Grain Distributions in Rocks Containing Single Domain Grains

Abstract

A method of estimating the grain volume-coercive force distribution n(H_co, v) for rocks which can be treated by the Néel theory of non-interacting single domain grains is described. The method involves the analysis of a. c. demagnetization curves of partial TRM's produced in various temperature ranges. Experimental curves reported by Everitt (1961) for a sample containing single domain grains of maghemite have been analysed and a partial outline of n(H_co, v) for this sample found. The accuracy of the method was tested by re-solving for the original a. c. demagnetization curves and was found to be reasonably good. Prediction of results of other experiments carried out on the same sample, however, gave only semiquantitative agreement with experiment. Some possible explanations of the observed discrepancies are considered.
When grain interactions are important, the coercive force of a grain in increasing fields, a, becomes different from the coercive force in decreasing fields, b. It is then useful to find the grain distribution n(a, b). A method of deducing n(a, b) from n(H_co, v) is described and applied to the sample used above. The results are found to resemble actual distributions found by Bate (1962) for synthetic samples containing single domain maghemite particles.