Frustrated rare earth magnetism: Spin glasses, spin liquids and spin ices in pyrochlore oxides

Rare earth-based materials have played a central role in recent efforts to understand the very unconventional behavior of geometrically frustrated magnetic materials. In particular, rare earth transition metal oxides with the pyrochlore and related structures have been investigated extensively. In the pyrochlore structure both the rare earth and the transition metal sublattices have a topology consisting of corner-sharing tetrahedra and are, thus, geometrically frustrated. Here, we will review progress over the past several years concerning the rare earth titanates, R2Ti2O7, which show a remarkable sensitivity to the electronic structure, specifically, the crystal field ground state of the R3+ ion. For example, the materials Gd2Ti2O7, Tb2Ti2O7, Dy2Ti2O7 and Ho2Ti2O7 exhibit a wide variety of ground states including unconventional long range order (Gd), a spin liquid state (Tb), and spin ice states (Dy, Ho). Er2Ti2O7 shows long range order but perhaps by the "order by disorder" mechanism and the Yb2Ti2O7 ground state may also be spin liquid like but is presently controversial. (c) 2005 Elsevier B.V. All rights reserved.