  The quasi-static strain (QSS) is the product generated by the lattice thermal
expansion after ultrafast photo-excitation and the effects of thermal and QSS
are inextricable. Nevertheless, the two phenomena with the same relaxation
timescale should be treated separately because of their different fundamental
actions to the ultrafast spin dynamics. By employing ultrafast Sagnac
interferometry and magneto-optical Kerr effect, we quantitatively prove the
existence of QSS, which has been disregarded, and decouple two effects
counter-acting each other. Through the magnetoelastic energy analysis, rather
we show that QSS in ferromagnets plays a governing role on ultrafast spin
dynamics, which is opposite to what have been known on the basis of thermal
effect. Our demonstration provides an essential way of analysis on ultrafast
photo-induced phenomena.
