  Both helicity-independent and helicity-dependent all-optical switching
processes driven by single ultrashort laser pulse have been experimentally
demonstrated in ferrimagnetic alloys as GdFeCo. Although the switching has been
previously reproduced by atomistic simulations, the lack of a robust
micromagnetic framework for ferrimagnets limits the predictions to small
nano-systems, whereas the experiments are usually performed with lasers and
samples of tens of micrometers. Here we develop a micromagnetic model based on
the extended Landau-Lifshitz-Bloch equation, which is firstly validated by
directly reproducing atomistic results for small samples and uniform laser
heating. After that, the model is used to study ultrafast single shot
all-optical switching in ferrimagnetic alloys under realistic conditions. We
find that the helicity-independent switching under a linearly polarized laser
pulse is a pure thermal phenomenon, in which the size of inverted area directly
correlates with the maximum electron temperature in the sample. On the other
hand, the analysis of the helicity-dependent processes under circular polarized
pulses in ferrimagnetic alloys with different composition indicates qualitative
differences between the results predicted by the magnetic circular dichroism
and the ones from inverse Faraday effect. Based on these predictions, we
propose experiments that would allow to resolve the controversy over the
physical phenomenon that underlies these helicity-dependent all optical
processes.
