Ultrafast Control of Magnetic Anisotropy by Resonant 4f Electronic and Phonon Excitations in Sm0.7Er0.3FeO3

We report on the ultrafast dynamics of spin reorientation using different excitation mechanisms in the orthoferrite Sm
0.7
Er
0.3
FeO
3
. First, intense few-cycle mid-infrared pulses tuned to optical phonon resonance are exploited to excite the lattice system. A finite time delay of the spin reorientation is observed at its initial dynamics, displaying the energy transfer between lattice and 4f system. In contrast, an instantaneous response is found when the f-f electronic transition of Sm
³⁺
is resonantly excited. Our results indicate that 4f electronic pumping can alter the magnetic anisotropy due to the modification of 4f-3d exchange at femtosecond time scales, without involving lattice thermalization.