Spin order on pyrochlore lattices studied by the Self-Consistent Cluster Approach

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In this paper we present a study of magnetic order of quantum Heisenberg spins on pyrochlore type lattices: a lattice of corner sharing tetrahedra. We use the Cluster Variation Approximation with the tetrahedron as basic cluster. We find that in case of anti-ferromagnetic nearest--neighbour exchange there is no long--range spin order at any finite temperature for any spin s. For ferromagnetic exchange and s=1/2 the spin system remains in a non-magnetic phase for all temperatures, while for s > 1/2 there is a finite order temperature. We derive the ground state phase diagram as a function of exchange anisotropy and tetragonal anisotropy and find regions of re--entrant behaviour. Magnetic fields can induce one or more magnetic transitions. Schottky--like anomalies appear in the specific heat, with on top signs of re--entrant behaviour. The limits of this Cluster Variation Approximations are discussed.