High spin frustration in Co based Swedenborgites

Abstract

The spin correlations in a series of cobaltate polycrystalline samples with Swedenborgite structure, ABaCo3BO7 (A = Y, Ca, and B = Co, Fe, Al, Zn), were studied by means of diffuse polarized neutron scattering. The alternate stacking of kagome and triangular Co layers forms a tetrahedral Co-network with geometrical frustration of antiferromagnetically (AF) coupled spins, which typically suppresses long-range order even at low temperatures (~1K) despite Curie-Weiss temperatures in the order of 1000 K. The generic diffuse magnetic scattering as found in Y0.5Ca0.5BaCo4O7 reveals two dimensional (2D) spin correlations on the kagome sublattices towards the entropically favoured sqrt3 × sqrt3 structure and suggests a decoupling of layers by low spin states S = 0 on triangular sites. Co-substitution by Al and Zn yields similar diffuse magnetic scattering, however, spin dilution results in even more disordered spin liquid or spin glass states. With B = Fe or Co, differences in the magnetic scattering evolve, indicating the onset of spin correlations perpendicular to the kagome layers and non-vanishing spin states on the triangular sites. Geometrical frustration prohibits any 3D long-range order when the characteristic 2D spin correlations with the sqrt3 × sqrt3 motif is present. We observe long-range order only in YBaCo4O7 and CaBaCo3FeO7 showing rather distinct ordering wave-vectors.