  Ultrafast heating of FeRh by a femtosecond laser pulse launches a
magneto-structural phase transition from an antiferromagnetic to a
ferromagnetic state. Aiming to reveal the ultrafast kinetics of this
transition, we studied magnetization dynamics with the help of the
magneto-optical Kerr effect in a broad range of temperatures (from 4 K to 400
K) and magnetic fields (up to 25 T). Three different types of ultrafast
magnetization dynamics were observed and, using a numerically calculated H-T
phase diagram, the differences were explained by different initial states of
FeRh corresponding to a (i) collinear antiferromagnetic, (ii) canted
antiferromagnetic and (iii) ferromagnetic alignment of spins. We argue that
ultrafast heating of FeRh in the canted antiferromagnetic phase launches
practically the fastest possible emergence of magnetization in this material.
The magnetization emerges on a time scale of 2 ps, which corresponds to the
earlier reported time-scale of the structural changes during the phase
transition.
