Abstract
Femtosecond photoexcitation can abruptly redistribute electrons and trigger a series of transient nonequilibrium processes, among which ultrafast interatomic forces play a pivotal role in determining the structural and functional characteristics of solids. While ultrafast interatomic forces and their associated lattice dynamics have been extensively examined in semiconductors, experimental investigations of these nonequilibrium dynamics in metals remain lacking. To address this scientific gap, herein the direct observation of femtosecond-scale variations in photoinduced ultrafast interatomic forces within wrinkled giant magnetostrictive FeGa thin films is presented. At the onset of demagnetization, a transient signal emerges, lasting ≈400 fs, with its orientation is influenced by the external magnetic field. Theoretical analysis indicates that this signal arises from the swift release of internal stress prompted by the suppression of magnetostriction during ultrafast demagnetization. Owing to magnetization-induced stress anisotropy, this transient alteration in the interatomic potential introduces additional birefringence to the probe light. Consequently, this signal is attributed to a transient distortion of interatomic forces induced by the abrupt electron redistribution, establishing a nonequilibrium force state before any observable lattice expansion. These findings provide direct evidence for the existence of sub-picosecond interatomic forces and suggest a novel approach to control metal lattice dynamics through ultrafast magnetostriction.