Broadband optical spectroscopy of stochastic ferromagnetic fluctuations across a spin-reorientation transition*

We use the magneto-optical Kerr effect to measure frequency spectra of stochastic magnetic fluctuations between 1 kHz and 1 MHz in a Pt/Co/Pt trilayer with anisotropy that has been laterally graded through a spin-reorientation transition. On regions of the film with strong perpendicular anisotropy, the fluctuation power obeys a power law over three orders of magnitude in frequency. Fluctuation spectra measured in regions with weaker anisotropy show a cutoff frequency below which the spectra are approximately frequency independent. We identify this cutoff as characterizing the maximum lifetime of ferromagnetic states, and find that it is sensitive to magnetic anisotropy, temperature, and applied magnetic field. Finally, the fluctuations show higher-order correlations which reveal temporal symmetry breaking, appearing upon application of magnetic field. These measurements establish magnetization fluctuation spectroscopy as a sensitive tool for characterizing ferromagnetic behavior and phase transitions.