131
Mater. Res. Soc. Symp. Proc. Vol. 1292 © 2011 Materials Research Society
DOI: 10.1557/opl.2011.20
Preparation and Characterization of the Single-Layered Cobaltate La
2-x
Ca
x
CoO
4
D. Fuchs,
1
M. Merz
1
, R. Schneider,
1
and H. v. Löhneysen
1,2
1
Karlsruher Institut für Technologie, Institut für Festkörperphysik, 76021 Karlsruhe, Germany
2
Karlsruher Institut für Technologie, Physikalisches Institut, 76131 Karlsruhe, Germany
ABSTRACT
Polycrystalline samples of the single-layered cobaltate La
2-x
Ca
x
CoO
4
were prepared in a
wide doping range of 0 x 1.5. Structural properties were characterized at room temperature.
The orthorhombic distorted structure of the mother compound La
2
CoO
4
changes to a tetragonal
structure for x = 0.5 and then becomes orthorhombic again for x > 0.5. The magnetic properties
were investigated in the temperature range from 5 K T 300 K. With increasing hole-doping a
successive decrease of antiferromagnetic exchange is observed for x 0.5 whereas an increase of
ferromagnetic exchange evolves for x 0.5.
.
INTRODUCTION
In La
2
MO
4
(M = Co, Ni, Cu), tilt distortions of the MO
6
octahedra, metal –insulator
transitions, charge segregation instabilities and spin-glass phases are strongly affected by
chemical composition and pressure and hence can be modulated rather easily. The parent
compound La
2
CoO
4
shows antiferromagnetic ordering in the low-temperature orthorhombic
(LTO) distorted structure with a Néel temperature of T
N
= 275 K [1]. Hole-doping with Sr
suppresses the long-range antiferromagnetic order and lifts the orthorhombic distortion [2].
La
2-x
Sr
x
CoO
4
has also been extensively studied with respect to a spin-state transition. The spin-
degree of freedom found in the cobaltates can lead to different spin-states, i. e., low-spin (LS),
intermediate-spin (IS) and high-spin (HS) state, depending on the delicate balance between the
crystal-field splitting of the Co 3d t
2g
and e
g
states and the Hund`s rule coupling [3,4].
Ferromagnetic correlations are observed for x > 0.5 where the maximum of T
C
= 250 K is
observed for the end member Sr
2
CoO
4
[5]. In contrast to the doping with Sr, Ca-substituted
La
2-x
Ca
x
CoO
4
has been investigated only in a narrow concentration range of 0.3 x 0.8 by
Horigane et al.[6 -8]. To shed more light on the magnetic phase diagram of La
2-x
Ca
x
CoO
4
we
have prepared polycrystalline samples in a wide concentration range (0 x 1.5). Room-
temperature structural properties as well as magnetic properties are presented.
EXPERIMENT
Polycrystalline samples of the composition La
2-x
Ca
x
CoO
4
with x = 0, 0.5, 1, and 1.5 were
prepared by standard solid-state reaction using La
2
O
3
, CaCO
3
and CoO powders with a purity
99.9% as starting materials. First, the powders were mixed in stoichiometric composition and
calcinated at 950° C for 24 hours. Before sintering, the powders were milled in a ball-mill for 30
minutes. The first sintering step was carried out at 1150°C for 40 hours under ambient