Laser-induced THz emissivity as a multiscale probe of the emergence of magnetization in a FeRh/Pt bilayer

FeRh is known to become ferromagnetic upon heating has long been well known. It is surprising, however, that depending on the employed experimental technique the measured speeds of the emergence of magnetization in FeRh can differ by 3 orders of magnitude while the mechanism of this emergence remains unclear. Here we resolve this controversy by using laser-induced THz emissivity as a multiscale probe of the magnetization. In particular, we show that the net magnetization emerges only after 20 ps and rises on a timescale of 100 ps. We find that laser-induced ferromagnetic domains nucleate in the bulk of FeRh and propagate at a speed at the scale of 40 m/s.