Uncovering field-induced magnetic phase transition by direct observation of the crystal electric-field splitting in a rare-earth magnetic insulator

Abstract

An indispensable step toward understanding magnetic interactions in rare-earth magnets is to determine the spatially anisotropic single-ion properties set by crystal electric field (CEF) physics. The CEF Hamiltonian yields a discrete energy spectrum governed by a set of parameters reflecting the local site symmetry of the magnetic ion. However, experimentally determining these parameters, especially for ones at low-symmetry sites remains highly challenging. In this work, we directly measure the CEF level splittings of CsErSe2 under magnetic fields using optical spectroscopy. This enables us to determine the CEF parameters and to predict the metamagnetic-like transition arising from a level-crossing in the ground state. We also identify a level-crossing in the first excited state that leads to a non-monotonic Zeeman splitting, which strongly influences the temperature and field dependence of the magnetization. Our results highlight the capacity of single-ion physics to drive rich and unanticipated phenomena in rare-earth magnetic insulators under applied magnetic field.