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| Format: | Preprint |
| Published: |
2025
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| Online Access: | https://arxiv.org/abs/2501.01400 |
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| _version_ | 1866912175540404224 |
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| author | Bramwell, Steven T. |
| author_facet | Bramwell, Steven T. |
| contents | Zero-field magnetic noise, characterised by the magnetic autocorrelation function $S_s(t)$, has been observed, perhaps surprisingly, to depend on sample shape $s$. The reasons for this are identified and general expressions are derived that relate the autocorrelation functions for systems of different shape to an underlying `intrinsic' form. Assuming the flcutuatiopn-dissipation theorem, it is shown that, for any noise that relaxes monotonically, the effect of sample shape is to reduce both the noise amplitude and mean relaxation time by a factor of $1+Nχ_i$, where $N$ is the demagnetizing factor and $χ_i$ the intrinsic susceptibility. In frequency space, where $S_s(t)$ Fourier transforms into the power spectrum $S_s(ω)$, the above two factors combine to suppress the zero frequency amplitude of $S_s(ω)$ by $(1+Nχ_i)^2$, while at high frequency, sample shape dependence becomes negligible. These results suggest simple and robust experimental tests of the fluctuation--dissipation theorem in magnetic systems that may be useful in distinguishing bulk from surface effects. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2501_01400 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Sample shape dependence of magnetic noise Bramwell, Steven T. Statistical Mechanics Zero-field magnetic noise, characterised by the magnetic autocorrelation function $S_s(t)$, has been observed, perhaps surprisingly, to depend on sample shape $s$. The reasons for this are identified and general expressions are derived that relate the autocorrelation functions for systems of different shape to an underlying `intrinsic' form. Assuming the flcutuatiopn-dissipation theorem, it is shown that, for any noise that relaxes monotonically, the effect of sample shape is to reduce both the noise amplitude and mean relaxation time by a factor of $1+Nχ_i$, where $N$ is the demagnetizing factor and $χ_i$ the intrinsic susceptibility. In frequency space, where $S_s(t)$ Fourier transforms into the power spectrum $S_s(ω)$, the above two factors combine to suppress the zero frequency amplitude of $S_s(ω)$ by $(1+Nχ_i)^2$, while at high frequency, sample shape dependence becomes negligible. These results suggest simple and robust experimental tests of the fluctuation--dissipation theorem in magnetic systems that may be useful in distinguishing bulk from surface effects. |
| title | Sample shape dependence of magnetic noise |
| topic | Statistical Mechanics |
| url | https://arxiv.org/abs/2501.01400 |