Optical spectroscopy ofPrFe3(BO3)4: Crystal-field and anisotropic Pr-Fe exchange interactions

Abstract

High-resolution polarized optical absorption spectra of ${\text{PrFe}}_{3}{({\text{BO}}_{3})}_{4}$ in the paramagnetic and antiferromagnetic phases are reported. The measured energies of the crystal-field (CF) levels within the $4{f}^{2}$ configuration of ${\text{Pr}}^{3+}$ in the paramagnetic ${\text{PrFe}}_{3}{({\text{BO}}_{3})}_{4}$ are described by the CF model that involves the $4{f}^{2}/4f5d$ and $4{f}^{2}/4f6p$ configuration interactions. Ordering of Fe spins along the crystalline $c$ axis below ${T}_{N}=32\text{ }\text{K}$ is confirmed by the analysis of the spectra of ${\text{Er}}^{3+}$ introduced as a probe into ${\text{PrFe}}_{3}{({\text{BO}}_{3})}_{4}$. To account for the observed changes in the optical spectra of ${\text{Pr}}^{3+}$ at temperatures below ${T}_{N}$, in particular, for the shift of the CF levels, splitting of the CF doublets, and the appearance of forbidden lines, the Pr-Fe exchange Hamiltonian defined by seven parameters is considered. The theoretical approach has been tested by calculating the temperature dependence of the magnetic susceptibility. A good agreement between theory and optical and magnetic experimental data is found demonstrating the validity of the model used. The obtained results confirm that the model of the iron dimers inside the spiral chains of ${\text{Fe}}^{3+}{({\text{O}}^{2\ensuremath{-}})}_{6}$ octahedrons introduced by us earlier for ${\text{NdFe}}_{3}{({\text{BO}}_{3})}_{4}$ and modified in the present work may serve as a basis for analyzing the low-temperature properties of other rare-earth iron borates.