
by the National project 421101/2002 of
Vietnam and National University Project
QGTD-00-01.

507

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