Gyromagnetic remanence and anisotropy in single-domain particles, rocks, and magnetic recording tape

Experimental results are described for the gyromagnetic remanence acquired by two rock samples and a sample of magnetic tape arbitrarily orientated in a non-magnetic holder. The remanence (gyroremanent magnetization—GRM) is produced in standard ‘demagnetization’ equipment when the sample is exposed without rotation to a strong alternating field in the absence of any other fields. This effect has already been qualitatively explained (Stephenson 1980 c) in terms of an effective transient biasing field resulting from the intrinsic angular momentum associated with the moments of single-domain particles as they are forced to undergo irreversible flips in the strong applied field. By constructing a theoretical model of a three-dimensional anisotropic distribution of uniaxial particles, a consideration of the average flip of the assembly using this transient field approach enables expressions for the dependence of GRM on field-axis orientation to be derived. Use of these expressions in conjunction with a computer program which obtains a least-squares fit between experimental and theoretical GRM curves enables the anisotropy axes of the rocks and the tape orientation to be found. An experimental observation not accounted for by the above approach is that a component of GRM can be present along the field axis. This, however, can be understood by a theoretical analysis of the flip of the moment of a single-domain particle whose internal energy is represented, not by an ellipsoid of revolution, but by an ellipsoid with three unequal axes. The analysis of such a particle indicates that GRM should not be acquired by ellipsoids of revolution, nor by ellipsoids which have one energy very much greater than the other two (i.e. a moment restricted to one plane). It indicates that there is an optimum ratio between the intermediate and hard magnetization axes of the particle at which the acquisition of GRM should be a maximum.