Ising Pyrochlore Magnets: Low Temperature Properties, Ice Rules and Beyond

Pyrochlore magnets are candidates for spin-ice behavior. We present theoretical simulations of relevance for the pyrochlore family R sub 2 Ti sub 2 O sub 7 (R=rare earth) supported by magnetothermal measurements on selected systems. By considering long ranged dipole-dipole as well as short-ranged superexchange interactions we get three distinct behaviors: (1) an ordered doubly degenerate state, (2) a highly disordered state with a break transition to paramagnetism, (3) a partially ordered state with a sharp transition to paramagnetism. Thus these competing interactions can induce behavior very different from conventional "spin ice". Closely corresponding behavior is seen in the real compounds - in particular Ho sub 2 Ti sub 2 O sub 7 corresponds to case (3) which has not been discussed before, rather than (2) as suggested earlier.