Crystal Field Excitations in Multiferroic HoMnO3

Antiferromagnetic and ferroelectric order coexist in hexagonal HoMnO₃, and strong coupling between these two order parameters has been previously observed. Neutron scattering measurements of the low-energy excitations in HoMnO₃ reveal a complex spectra of Ho^3+ crystal-field excitations which depend upon both temperature and applied magnetic field. These crystal-field changes are correlated with changes in the magnetic symmetry of the Mn^3+ magnetic sublattice. Measurements of the magnon excitations near these crystal field levels indicates strong coupling between the Mn^3+ moments and the Ho^3+ crystal field levels. This coupling may play a critical role in explaining the interaction of ferroelectricity and antiferromagnetism in HoMnO₃.