Broadband spectroscopy of thermodynamic magnetization fluctuations through a ferromagnetic spin- reorientation transition
a L. Balk, F Li, Ian J. Gilbert, John Unguris, N A. Sinitsyn, S A. Crooker
We use scanning optical magnetometry to study the broadband frequency spectra of spontaneous magnetization fluctuations, or "magnetization noise", in an archetypal ferromagnetic that can be smoothly tuned through a spin reorientation transition (SRT). The SRT is achieved by laterally varying the magnetic anisotropy across an ultrathin Pt/Co/Pt trilayer, from the perpendicular to in-plane direction, via graded Ar+ irradiation. In regions exhibiting perpendicular anisotropy, the power spectrum of the magnetization noise, S(), exhibits a remarkably robust 3=2 power law over frequencies from 1 kHz - 1MHz. As the SRT region is traversed, however, S() spectra develop a steadily-increasing critical frequency, 0, below which the noise power is spectrally flat, indicating an evolving low-frequency cutoff for magnetization fluctuations. The magnetization noise depends strongly on applied in- and out-of- plane magnetic fields, revealing local anisotropies and also a fi eld-induced emergence of fluctuations in otherwise stable ferromagnetic films. Finally, we demonstrate that higher-order correlators can be computed from the noise. These results highlight broadband spectroscopy of thermodynamic fluctuations as a powerful tool to characterize the interplay between thermal and magnetic energy scales, and as a means of characterizing phase transitions in ferromagnets.