Hidden-anisotropy-induced $\pi$ phase shift in all-optical magnetization precession

Laser-induced magnetization precession of an in-plane magnetized Pt/Co/Pt film with perpendicular interface anisotropy was studied using time resolved magneto-optical Kerr effect. An additional $\pi$ shift in the phase of precession is needed to describe the measured data if only the demagnetization energy is considered. Based on an augmented microscopic model description of the laser-induced magnetization dynamics, the additional $\pi$ phase is found to be rendered by the dependence on the phonon temperature of the hidden interface anisotropy, in contrast to the dependence on the electron temperature of the demagnetization energy. The observation that the phase of precession is affected by both the electron and the phonon temperature warrants a detailed knowledge about the forms of anisotropy present in the system under investigation for a holistic description of laser-induced magnetization precession.