Balk A L, Li F, Gilbert I, Unguris J, Sinitsyn N A, Crooker S A
National High, Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
Phys Rev X. 2018;8. doi: 10.1103/PhysRevX.8.031078.
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, (), exhibits a remarkably robust power law over frequencies from 1 kHz to 1 MHz. As the SRT region is traversed, however, () spectra develop a steadily-increasing critical frequency, , 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.
我们使用扫描光学磁力测量法来研究一种典型铁磁薄膜中自发磁化涨落（即“磁化噪声”）的宽带频谱，该铁磁薄膜可通过自旋重取向转变（SRT）进行平滑调节。通过分级氩离子辐照，在超薄Pt/Co/Pt三层膜中横向改变磁各向异性，从垂直方向到面内方向，从而实现SRT。在表现出垂直各向异性的区域，磁化噪声的功率谱(S_{zz}(f))在1 kHz至1 MHz的频率范围内呈现出显著稳健的幂律。然而，当穿过SRT区域时，(S_{zz}(f))频谱会出现一个不断增加的临界频率(f_c)，在该频率以下，噪声功率在频谱上是平坦的，这表明磁化涨落的低频截止在不断演变。磁化噪声强烈依赖于施加的面内和面外磁场，揭示了局部各向异性，同时也表明在原本稳定的铁磁薄膜中，磁场会诱导涨落的出现。最后，我们证明可以从噪声中计算高阶关联函数。这些结果突出了热力学涨落的宽带光谱学，它是表征热能和磁能尺度之间相互作用的有力工具，也是表征铁磁体中相变的一种手段。
