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

We considered the effect of optical pumping at the $f-f$ transitions frequency on $d-f$ superexchange in the rare-earth antiferromagnetic ferroborate ${\mathrm{Nd}}_{0.5}{\mathrm{Gd}}_{0.5}{\mathrm{Fe}}_3{\left({\mathrm{BO}}_3\right)}_4$ between ${\mathrm{Nd}}^{3+}$ ions in the excited states of the ${}^{}{G}_{5/2},{}^{}{G}_{7/2}$, and ${\mathrm{Fe}}^{3+}$ in the ground state ${}^{}{A}_1$. In the ferroborate, $d-f$ 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 $f-f$ transitions ${}^{}{I}_{9/2}\leftrightarrow ({}^{}{G}_{5/2}+{}^{}{G}_{7/2})$ in the ${\mathrm{Nd}}^{3+}$ ion, the nature of the $d\text{−}f$ 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 $d\text{−}f$ AFM interaction to AFM ordering under the FM interaction in the basal hexagonal plane.

DOI: https://journals.aps.org/prb/abstract/10.1103/PhysRevB.104.064445