  Linearly polarized spectra of far-infrared (IR) transmission in HoMn2O5
multiferroic single crystals have been studied in the frequency range between
8.5 and 105 cm-1 and for temperatures between 5 K and 300 K. Polarization of
IR-active excitations depends on the crystallographic directions in HoMn2O5 and
is sensitive to the magnetic phase transitions. We attribute some of the
infrared-active excitations to electric-dipole transitions between ligand-field
split states of Ho3+ ions. For light polarization along crystalline b-axis, the
oscillator strength of electric dipoles at low frequencies (10.5, 13, and 18
cm-1) changes significantly at the commensurate-incommensurate
antiferromagnetic phase transition at T3 = 19 K. This effect shows a strong
correlation with the pronounced steps of the b-directional static dielectric
function. We propose that the ligand field (LF) on Ho3+ connects the magnetism
and dielectric properties of this compound through coupling with the Mn spin
structure. We comment on the possibility for composite excitations of magnons
and excited LF states.
