Crystal field effects in rare-earth ferroborates RFe3(BO3)4 with R = Nd, Tb, Dy, or Er

Crystal field effects in rare-earth ferroborates RFe₃(BO₃)₄ with various rare-earth elements are studied theoretically. The rare-earth magnetoelastic interaction Hamiltonian is written in a multipole approximation. The field and temperature dependences of the multipole moments and strain susceptibilities of rare-earth ions in the ferroborate structure are calculated. A comparative analysis of anomalies in the thermal expansion and elastic constants of rare-earth ferroborates with Tb and Dy ions having identical magnetic structure but differing in the degree of anisotropy of the rare-earth ion is performed.