Observation of incoherent picosecond magnetisation dynamics in micron sized Ni81Fe19 elements by time resolved scanning Kerr effect microscopy

We have studied small amplitude magnetisation precession in a square Ni81Fe19 element by time resolved scanning Kerr effect microscopy. A single precessional mode was observed with a 1 mum diameter probe spot placed at the centre of the element. Using a coherent rotation model, the presence of a four-fold anisotropy of about 30 Oe was deduced. Time resolved images show that the dynamic magnetisation is initially nonuniform at the edges of the element, with the nonuniformity then extending towards the centre. The dynamic magnetisation is uniform at the centre of the element over the measurement time, justifying the use of the coherent rotation model in the analysis of the precession frequency