Ultrafast Control of Magnetic Anisotropy by Resonant Excitation of 4 f Electrons and Phonons in Sm 0.7 Er 0.3 FeO 3

We compare the ultrafast dynamics of the spin reorientation transition in the orthoferrite ${\mathrm{Sm}}_{0.7}{\mathrm{Er}}_{0.3}{\mathrm{FeO}}_{3}$ following two different pumping mechanisms. Intense few-cycle pulses in the midinfrared selectively excite either the $f\text{\ensuremath{-}}f$ electronic transition of ${\mathrm{Sm}}^{3+}$ or optical phonons. With phonon pumping, a finite time delay exists for the spin reorientation, reflecting the energy transfer between the lattice and $4f$ system. In contrast, an instantaneous response is found for resonant $f\text{\ensuremath{-}}f$ excitation. This suggests that $4f$ electronic pumping can directly alter the magnetic anisotropy due to the modification of $4f\text{\ensuremath{-}}3d$ exchange at femtosecond timescales, without involving lattice thermalization.