The crystal field and exchange interactions in Yb₂Ti₂O₇

In the pyrochlore-structure compounds R₂Ti₂O₇, the rare-earth (R) sublattice forms a network of corner-sharing tetrahedra such that the magnetic interactions may be geometrically frustrated. The low-temperature magnetic properties of these compounds are fashioned both by the frustration and by the intrinsic properties of the rare earth, that is, by the degeneracy and anisotropy of the rare-earth crystal-field ground state and by the nature, size and strength of the inter-ionic magnetic coupling. For Yb₂Ti₂O₇, we combine ¹⁷⁰Yb Mössbauer spectroscopy, ¹⁷²Yb perturbed angular correlation, magnetization and susceptibility measurements to establish the Yb³⁺ crystal-field level scheme and to show that the crystal-field ground state is a well isolated Kramers doublet having a planar anisotropy. The main contribution to the Yb³⁺-Yb³⁺ coupling is the exchange interaction which is ferromagnetic. We describe the frustration-related low temperature (<1 K) properties of Yb₂Ti₂O₇ in a separate publication.