Imaging the dephasing of spin wave modes in a square thin film magnetic element

We have used time-resolved scanning Kerr effect microscopy to study dephasing of spin wave modes in a square ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ element of 10 \ensuremath{\mu}m width and 150 nm thickness. When a static magnetic field H was applied parallel to an edge of the square, demagnetized regions appeared at the edges orthogonal to the field. When H was applied along a diagonal, a demagnetized region appeared along the opposite diagonal. Time-resolved images of the out-of-plane magnetization component showed stripes that lie perpendicular to H and indicate the presence of spin wave modes with wave vector parallel to the static magnetization. The transient Kerr rotation was measured at different positions along an axis parallel to H, and the power spectra revealed a number of different modes. Micromagnetic simulations reproduce both the observed images and the mode frequencies. This study allows us to understand an anisotropic damping observed at the center of the square element in terms of dephasing of the resonant mode spectrum.