Single‐domain theory of remanent magnetization

Abstract

The basic principles underlying the single‐domain theory developed by Néel are examined using a modem statistical mechanics approach. It is shown that the quasi‐equilibrium theory of Néel is in good shape but there are problems with the dynamic part of the theory. The physical assumptions made with regard to “thermal fluctuations” appear oversimplified, and Néel's theory, and a similar theory by Brown, violate the detailed balance theorem of statistical mechanics. These problems deal only with the attempt frequency factor used in estimations of the relaxation time and not with the important exponential factor, which appears to have been correctly determined by Néel. In the absence of a convincing theory for the attempt frequency factor, it is recommended that experimental values be used (10 7 to 10 8 s −1 for magnetite). Although Néel's theory does appear to be correct in identifying the critical exponent dependence of the relaxation time as a function of activation energy and temperature, the activation energy values used by paleomagnetists in applying Néel's theory are often incorrect, as has been previously pointed out by Smith. Consistent with Smith's suggestion, lower activation energies are obtained by modeling noncoherent reversal modes with the passage of a domain wall. This leads to a significant lowering of the activation energy in most cases. However, the resulting activation energy still appears to be somewhat too high based on experimental evidence, indicating that either defects play an important role and/or other modes of noncoherent rotation occur (including other possible domain wall structures) in practice.