A generalized theory of thermoremanence, covering the transition from single domain to multi-domain magnetic grains

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Abstract
Single crystal magnetic grains are found to have appreciable permanent (pseudo-single domain) moments by virtue of the Barkhausen discreteness in the positions of their domain walls. These moments are proportional to the 3/2 power of grain diameter and are responsible for the low-field thermoremanence of grains with diameters between 0.03 μ and approximately 5 μ. Smaller grains are true single domains, larger ones behave as true multi-domains with properties accurately described by an earlier theory. A graph of thermoremanence against grain size shows a strong maximum at 0.3 μ, at which TRM is 40 times as strong as for large grains. Under demagnetizing conditions the permanent moments of grains up to several hundred microns across become important. They acquire saturation alignment in extremely small stray fields, giving observable specimen magnetizations quite out of proportion to the fields, and may also interact to cause mutual alignment even in the absence of stray fields. This is considered to be the origin of observed random moments in demagnetized rocks. Measured random moments allow the pseudo-single domain grain moments to be estimated quantitatively and applied to the theory of TRM for all grains larger than 0.03 μ.

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