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

TL;DR

This study uses broadband optical magnetometry to analyze magnetization fluctuations in a ferromagnetic film across a spin reorientation transition, revealing critical spectral behaviors and the influence of magnetic fields.

Contribution

It demonstrates broadband spectral analysis of thermodynamic magnetization fluctuations through a controlled spin reorientation transition in a ferromagnetic film.

Findings

Magnetization noise follows a ν^{-3/2} power law in perpendicular anisotropy regions.

A critical frequency ν_0 emerges during the transition, indicating a low-frequency cutoff.

Magnetization noise is strongly affected by applied magnetic fields, revealing local anisotropies.

Abstract

We use scanning optical magnetometry to study the broadband frequency spectra of spontaneous magnetization fluctuations, or "magnetization noise", in an archetypal ferromagnetic film 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, $S(\nu)$, exhibits a remarkably robust $\nu^{-3/2}$ power law over frequencies $\nu$ 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 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 field-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.