Magnetic exchange interaction between rare-earth and Mn ions in   multiferroic hexagonal manganites
  We report a study of magnetic dynamics in multiferroic hexagonal manganite HoMnO3 by far-infrared spectroscopy. Low-temperature magnetic excitation spectrum of HoMnO3 consists of magnetic-dipole transitions of Ho ions within the crystal-field split J=8 manifold and of the triangular antiferromagnetic resonance of Mn ions. We determine the effective spin Hamiltonian for the Ho ion ground state. The magnetic-field splitting of the Mn antiferromagnetic resonance allows us to measure the magnetic exchange coupling between the rare-earth and Mn ions. 
D. Talbayev, A. D. LaForge, S. A. Trugman, N. Hur, A. J. Taylor, R. D.   Averitt, D. N. Basov