  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.
