Temperature dependence of the ESR linewidth in the paramagnetic phase „T > T
C
…
of R
1 x
B
x
MnO
3
„R La,Pr; B Ca,Sr…
C. Rettori, D. Rao, J. Singley, D. Kidwell, and S. B. Oseroff
San Diego State University, San Diego, California 92182
M. T. Causa
Centro Ato ´mico Bariloche and Instituto Balseiro 8400, San Carlos de Bariloche, Argentina
J. J. Neumeier and K. J. McClellan
Los Alamos National Laboratory, Los Alamos, New Mexico 87545
S-W. Cheong
AT&T Bell Laboratories, Murray Hill, New Jersey 07974
S. Schultz
University of California, San Diego, California 92037
Received 6 June 1996
Electron spin resonance ESR experiments in the paramagnetic phase of R
1-x
B
x
MnO
3+
( R =La,Pr;
B =Ca,Sr show, for 1.1 T
C
T 2 T
C
, a linear T increase of the resonance linewidth, H, in powders,
ceramic pellets, and single crystals. Above 2 T
C
a slowdown in the T increase of H is observed. The data
resemble the results found in other ferromagnetic insulators where the spin-lattice relaxation involves a single-
phonon process. We find that the one-phonon process may account for the linear T dependence of the linewidth
observed up to 2 T
C
. A large T dependence of the resonance intensity above T
C
was found in all the samples
studied, suggesting the existence of spin clusters in these compounds over a wide range of temperature.
S0163-18299705805-0
I. INTRODUCTION
The discovery of huge negative magnetoresistance MR,
now termed colossal magnetoresistance CMR, in the doped
manganites R
1 -x
B
x
MnO
3+
R =La, Pr, Nd, etc.; B =Ca, Sr,
Ba, Pb with a perovskite structure has recently become a
subject of intense interest.
1
The increased interest in the
study of these systems arises from the high correlation found
between their structural, transport, and magnetic properties at
T
C
, the Curie temperature.
2
The proximity of the
paramagnetic-ferromagnetic FM phase transition,
3
and the
semiconductor-metal phase transition lead to giant MR ef-
fects at T
C
.
1,2
Further improvements in the MR sensitivity,
particularly at low magnetic fields and room temperatures,
may make these systems suitable for commercial magnetic
sensing applications.
2,3
The structure and basic behavior of the doped manganites
has been established long ago.
4
A model for the transition to
a FM-metallic phase was postulated by Zener
5
and refined
later by deGennes using the double-exchange DE mecha-
nism. In this model the conductivity is established by an
itinerant Mn d -electron hopping between Mn
3+
and Mn
4+
e
g
states. The carriers are strongly exchange coupled to the
localized electrons FM aligned in the d -core t
2 g
states ac-
cording to Hund’s rule S=
3
2
. However, recent calculations
6
indicated that the DE model alone cannot quantitatively ac-
count for the measured CMR values. The formation of a
spin-polaronic band in the paramagnetic phase arising from
Jahn-Teller splitting of the outer e
g
states, has been invoked
to explain the experimental results.
6,7
But the true nature of
the semiconducting-paramagnetic phase and the metallic-FM
phase is still a subject under intense investigation.
8
We have previously reported
9
that these systems have a
strong ESR line in the paramagnetic phase ( T T
C
). In par-
ticular, for those compounds that undergo a FM transition,
the intensity of the resonance grows approximately exponen-
tially as one approaches T
C
from above. The increase in
intensity is much more rapid than the T
-1
Curie-like behav-
ior expected for a single-ion excitation either Mn
3+
or
Mn
4+
, and it does not follow a ( T - )
-1
Curie-Weiss law
which is commonly observed for FM coupled ions. We have
suggested that the anomalous increase in intensity is due to
the formation of a complex of Mn
3+
– Mn
4+
spin clusters.
9
The average effective spin S increases as we approach T
C
.
This growth reflects the increase of the number of Bohr mag-
netons measured in dc-magnetization experiment as T
decreases.
9,10
In this paper we have extended our previous work
9
to a
larger number of systems of the R
1 -x
B
x
MnO
3+
R =La,Pr;
B =Ca,Sr; 0x 0.5 family in the form of powders, ceramic
pellets, and single crystals. We have studied the T depen-
dence of the linewidth, H , for T 1.1T
C
. We find that, for
1.1T
C
T 2 T
C
, H increases linearly with T over a large
range of x . Such behavior is unusual for materials that, as in
our case, are semiconductors or insulators. Above 2 T
C
a
slowdown in the slope of H is found. We attempt to de-
scribe our results in terms of a single-phonon spin-lattice
PHYSICAL REVIEW B 1 FEBRUARY 1997-I VOLUME 55, NUMBER 5
55 0163-1829/97/555/30834/$10.00 3083 © 1997 The American Physical Society