Magneto-optical Kerr effects in perovskite-type transition-metal oxides:
La
1 x
Sr
x
MnO
3
and La
1 x
Sr
x
CoO
3
S. Yamaguchi, Y. Okimoto, K. Ishibashi, and Y. Tokura
Department of Applied Physics, University of Tokyo, Tokyo 113, Japan
Received 5 January 1998; revised manuscript received 23 March 1998
Magneto-optical Kerr effects have been investigated for perovskite-type hole-doped oxides
La
1-x
Sr
x
MnO
3
(0 x 0.3) and La
1-x
Sr
x
CoO
3
(0 x 0.3) in the photon energy range between 0.9 and 5.3
eV at room temperature. In La
1-x
Sr
x
MnO
3
, charge-transfer-type Kerr spectra are observed for the lowest
( 1.2 eV) and the second-lowest ( 3.5 eV) optical transitions that are likely to arise from electron excita-
tions from the O 2 p to the majority-spin e
g
band and to the minority-spin t
2g
band, respectively. In
La
1-x
Sr
x
CoO
3
, a crystal-field-transition-type Kerr spectrum is observed that is assigned to the electron exci-
tation from the bonding t
2g
state to the antibonding e
g
state allowed by the strong hybridization effect between
Co 3 d and O 2 p states. Different characters in the Hund’s-rule coupling and covalency seem to cause such
a difference in magneto-optical properties between ferromagnetic Mn and Co oxides.
S0163-18299805336-3
I. INTRODUCTION
Close interplay between doped holes and local spins in 3 d
transition-metal oxides is of current interest, since the re-
markable magnetotransport and magnetostructual properties
have recently been discovered in perovskite-type hole-doped
manganese and cobalt oxides.
1–8
In particular, colossal mag-
netoresistance CMR and related striking magnetotransport
phenomena have been observed for the manganese oxides
with variation of rare-earth or alkaline-earth elements on the
perovskite A site.
6,7,9–15
These properties are due partly to
strong Hund’s-rule coupling between the doping-induced
carriers and the localized spins, which gives rise to the
ferromagnetic-metallic states in terms of the double-
exchange mechanism.
16–18
The parent compound LaMnO
3
is a charge-transfer-type
CT insulator
19,20
in the Zaanen-Sawatzky-Allen scheme,
21
in which the lowest-lying gap transition corresponds to the
CT excitation from the O 2 p to the Mn 3 d state. The Mn
3+
ion has nominally 3 d
4
electrons with high-spin configuration
( t
2 g
3
e
g
1
). The t
2 g
electrons hybridize less with O 2 p states
and hence may be viewed as local spins ( S =
3
2
). The e
g
electron, on the other hand, strongly hybridizes with O 2 p
and organizes either an itinerant or localized state depending
on the e
g
band filling ( n =1 -x ), temperature, and magnetic
field. An increase of hole doping ( x ), or, equivalently, a
decrease of the e
g
band filling ( n ), gives rise to the phase
change from an antiferromagnetic insulator to a ferromag-
netic metal.
22
The strong interaction J
H
Hund’s-rule cou-
pling between an itinerant e
g
electron and t
2 g
local spin
rules the electronic properties and dynamics of the e
g
carri-
ers critically depends on the magnetic state of this com-
pound. Kubo and Ohata
23
derived the temperature- and
magnetic-field dependence of the resistivity using the Kondo
lattice ( s -d ) model with ferromagnetic coupling ( J
H
0).
Furukawa
24,25
also calculated the magnetization dependence
of the resistivity as well as the optical conductivity spectrum
using the Kubo formula in the limit of the classical spin ( S
=) and the infinite dimension ( D = ), and presented a
mean-field interpretation for the large negative magnetoresis-
tance in hole-doped manganese. Optical conductivity
studies
25,26
revealed that the interband transitions between
the exchange-split e
g
bands exhibit nearly complete weight
transfer to the intraband ones within the spin-polarized con-
duction band.
The oxide compounds with such a double exchange inter-
action are not restricted to the Mn oxides. The ferromagnetic
state of the hole-doped Co oxides with perovskite structure,
e.g., La
1 -x
Sr
x
CoO
3
, has also been assigned as mediated by
the double exchange interaction, although there is some com-
plication related to the problem of the spin-state
transition.
8,27–33
The parent compound LaCoO
3
is also a
charge-transfer-type insulator,
21
the charge gap of which is
fairly small ( 0.2 eV) Ref. 19 compared with LaMnO
3
.
The electron configuration of Co
3+
ion in LaCoO
3
is nomi-
nally 3 d
6
. The crystal-field splitting (10Dq ) between the t
2 g
and e
g
states and the Hund’s-rule coupling energy is compa-
rable, which seems to be a major origin of the thermally
induced spin-state transition from the low-spin ( S =0: t
2 g
6
)
state to the intermediate-spin ( S =1: t
2 g
5
e
g
1
) or the high-spin
( S =2: t
2 g
4
e
g
2
) state.
28,32–37
Such a subtle energy balance
seems to cause successive magnetic and resistive phase
transformations that are observed in LaCoO
3
with change of
temperature.
27,28,31,34,38,39
Hole doping for LaCoO
3
, e.g., in
La
1 -x
Sr
x
CoO
3
, causes the low-spin to the intermediate-spin
or high-spin transition of the Co site, producing the mag-
netic polarons
33
with the strong hybridization between 3 d
and O 2 p states. The ferromagnetic ground state with me-
tallic conduction is realized with further doping of
x ( 0.18),
40–42
showing the maximal Curie temperature
( T
C
=220 K) around x =0.3.
40
The carriers in
La
1 -x
Sr
x
CoO
3
may be more itinerant, due to the increased
hybridization between the Co 3 d and O 2 p states, than in
the Mn-oxide analogs, yet the strong coupling between the
carriers and t
2 g
spins is persistent, producing the
PHYSICAL REVIEW B 15 SEPTEMBER 1998-I VOLUME 58, NUMBER 11
PRB 58 0163-1829/98/5811/68629/$15.00 6862 © 1998 The American Physical Society