Transformation of f-f absorption bands of holmium ions in Ho0.75Nd0.25Fe3(BO3)4 crystal during spin-reorientation transition

Polarized absorption spectra of the Ho_0.75Nd_0.25Fe₃(BO₃)₄ crystal in the region of ⁵I₈→⁵F₅, ⁵S₂ + ⁵F₄, ⁵F₃ and ⁵F₂ absorption bands of Ho ions were studied as a function of temperature in the range of 5–20 K. The absorption spectra were decomposed into Lorentzian or Gaussian components. Below 203 K, the local symmetry of the holmium ion is C₂. However, an analysis of the polarization of the absorption lines from the point of view of the selection rules showed that in the easy-axis state of the crystal (T < T_R = 6.9 K), the local symmetry of the holmium ion is close to D₃. At T > T_R, a distortion of the local symmetry appears, which increases with increasing temperature. Shifts in the positions of absorption lines are observed at the spin-reorientation transition. The shifts are different for different lines. This means that the local magnetic anisotropy of the holmium ion, the Ho–Fe exchange energy, and the local crystal field are different in different excited states of holmium. Significant and different changes in the intensity of absorption lines depending on temperature were observed both during the reorientation transition and above it, which is a consequence of changes of the local crystal field in excited states with temperature.