Effect of optical f−f excitations on the Nd-Fe exchange interaction in Nd ferroborate with multiferroic properties

Abstract

We considered the effect of optical pumping at the $f\ensuremath{-}f$ transitions frequency on $d\ensuremath{-}f$ superexchange in the rare-earth antiferromagnetic ferroborate ${\mathrm{Nd}}_{0.5}{\mathrm{Gd}}_{0.5}{\mathrm{Fe}}_{3}{({\mathrm{BO}}_{3})}_{4}$ between ${\mathrm{Nd}}^{3+}$ ions in the excited states of the ${}^{4}{G}_{5/2}, {}^{2}{G}_{7/2}$, and ${\mathrm{Fe}}^{3+}$ in the ground state ${}^{6}{A}_{1}$. In the ferroborate, $d\ensuremath{-}f$ superexchange is directly related to the strong magnetoelectric coupling observed in the ground state of the material. We show that under optical pumping at the frequency of $f\ensuremath{-}f$ transitions ${}^{4}{I}_{9/2}\ensuremath{\leftrightarrow}({}^{4}{G}_{5/2}+{}^{2}{G}_{7/2})$ in the ${\mathrm{Nd}}^{3+}$ ion, the nature of the $d\text{\ensuremath{-}}f$ interaction changes to FM. The phenomena can be explained by the occupation of the excited ${\mathrm{Nd}}^{3+}$ ``spin pure'' states with spin $1/2$, or their mixing to the optically excited states with spin $3/2$ due to the spin-orbit interaction. Near optically excited ions ${\mathrm{Nd}}^{3+}$, magnetic frustrations change from FM ordering under the $d\text{\ensuremath{-}}f$ AFM interaction to AFM ordering under the FM interaction in the basal hexagonal plane.