Induced quadrupole effects near a crossover in a tetragonal TbLiF4 sheelite in a strong magnetic field up to 50 T

The anomalies of magnetic properties of TbLiF₄ caused by the interaction of the energy levels of a rare-earth ion in a strong magnetic field up to 50 T directed along the 100] and 110] axes are studied experimentally and theoretically. The jumps in magnetization M(H) and the maxima of the differential magnetic susceptibility dM(H)/dH are found in critical fields H _c = 28 and 31 T, where the lower component of the excited doublet approaches the ground-state singlet of a Tb³⁺ ion. Based on the crystal-field model with known interaction parameters, we calculated the Zeeman effect and the magnetization and magnetic susceptibility curves for the TbLiF₄ crystal, which adequately describe magnetic anomalies and critical parameters of a crossover. It is shown that the jumpwise change in the α- and γ-symmetry quadrupole interactions in TbLiF₄ caused by changes in the corresponding quadrupole moments during the crossing of energy levels leads, in accordance with experiments, to a decrease in the critical field H _c by approximately 4 T and an increase in the maximum of the differential susceptibility dM(H)/dH near the crossover more than twofold. This behavior can be considered as an analog of the induced quadrupole transition caused by a change of the ground state of the rare-earth ion during crossover.