Magnetic ordered phase in La
0.6
Sr
0.4
MnO
3
ferromagnets
L. B. Steren,* M. Sirena, and J. Guimpel
Centro Ato ´mico Bariloche and Instituto Balseiro, Comisio ´n Nacional de Energı ´a Ato ´mica, 8400 S. C. de Bariloche, R. N., Argentina
Received 8 July 2001; published 20 February 2002
The ferromagnetic phase of La
0.6
Sr
0.4
MnO
3
thin films has been investigated through measurements of
magnetization loops at different temperatures, zero-field-cooled and field-cooled magnetization curves obtained
under different magnetic fields. We have found that the main sources of the ‘‘bulk’’coercivity in manganite
films are the film/substrate interface and film surface. The temperature dependence of the coercivity is de-
scribed by a ‘‘strong domain-wall pinning’’ model, independently of the thickness and substrate. The
magnetization-vs-temperature curves, measured under different magnetic fields (10 OeH2.5 kOe), have
been explained in terms of the magnetic hysteresis of the films. We find no evidence of glass states or of the
existence of single-domain clusters, as suggested by other authors.
DOI: 10.1103/PhysRevB.65.094431 PACS numbers: 75.60.-d, 75.70.-i
I. INTRODUCTION
In recent years, manganite oxides have attracted a lot of
attention due to their potential application as magnetoresis-
tive sensors. A
1 -x
B
x
MnO
3
( A =La, B =Sr,Ba,Ca) com-
pounds exhibit a wide variety of magnetic and electric trans-
port properties, depending on the concentration x.
1
In the
region 0.2x 0.5 these compounds present ferromagnetic
order and an insulator-metal transition near the Curie tem-
perature, while the undoped parent compound LaMnO
3
is an
antiferromagnetic insulator. Zenner
2
proposed the existence
of a double-exchange interaction between Mn
3 +
and Mn
4 +
to explain both the ferromagnetic order and the metallic
character of the doped compounds. Many questions have
been opened about the interplay of this ferromagnetic term
with the antiferromagnetic superexchange in manganite com-
pounds. de Gennes
3
proposed the existence of a frustrated
phase in an intermediate-concentration range, based on the
competition between both interactions. The important mag-
netohistory effect observed in the ferromagnetic manganites
has given rise to a wide variety of interpretations for the
magnetic phases of these compounds.
4,5
Ju and Sohn discuss
in Ref. 4 zero-field-cooled ZFC and field-cooled FC mag-
netization curves and the thermal dependence of coercitivity
in terms of spin freezing and magnetic inhomogeneity. These
authors studied the variation of the magnetization curves
with oxygen content of different bulk and film samples of
La-Ca-Mn-O and La-Ba-Mn-O, and interpreted their results
in the frame of a ‘‘spin-clustered’’ system. As these com-
pounds are highly disordered, with Mn
3 +
-Mn
4 +
pairs dis-
tributed randomly, the idea of clusters embedded in a non-
magnetic matrix has been introduced for the explanation of
some experimental results. However, the temperature depen-
dence of the coercive field does not follow the expected law
for blocked cluster systems. Moreover, the blocking tempera-
ture deduced from the fit does not correspond to the maxi-
mum in the ZFC curves. Li and coworkers
5
introduced the
concept of a ‘‘cluster-glass’’ phase in La
0.7
Sr
0.3
Mn
0.7
Co
0.3
O
3
.
In spite of the fact that for Co doping the interpretation of the
experimental data is complicated due to the coexistence of
Co in different electronic states, the features observed in
magnetization measurements and attributed to a field-
induced transition from a cluster-glass to a ferromagnetic
phase could not be taken as conclusive on this subject. It is
important to remark that irreversibilities between zero-field-
cooled and field-cooled magnetization curves have been ob-
served in both ferromagnetic bulk
5,6
and film samples,
4
so
they cannot be only assigned to substrate influence on the
magnetic properties.
We believe that without performing a complete experi-
mental study of the magnetism of the samples it is rather
difficult to identify complex magnetic phases, like spin or
cluster glasses. In this work, the field and temperature
dependences of the magnetization of magnetoresistive
La
0.6
Sr
0.4
MnO
3
thin films have been studied for different
magnetic histories in order to understand the nature of the
magnetic order in these compounds. Bulk La
1 -x
B
x
MnO
3
(0.2x 0.5; B:Sr,Ba,Ca) is a relatively soft material H
c
10 Oe Ref. 7. Due to the fact that ‘‘low-field’’magnetic
measurements are usually performed in fields larger than 10
Oe, small magnetohistory effects have been reported for bulk
compounds. However, in thin films, the coercive field is
larger
8
and a rich variety of behaviors is observed, depending
on the applied field and temperature.
II. EXPERIMENTAL DETAILS
The films were grown by dc magnetron sputtering from a
stoichiometric ceramic target of nominal composition
La
0.6
Sr
0.4
MnO
3
LSMO. Films with thickness t, ranging
from 5 nm to 500 nm, were grown both on 100 MgO and
100 SrTiO
3
single-crystalline substrates. The growth tem-
perature was 660° C. After deposition the film was cooled
down slowly to room temperature in a 100 Torr O
2
partial
pressure.
The film composition was measured by energy dispersive
x-ray analysis EDAX. The results show that the composi-
tion of the films is that of the target within 10% and that it is
homogeneous along the sample. The crystalline structure of
the samples was characterized by x-ray diffraction spectros-
copy. X-ray diffraction patterns indicate that the films are
strongly textured, with the 001 pseudocubic axis in the film
normal direction.
9
The lattice-parameter-thickness depen-
dence shows evidence of strains, for SrTiO
3
STO sub-
PHYSICAL REVIEW B, VOLUME 65, 094431
0163-1829/2002/659/0944316/$20.00 ©2002 The American Physical Society 65 094431-1