2002/08/06 by Wiebe Geertsma, Geertsma, Wiebe
Physics and Astronomy · #FOS: Physical sciences #Materials Science (cond-mat.mtrl-sci) #Strongly Correlated Electrons (cond-mat.str-el) #cond-mat.mtrl-sci #cond-mat.str-el
paper · pdf · doi:10.48550/arxiv.cond-mat/0208119
Latex 50 pgs; 6 tables; 18 figures (eps)
arxiv created 2002/08/06 · arxiv updated 2009/11/30
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.