Characterization of the crystal field scheme of a new family of geometrically frustrated rare earth spinels.

The pyrochlore lattice is known for having a large variety of exotic, frustration-driven phases, the specifics of which are determined by the symmetry of the local moments and their interactions. Here we present on a new family of compounds MgRE₂Se₄ (RE = Ho, Tm, Er, and Yb), which form in the spinel structure, were the rare earth atoms are a pyrochlore sublattice. In this presentation we will show inelastic neutron scattering measurements of the crystal electric field (CEF) excitations. We fit this neutron data to find the most likely CEF Hamiltonian for each compound in the family and comment on the possible consequences of the result. We show that MgTm₂Se₄ has a spin singlet ground ground state with a low lying signlet excited state, similar to an Ising moment in a transverse field. MgHo₂Se₄ has an Ising ground state with several additional thermally accessible levels. MgEr₂Se₄ is shown to have Ising moments and is a spin ice compound. MgYb₂Se₄ is shown to be an effective spin 1/2 system with strong spin anisotropy.