The detection of fine particles of magnetite using anhysteretic and rotational remanent magnetizations

Abstract

The dependence of anhysteretic remanent magnetization (ARM) and rotational remanent magnetization (RRM) on particle size has been determined for magnetite particles covering the approximate size range 0.2 to 90 μm. Both types of remanence depend on particle size with the smallest particles acquiring the highest remanence. For comparison a sample containing γFe2O3 particles from magnetic recording tape has also been included. By measuring the ratio RRM/ARM and multiplying this by the steady field used to produce the ARM, an effective field, Bg, which can be considered to produce the RRM, can be obtained. Bg ranges from 300 to 3 μT for particles in the range 0.2 to 45 μm at a rotation rate of twice the alternating field frequency (in a peak field of 80 mT) and varies approximately as the inverse of the particle size. The RRM under the same conditions varies as the inverse 1.3 power of particle size. A determination of the RRM and effective field Bg for rocks, therefore, offers a potentially quick, non‐destructive method of obtaining an estimate of the average size and approximate concentration of any magnetite particles which may be present. This method, however, must be used with some caution since the role (if any) played by small parasitic particles adhering to their larger host particles, which appear in electron micrographs of the samples, is at present unclear. RRM, however, at the very least, provides a quick method of detecting the presence of fine particles < 1 μm in size. Copyright © 1986, Wiley Blackwell. All rights reserved