  Resonant absorption of a photon by bound electrons in a solid can promote an
electron to another orbital state or transfer it to a neighboring atomic site.
Such a transition in a magnetically ordered material could affect the magnetic
order. While this process is an obvious road map for optical control of
magnetization, experimental demonstration of such a process remains
challenging. Exciting a significant fraction of magnetic ions requires a very
intense incoming light beam, as orbital resonances are often weak compared to
above-band-gap excitations. In the latter case, a sizeable reduction of the
magnetization occurs as the absorbed energy increases the spin temperature,
masking the non-thermal optical effects. Here, using ultrafast x-ray
spectroscopy, we were able to resolve changes in the magnetization state
induced by resonant absorption of infrared photons in Co-doped yttrium iron
garnet, with negligible thermal effects. We found that the optical excitation
of the Co ions affects the two distinct magnetic Fe sublattices differently,
resulting in a transient non-collinear magnetic state. The present results
indicate that the all-optical magnetization switching most likely occurs due to
the creation of a transient, non-collinear magnetic state followed by coherent
spin rotations of the Fe moments.
