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Title: Broadband Spectroscopy of Thermodynamic Magnetization Fluctuations through a Ferromagnetic Spin-Reorientation Transition

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 ( ν ) exhibits a remarkably robust ν - 3 / 2 power law over frequencies ν from 1 kHz to 1 MHz. As the SRT region is traversed, however, S ( ν ) spectra develop a steadily increasing critical frequency ν 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.

Authors:
 [1];  [1];  [2];  [2];  [1];  [1]
  1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
  2. National Inst. of Standards and Technology (NIST), Gaithersburg, MD (United States). Center for Nanoscale Science and Technology
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org.:
USDOE; LANL Laboratory Directed Research and Development (LDRD) Program; National Science Foundation (NSF)
OSTI Identifier:
1471805
Alternate Identifier(s):
OSTI ID: 1479988
Report Number(s):
LA-UR-18-22421
Journal ID: ISSN 2160-3308
Grant/Contract Number:  
AC52-06NA25396; DMR-1157490
Resource Type:
Journal Article: Published Article
Journal Name:
Physical Review. X
Additional Journal Information:
Journal Volume: 8; Journal Issue: 3; Journal ID: ISSN 2160-3308
Publisher:
American Physical Society
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; High Magnetic Field Science

Citation Formats

Balk, A. L., Li, F., Gilbert, I., Unguris, J., Sinitsyn, N. A., and Crooker, S. A. Broadband Spectroscopy of Thermodynamic Magnetization Fluctuations through a Ferromagnetic Spin-Reorientation Transition. United States: N. p., 2018. Web. doi:10.1103/PhysRevX.8.031078.
Balk, A. L., Li, F., Gilbert, I., Unguris, J., Sinitsyn, N. A., & Crooker, S. A. Broadband Spectroscopy of Thermodynamic Magnetization Fluctuations through a Ferromagnetic Spin-Reorientation Transition. United States. doi:10.1103/PhysRevX.8.031078.
Balk, A. L., Li, F., Gilbert, I., Unguris, J., Sinitsyn, N. A., and Crooker, S. A. Fri . "Broadband Spectroscopy of Thermodynamic Magnetization Fluctuations through a Ferromagnetic Spin-Reorientation Transition". United States. doi:10.1103/PhysRevX.8.031078.
@article{osti_1471805,
title = {Broadband Spectroscopy of Thermodynamic Magnetization Fluctuations through a Ferromagnetic Spin-Reorientation Transition},
author = {Balk, A. L. and Li, F. and Gilbert, I. and Unguris, J. and Sinitsyn, N. A. and Crooker, S. A.},
abstractNote = {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(ν) exhibits a remarkably robust ν-3/2 power law over frequencies ν from 1 kHz to 1 MHz. As the SRT region is traversed, however, S(ν) spectra develop a steadily increasing critical frequency ν0, 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.},
doi = {10.1103/PhysRevX.8.031078},
journal = {Physical Review. X},
issn = {2160-3308},
number = 3,
volume = 8,
place = {United States},
year = {2018},
month = {9}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record at 10.1103/PhysRevX.8.031078

Citation Metrics:
Cited by: 2 works
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Figures / Tables:

FIG. 1. FIG. 1.: (a) Experimental schematic. A scanning broadband optical magnetometer based on polar MOKE measures the spontaneously fluctuating out-of-plane magnetizationmore » $\delta$$m$$z$ within a small (4 $μ$m) spot on the sample. The power spectral density of this magnetization noise $S$($ν$) is continuously computed and averaged in real time (PBS: polarizing beamsplitter, FFT: fast Fourier transform). The sample is a Pt/Co/Pt trilayer irradiated with a laterally varying dose of Ar+, generating a lateral gradient of interfacial magnetic anisotropy. Red and blue colors depict Co and Pt layers; arrows depict the net magnetization direction, with yellow arrows indicating the region where the spin reorientation transition (SRT) occurs. (b) The magnetic contrast image of the trilayer surface (adapted from Ref. [32]) shows the mazelike domain patterns that form in graded trilayers of this type. The patterns become blurry near the SRTas fluctuations increase and occur faster than the image acquisition time. Conventional MOKE magnetization measurements $m$$z$($B$$z$) show that the coercive field $μ$0$H$$c$ → 0 as the SRT is approached. (c) Characteristic Kerr rotation fluctuations $\delta$θ$K$($t$) reveal the thermodynamic magnetization fluctuations $\delta$$m$$z$($t$). (d) $S$($ν$) falls as a $ν$−3/2 power law over the measured 1 kHz–1 MHz range in regions of the trilayer exhibiting perpendicular magnetization. Discussion of measurement uncertainties in this and subsequent figures can be found in the Supplemental Material [35].« less

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