Resonant Pumping of d-d Crystal Field Electronic Transitions as a Mechanism of Ultrafast Optical Control of the Exchange Interactions in Iron Oxides

The microscopic origin of ultrafast modification of the ratio between the symmetric ($J$) and antisymmetric ($D$) exchange interaction in antiferromagnetic iron oxides is revealed, using femtosecond laser excitation as a pump and terahertz emission spectroscopy as a probe. By tuning the photon energy of the laser pump pulse we show that the effect of light on the $D/J$ ratio in two archetypical iron oxides ${\mathrm{FeBO}}_3$ and ${\mathrm{ErFeO}}_3$ is maximized when the photon energy is in resonance with a spin and parity forbidden $d-d$ transition between the crystal-field split states of ${\mathrm{Fe}}^{3+}$ ions. The experimental findings are supported by a multielectron model, which accounts for the resonant absorption of photons by ${\mathrm{Fe}}^{3+}$ ions. Our results reveal the importance of the parity and spin-change forbidden, and therefore often underestimated, $d-d$ transitions in ultrafast optical control of magnetism.