ARTICLE IN PRESS
Magnetic, transport and 151Eu Mössbauer studies on partially doped
La1x EuxSr0:2MnO3 (0:04pxp0:32) compounds
Rakesh Kumara, D.S. Ranab, C.V. Tomya,, P.L. Pauloseb, S.K. Malikb
a
Department of Physics, Indian Institute of Technology, Mumbai 400 076, India
b
Tata Institute of Fundamental Research, Mumbai 400 005, India
Abstract
Electrical resistivity, magnetic susceptibility and 151Eu Mössbauer studies have been carried out on La1x Eux Sr0:2 MnO3
(0:04pxp0:32) compounds. As Eu substitution for La increases, the Curie temperature (T C ) decreases from 261 K for x ¼ 0:04 to
187 K for x ¼ 0:16 along with a decrease in the metal–insulator transition temperature. However, the x ¼ 0:32 sample shows an increase
in T C to 224 K along with an increase in the metal–insulator transition. The 151Eu Mössbauer studies show that all the Eu are in
3 þ ð4f 6 ; J ¼ 0Þ nonmagnetic state. Broadening of the Mössbauer spectra below T C indicates a transferred magnetic hyperfine field at the
Eu site.
PACS: 75.47.Gk; 76.80.+y
Keywords: Magnetization; Colossal magnetoresistance; Mössbauer effect
The perovskite oxides, La1x Ax MnO3 (A ¼ alkaline
earth metal), have been of great interest as they show
colossal magnetoresistance near the metal–insulator transition temperature (T p ). The magnetic and transport properties of these materials are greatly influenced by parameters
such as Mn3þ =Mn4þ ratio, divalent/trivalent cation size,
etc. The compound La0:8 Sr0:2 MnO3 orders ferromagnetically with a Curie temperature (T C ) of 309 K [1]. We have
studied the effect of substitution of smaller size Eu cation
for La in La0:8 Sr0:2 MnO3 on its magnetic and transport
properties.
The La1x Eux Sr0:2 MnO3 samples, with x ¼ 0:04, 0.08,
0.16 and 0.32, were synthesized using the standard solidstate reaction method. Final sintering of the pelletized
powders was carried out at 1250 C for 24 h. Rietveld
analysis of the X-ray diffraction patterns shows the
compounds to form in single phase orthorhombic structure
(space group—Pnma, No. 62). The unit cell volume
decreases linearly with increase in Eu content.
The magnetic measurements on the title compounds
were carried out using a SQUID magnetometer (Quantum
Design, USA) in the temperature range of 2–300 K. All the
samples show ferromagnetic ordering (Fig. 1). The Eu
substitution upto x ¼ 0:16 initially lowers the Curie
temperature (T C ), but on further increase in Eu content
(x ¼ 0:32), the T C is found to increase (Table 1).
Fig. 2 shows the temperature variation of resistivity of
La0:8x Eux Sr0:2 MnO3 (x ¼ 0:08, 0.16 and 0.32) samples in
zero field and in a field of 9 T. Three prominent features are
observed in the resistivity data for x ¼ 0:08 sample: (i) a
peak around T C denoted as T p (ii) a very broad hump
below T C and (iii) an increase in the resistivity at still lower
temperatures, below about 50 K. Curiously all these three
features get diminished in magnitude by the application of
a magnetic field. The magnetoresistance [defined as
MRð%Þ ¼ ððRð0Þ RðHÞÞ=Rð0ÞÞ 100] measured in a field
of 9 T shows two peaks, one at 236 K and other at 165 K.
We denote the high-temperature peak as T p1 and the low
temperature peak as T p2 . Similar features are observed in
the MR data of other concentrations (Fig. 2) also.
The peak at T p1 is clearly related to the metal–insulator
transition because of its proximity to T C , while the origin
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508
Table 1
Variation of T C and T p with x in La0:8x Eux Sr0:2 MnO3 compounds. T p1 and T p2 correspond to the temperature of peaks observed in MR(%) measured in
9T
T C (K)
T p (0 T) (K)
T p1 (9 T) (K)
T p2 (9 T) (K)
0.04
0.08
0.16
0.32
261
226
187
224
272
232
–
–
272
236
205
248
205
165
107
122
1.2
1.0
0.8
0.6
0.4
0.2
0.0
600
500
400
300
200
100
0
500
MR (%)
0T
9T
La0.64Eu0.16Sr0.2MnO3
0T
9T
100
La0.48Eu0.32Sr0.2MnO3
0
of the peak at T p2 probably lies in a magnetic phase
segregation leading to magnetic inhomogeneity. In the
resistivity data of samples with x ¼ 0:16 and 0.32, the
metal–insulator transition at T p gets suppressed and the
only the peak corresponding to the broad hump remains.
However, these two peaks are very distinct in the MR data.
The T C decreases from 261 K for x ¼ 0:04 to 187 K for
x ¼ 0:16 sample with increase in the Eu content, but
increases to 224 K for the x ¼ 0:32 sample. The reason for
the sudden increase in T C is not clear and needs further
investigations. The values of T C , T p , T p1 and T p2 are listed
in Table 1. The overall resistivity increases with increasing
Eu content in La1x Eux Sr0:2 MnO3 compounds which may
be interpreted in terms of the decreased bandwidth as a
result of replacement of the bigger size La by a smaller size
Eu ion. However, the increase in the resistivity at low
temperatures may be understood on the basis of the large
mismatch in ionic radii between La3þ and Sr2þ ions, which
leads to the localization of charge carriers [2].
The 151Eu Mössbauer absorption spectra in the title
compounds get broadened as the temperature is lowered
below T C , while the isomer shift remains unchanged. A
typical spectrum is shown in Fig. 3. A single line with an
Isomer shift of 0.2 mm/s is observed which indicates a 3+
100
200
T (K)
MR (%)
ρ (Ω-cm)
0T
9T
200
0
50
La0.72Eu0.08Sr0.2MnO3
H = 9T
40
80
70
60
50
40
30
20
Tp2
H = 9T
Tp2
40
20
Tp1
La0.56Eu0.24Sr0.2MnO3
H = 9T
0
300
Tp1
La0.64Eu0.16Sr0.2MnO3
60
400
Fig. 1. Zero-field-cooled magnetization for La0:8x Eux Sr0:2 MnO3 of
x ¼ 0:04, 0.8, 0.16 and 0.32.
60
30
La0.72Eu0.08Sr0.2MnO3
300
Tp1
Tp2
70
Tp
MR (%)
ρ (Ω-cm)
ρ (Ω-cm)
x
0
100
200
T (K)
300
Fig. 2. Resistivity of La0:8x Eux Sr0:2 MnO3 for x ¼ 0:08, 0.16 and 0.32 in
zero field and in 9 T field along with MR (%) in 9 T field.
Fig. 3. 151Eu Mössbauer spectrum of La0:64 Eu0:16 Sr0:2 MnO3 at 4.2 K.
Solid line indicates the single Lorentzian fit.
electronic state for the Eu ion [3]. The Eu3þ ion has the
electronic configuration 4f 6 corresponding to J ¼ 0 and
hence is nonmagnetic. The broadening in absorption
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509
spectra amounts to a transferred magnetic hyperfine field
42 KOe at the Eu site from the ordered Mn ions.
The part of the work at IIT Bombay was supported by
CSIR, New Delhi, India.
References
[1] A. Urushibara, et al., Phys. Rev. B 51 (1995) 14103.
[2] A. Maignan, et al., Solid State Commun. 107 (1998) 363.
[3] I. Nowick, et al., J. Magn. Magn. Mater. 237 (2001) 1.