  In recent years, there has been an intense interest in understanding the
microscopic mechanism of thermally induced magnetization switching driven by a
femtosecond laser pulse. Most of the effort has been dedicated to periodic
crystalline structures while the amorphous counterparts have been less studied.
By using a multiscale approach, i.e. first-principles density functional theory
combined with atomistic spin dynamics, we report here on the very intricate
structural and magnetic nature of amorphous Gd-Fe alloys for a wide range of Gd
and Fe atomic concentrations at the nanoscale level. Both structural and
dynamical properties of Gd-Fe alloys reported in this work are in good
agreement with previous experiments. We calculated the dynamic behavior of
homogeneous and inhomogeneous amorphous Gd-Fe alloys and their response under
the influence of a femtosecond laser pulse. In the homogeneous sample, the Fe
sublattice switches its magnetization before the Gd one. However, the temporal
sequence of the switching of the two sublattices is reversed in the
inhomogeneous sample. We propose a possible explanation based on a mechanism
driven by a combination of the Dzyaloshiskii-Moriya interaction and exchange
frustration, modeled by an antiferromagnetic second-neighbour exchange
interaction between Gd atoms in the Gd-rich region. We also report on the
influence of laser fluence and damping effects in the all-thermal switching.
