Magnetic ordering effects in the Raman spectra of La
1 x
Mn
1 x
O
3
E. Granado, A. Garcı
´
a, J. A. Sanjurjo, C. Rettori, and I. Torriani
Instituto de Fı ´sica ‘‘Gleb Wataghin,’’ UNICAMP, 13083-970, Campinas-SP, Brazil
F. Prado, R. D. Sa
´
nchez, and A. Caneiro
Centro Ato ´mico Bariloche and Instituto Balseiro, Comisio ´n Nacional de Energı ´a Ato ´mica and Universidad Nacional de Cuyo, RA-8400
San Carlos de Bariloche, Argentina
S. B. Oseroff
San Diego State University, San Diego, California 92182
Received 21 January 1999; revised manuscript received 2 June 1999
Temperature-dependent Raman spectra of polycrystalline La
1-x
Mn
1-x
O
3
samples, referred to as
LaMnO
3+
for convenience, were correlated to their structural and magnetic properties for different . For
0.05, a softening of the 610 cm
-1
Raman mode is observed below the magnetic ordering temperature.
This softening is reduced as increases from 0.00 to 0.05. This behavior is not associated with lattice
anomalies, and is interpreted in terms of a spin-phonon coupling caused by a phonon modulation of the
superexchange integral. A quantitative analysis of this effect is presented. S0163-18299910737-9
In recent years, much effort has been devoted to under-
standing the physics involved in the transport and magnetic
phenomena of perovskite manganites. It was claimed that
the early double-exchange DE theory
1
cannot explain
quantitatively the large conductivity drop near the
ferromagnetic-paramagnetic FM-PM transitions for doped
compounds.
2
The presence of lattice polarons, possibly en-
hanced by the strong Jahn-Teller character of the Mn
3 +
ion,
has been invoked to explain the large change in electron
kinetic energy at T T
C
,
2
as well as other anomalous physi-
cal properties of these materials.
3–5
Such polarons are ex-
pected to be present when
h
-1
, where
h
is the electron
transfer time and
is the frequency of an optical-mode
lattice vibration associated with a dynamic Jahn-Teller
distortion,
6
i.e., a stretching mode. Therefore, lattice vibra-
tions possibly play an important role in the conduction
mechanism of this class of materials. Although the parent
compound LaMnO
3
is an antiferromagnetic AFM insulator,
the study of the lattice and magnetic properties of this mate-
rial may be taken as a starting point to a consistent under-
standing of the more complex physical properties of the
doped compounds.
Perovskites with an orthorhombic Pnma space group
present 24 Raman active modes (7 A
g
+5 B
1 g
+7 B
2 g
+5 B
3 g
). Lattice dynamical calculations and assignment of
the Raman modes of LaMnO
3
were done previously by Iliev
et al.
7
In a recent work, we reported a softening of the
604 cm
-1
stretching mode below T
N
for the orthorhombic
LaMnO
3.0
,
8
also observed by others.
9
This effect was asso-
ciated with spin-phonon interactions,
9
and exchange striction
effects.
8
Nevertheless, a detailed study that explains the soft-
ening of the 604 cm
-1
stretching mode in LaMnO
3
is still
missing. In this work, we present Raman and x-ray studies of
the LaMnO
3 +
controlled oxygen content series ( 0.07).
We conclude that the observed anomalous shifts for the
610 cm
-1
mode in the samples with 0.05 are due to a
phonon modulation of the xz -plane superexchange integral
and not to the lattice anomalies observed below the magnetic
transitions. From a first-neighbors contribution, we obtain
the value of the second derivative of the xz -plane superex-
change integral with respect to the stretching oxygen dis-
placements in the Mn-O( xz ) direction.
In this paper, for simplicity, the nomenclature LaMnO
3 +
is used. However, we should mention that the O
2 -
ions can-
not be accomodated interstitially in the lattice. Thus, from a
crystallographic point of view, the correct formula for this
series is La
1 -x
Mn
1 -x
O
3
with x /3.
10
General properties
of cation-deficient samples with the La
1 -x
Mn
1 -y
O
3
formula
have been systematically studied in the literature.
10–14
The LaMnO
3 +
samples studied in this work were grown
by different methods. Samples with =0.00, 0.03, 0.05, and
0.07 were prepared following the nitrate decomposition
method using Mn and La
2
O
3
as starting materials.
15
The
sample referred to as LaMnO
3
: Ar annealed was prepared by
standard ceramic methods, heating stoichiometric mixtures
of the corresponding oxides.
16
The structure and phase purity
of the samples were checked by x-ray powder diffraction.
Well resolved x-ray patterns were obtained with Cu K ra-
diation of a Rigaku conventional source and a high-
resolution diffractometer with low-temperature attachment.
Lattice parameters were obtained using a Rietveld refinement
program.
17
Magnetization dc/ac measurements have been
taken in a Quantum Design PPMS magnetometer. Details of
the Raman experimental setup are given elsewhere.
8
Except
for the x-ray experiments, all the measurements as a function
of T were done with increasing temperature.
A summary of the structural and magnetic properties of
the studied samples is given in Table I. The Ar annealed
sample and those with =0.00 and 0.03 are mainly AFM
with a FM component that increases with . For simplicity,
we will refer to all these samples as AFM. The sample with
=0.07 is FM. Finally, the sample with =0.05 presents a
complex magnetic behavior that can be ascribed to frustrated
FM or mixed FM/AFM phases.
15
All the samples studied in
this work present orthorhombic Pnma crystal structure be-
tween 15 and 300 K.
PHYSICAL REVIEW B 1 NOVEMBER 1999-I VOLUME 60, NUMBER 17
PRB 60 0163-1829/99/6017/118794/$15.00 11 879 ©1999 The American Physical Society