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We use scanning optical magnetometry to study the broadband frequency spectraof spontaneous magnetization fluctuations, or "magnetization noise", in anarchetypal ferromagnetic film that can be smoothly tuned through a spinreorientation transition (SRT). The SRT is achieved by laterally varying themagnetic anisotropy across an ultrathin Pt/Co/Pt trilayer, from theperpendicular to in-plane direction, via graded Ar$^+$ irradiation. In regionsexhibiting perpendicular anisotropy, the power spectrum of the magnetizationnoise, $S(\nu)$, exhibits a remarkably robust $\nu^{-3/2}$ power law overfrequencies $\nu$ from 1~kHz to 1~MHz. As the SRT region is traversed, however,$S(\nu)$ spectra develop a steadily-increasing critical frequency, $\nu_0$,below which the noise power is spectrally flat, indicating an evolvinglow-frequency cutoff for magnetization fluctuations. The magnetization noisedepends strongly on applied in- and out-of-plane magnetic fields, revealinglocal anisotropies and also a field-induced emergence of fluctuations inotherwise stable ferromagnetic films. Finally, we demonstrate that higher-ordercorrelators can be computed from the noise. These results highlight broadbandspectroscopy of thermodynamic fluctuations as a powerful tool to characterizethe interplay between thermal and magnetic energy scales, and as a means ofcharacterizing phase transitions in ferromagnets.

Broadband spectroscopy of thermodynamic magnetization fluctuations through a ferromagnetic spin-reorientation transition