Magnetoresistance of the spin-state-transition compound La
1 x
Sr
x
CoO
3
R. Mahendiran and A. K. Raychaudhuri
Department of Physics, Indian Institute of Science, Bangalore – 560 012, India
Received 1 April 1996
We have investigated the magnetoresistance MR of the perovskite oxide La
1-x
Sr
x
CoO
3
for 0 x 0.4
which shows a spin-state transition from low-spin Co
III
to high-spin Co
3+
as a function of temperature. In the
metallic compositions ( x 0.25) appreciable MR occurs only near the ferromagnetic Curie temperature. For
the most metallic composition x =0.4, there is a small positive contribution to the MR near the T
c
. In the
insulating samples ( x 0.2) the MR shows a large hysteresis which depends on the temperature. The value of
the MR is negative and large in the insulating compositions and shows a maximum near the temperature where
magnetic studies showed a spin-glass-like transition. We also find a strong role of the spin-state transition of
the Co
3+
ions in the electronic transport leading to a characteristic behavior of the MR in the Co
3+
-rich
insulating samples. S0163-18299601645-1
I. INTRODUCTION
The recent discovery of giant magnetoresistance GMR
in Mn-based oxides
1–11
L
1 -x
A
x
MnO
3
( L =La, Nd, Pr, etc.,
and A =Ca, Sr, Ba, Pb, etc. has led to further investigation
of the physical and chemical properties of these oxides in
detail. Unlike Mn-based oxides manganates, there are very
few reports on the magnetoresistance behavior of Co-based
oxides cobaltates.
12–14
Even though hole doping by substi-
tution of Sr
2 +
for La
3 +
) in antiferromagnetic AFM insu-
lating LaMnO
3
as well as in LaCoO
3
renders ferromag-
netism FM and metallicity, there are certain differences
between these two systems in spin structure. We elaborate
these differences below. The present MR results indicate that
these differences between the two systems should be taken
into account to ascertain whether MR arises from the same
origin.
Sr substitution in LaMnO
3
leads to the conversion of
Mn
3+
( t
2 g
3
e
g
1
, S =2) into Mn
4+
( t
2 g
3
, S =3/2). Magnetic
studies on this system show that there is a core spin arising
from 1/2-filled t
2 g
3
levels. Since the exchange energy is larger
than the crystal field energy, the high-spin state is stable in
Mn-based systems. No thermal variation of the spin state of
the Mn ion has been reported so far. A strong Hund’s rule
coupling of the core spin with the more mobile carriers hole
or electron in the e
g
orbitals determines most of the observ-
able behavior of the Mn-based system. However, in
LaCoO
3
, the Co ion is predominantly in low-spin-state
Co
III
( t
2 g
6
, S =0) at low temperature, and with increases in
temperature, a progressive conversion of low-spin Co
III
into
high-spin Co
3+
( t
2 g
4
e
g
2
, S =2) takes place.
15–18
This happens
because at low temperature a large crystal field stabilizes the
low-spin state. However, the energy difference between the
two spin states being low ( 0.03 eV, thermal excitation can
provide a transition to a high-spin configuration. In the tem-
perature range 100 K T 350 K, the ratio of high-spin to
low-spin Co reaches 50 : 50 with short-range ordering
of low-spin and high-spin Co ions
18
and above 600 K
LaCoO
3
shows metallic behavior.
16,17
On Sr substitution, tet-
ravalent Co ions are created. They can be low-spin Co
IV
( t
2 g
5
, S =1/2) or high-spin Co
4 +
( t
2 g
3
e
g
2
, S =5/2). Recent
electron spectroscopy
24
results show that high-spin Co
4 +
is
1 eV lower in energy than low-spin Co
IV
. Sr-substituted
La
1 -x
Sr
x
CoO
3
thus contains a mixture of low-spin Co
III
( t
2 g
6
), low-spin Co
IV
( t
2 g
5
), and some high-spin Co
3 +
( t
2 g
4
e
g
2
) and high-spin Co
4 +
( t
2 g
3
e
g
2
) depending on the tem-
perature as well as the value of x .
The absence of a half-filled t
2 g
orbital in the low-spin Co
ion gives rise to less strong Hund’s rule coupling of the
carrier to the core spin. This is an important fact that distin-
guishes the Co system from the Mn system. We show in this
paper that these aspects of the Co ions along with the spin-
state transition of the Co
3 +
ions are essential ingredients in
understanding the MR data.
The superexchange interaction Co
IV
-O-Co
3+
or
Co
4+
-O-Co
3+
is known to be ferromagnetic and the ex-
change intereactions between the ions with the same valency
state are antiferromagnetic.
15–18
Whether the ferromagnetism
in cobaltates is mediated by a double-exchange mechanism
or not is clearly not understood at present. However, the
absence of the half-filled t
2 g
level providing the core spin
and a strong Hund’s rule coupling, unlike the manganates,
make this mechanism a less likely possibility.
At this stage it will be worthwhile to consider the mag-
netic phase diagram of La
1 -x
Sr
x
CoO
3
as has been proposed
by different investigators
18–21
as shown in Fig. 1. As we will
see below the magnetic phase diagram helps us to understand
MR data as well as the zero-field resistivity data to be re-
ported in this paper. It can be seen that there are two distinct
regions in the phase diagram. For x 0.2–0.25 one sees the
onset of ferromagnetic transition denoted by T
c
) but the
resulting ferromagnetic state does not have a long-range or-
der. Rather it becomes like a cluster of ferromagnetic regions
embedded in a nonferromagnetic matrix. For x 0.1, one ob-
serves a spin-glass-like state at lower temperature and there
seems to exist a spin-state-transition temperature marked by
T
s
in this region. In the transition region 0.2x 0.1 the
behavior seems to be more complicated and there seems to
be disagreement between different investigators. We avoided
this region shaded in Fig. 1 in the present work. The data
PHYSICAL REVIEW B 1 DECEMBER 1996-II VOLUME 54, NUMBER 22
54 0163-1829/96/5422/160449/$10.00 16 044 © 1996 The American Physical Society