Picosecond large angle reorientation of the magnetization in Ni81Fe19 circular thin-film elements

Large angle picosecond reorientation of the magnetization has been studied in circular Ni81Fe19 thin-film elements of 30 µm diameter and 500 Å thickness by means of an optical pump–probe technique. The sample was pumped by an optically triggered magnetic field pulse and probed by a time resolved magneto-optical Kerr effect measurement. The temporal profile of the pulsed field and the in-plane uniaxial anisotropy of the element were first determined from measurements made in large static fields where the magnetization exhibited small amplitude ferromagnetic resonance oscillations. Measurements of large amplitude oscillations were then made in a smaller static field that was still larger than the in-plane uniaxial anisotropy field and sufficient to saturate the sample. Using the measured temporal profile of the pulsed field, the Landau–Lifshitz–Gilbert equation was used to model the motion of the magnetization as a coherent rotation process. The same values of the anisotropy and damping constants provided an adequate simulation of both the high and low field data. The magnetization was found to move through an angle of up to about 30° on subnanosecond time scales. The dependence of the reorientation upon the direction of the static applied field and observed deviations from the coherent precession model are discussed.