Abstract
The symmetry of local moments plays a defining role in the nature of exotic ground states stabilized in frustrated magnetic materials. We present inelastic neutron scattering (INS) measurements of the crystal electric field (CEF) excitations in the family of compounds MgR2Se4 (R∈{Ho,Tm,Yb}). These compounds form in the spinel structure, with the rare-earth ions comprising a highly frustrated pyrochlore sublattice. Within the symmetry constraints of this lattice, we fit both the energies and intensities of observed transitions in the INS spectra to determine the most likely CEF Hamiltonian for each material and comment on the ground-state wavefunctions in the local electron picture. In this way, we experimentally confirm MgTm2Se4 has a nonmagnetic ground state, and MgYb2Se4 has effective S=12 spins with g∥=5.188(79) and g⊥=0.923(85). The spectrum of MgHo2Se4 indicates a ground-state doublet containing Ising spins with g∥=2.72(46), though low-lying CEF levels are also seen at thermally accessible energies E=0.591(36), 0.945(30), and 2.88(7) meV, which can complicate interpretation. These results are used to comment on measured magnetization data of all compounds and are compared to published results on the material MgEr2Se4.
