  Ultra-fast magnetization switching induced by a single femtosecond laser
pulse, under no applied magnetic field has attracted a lot of attention during
the last 10 years because of its high potential for low energy and ultra-fast
memory applications. Single-pulse helicity-independent switching has mostly
been demonstrated for Gd based materials. It is now important to optimize the
pulse duration and the energy needed to switch a GdFeCo magnet depending on the
alloy thickness, concentration. Here we experimentally report state diagrams
showing the magnetic state obtained after one single pulse depending on the
laser pulse duration and fluence for various GdFeCo thin films with different
compositions and thicknesses. We demonstrate that these state diagrams share
similar characteristics: the fluence window for switching narrows for longer
pulse duration and for the considered pulse duration range the critical fluence
for single pulse switching increases linearly as a function of the pulse
duration while the critical fluence required for creating a multidomain state
remains almost constant. Calculations based on the atomistic spin model
qualitatively reproduce the experimental state diagrams and their evolution. By
studying the effect of the composition and the thickness on the state diagram,
we demonstrated that the best energy efficiency and the longest pulse duration
for switching are obtained for concentration around the magnetic compensation.
