Magnetoelasticity-driven phase inversion of ultrafast spin precession in Ni_xFe_100-x thin films

We present strong evidence for the deterministic role of magnetoelasticity in ultrafast spin dynamics of ferromagnetic Ni_xFe_100-x alloy films. Without a change in the crystal structure, we observed sudden π-phase inversion of the spin precession in the range of x = 88.5–98.2. In addition, it was found that the phase continuously changed and reversed its sign by varying the pump fluence. These cannot be explained simply by the temperature dependence of magnetocrystalline, demagnetizing, and Zeeman fields which have been conventionally considered so far in describing the spin dynamics. Through the temperature- and composition-dependent simulations adding the magnetoelastic field generated from the lattice thermal strain, we revealed that the conventional and magnetoelastic fields were competing around x = 95.8, where the spin dynamics showed the largest phase shift. For analytic understanding, we further show that the temperature-dependent interplay of the Curie temperature, saturation magnetization, and magnetostriction, which are demonstrated to be the most important macroscopic parameters, determines the ultrafast spin dynamics. Our extensive study emphasizes that magnetoelasticity is the key ingredient for fully understanding the driving mechanism of ultrafast spin dynamics.