Abstract
We report the structural and magnetic properties of a new class of cobaltates with the chemical
formula (BaSr)4-xLa2xCo4O15 (x = 0, 0.5 and 1). These compounds crystallize in a hexagonal
structure in which cobalt ions are distributed among two distinct crystallographic sites with di erent
oxygen coordination. Three Co{O tetrahedra and one octahedron are linked by shared oxygen
atoms to form Co4O15 clusters, which are packed together into a honeycomb-like network. Partial
substitution of Sr and/or Ba atoms by La allows one to adjust the degree of Co valence mixing,
but all compositions remain subject to a random distribution of charge. Magnetic susceptibility
together with neutron scattering measurements reveal that all studied specimens are characterized by competing ferro- and antiferro-magnetic exchange interactions that give rise to a three dimensional Heisenberg spin-glass state. Neutron spectroscopy shows a clear trend of slowing down of spindynamics upon increasing La concentration, suggesting a reduction in charge randomness in the doped samples.
