Infrared-active excitations related to Ho³⁺ ligand-field splitting at the commensurate-incommensurate magnetic phase transition in HoMn₂O₅

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 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 (LF) 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 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.