Detection of coherent and incoherent spin dynamics during the magnetic switching process using vector-resolved nonlinear magneto-optics

It is usually assumed that magnetic switching proceeds via coherent rotation under conditions of high symmetry. There is no a priori reason to expect an inhomogeneous response when a uniform magnetic torque is applied to a homogeneous ferromagnet. We test this assumption using vector- and time-resolved nonlinear magneto-optic measurements on a continuous Ni-Fe film. While coherent dynamics are observed when the magnetization M is initially oriented along the easy axis (the preferred axis of M in the absence of external fields), we find evidence for inhomogeneous spin dynamics when M is initially oriented perpendicular to the easy axis, which suggests the generation of incoherent spin waves during the magnetic reorientation process. The inhomogeneity is sufficient to reduce the spatially averaged magnetic moment within the measured area by almost 50%.