Quasi-static Strain Governing Ultrafast Spin Dynamics

The quasi-static strain (QSS) is the product induced by the lattice thermal expansion after ultrafast photo-excitation. Although the ultrafast spin dynamics driven by the QSS and thermal effects are barely distinguishable in time, they should be treated separately because of their different fundamental actions. By employing ultrafast Sagnac interferometry and the magneto-optical Kerr effect, we demonstrate quantitatively the existence of QSS and the decoupling of two effects counteracting each other in typical polycrystalline Co and Ni films. The Landau-Lifshitz-Gilbert and Kittel equations considering a magnetoelastic energy term showed that QSS, rather than the thermal energy, in ferromagnets plays a governing role in ultrafast spin dynamics. This demonstration provides a way to analyze ultrafast photo-induced phenomena.