Induced susceptibility anisotropy of igneous rocks caused by uniaxial compression.

Abstract

The effect of uniaxial compression on the initial susceptibility of a variety of igneous rocks has been studied to determine the dominant factors controlling their response. Hysteresis properties, Curie temperatures, and grain size distributions of the magnetic phases of the rocks have been determined to provide control data.The changes in susceptibility varied from 1 to 10 percent for a stress of 100 bars, with considerable variation in the form of induced anisotropy. In the sample with the coarsest magnetic phases, small decreases in susceptibility (1%) were seen both parallel and perpendicular to the applied stress. The dominance of finer, but still clearly multidomain phases gave rise to larger induced anisotropy (-10%) with the “normal” pattern of an increase perpendicular to stress accompanying a larger decrease in the direction parallel to stress. Samples with pseudo-single domain grain size magnetic phases showed anomalously large increases in the direction perpendicular to stress and only very small decreases in the parallel direction. Some of these samples also showed remarkable increases in susceptibility during loading and unloading. In the finest grain size range, the magnetic particles approached the single domain state and revealed a small (-1%) increase in the perpendicular direction accompanied by a similar decrease in the parallel direction.The dominant characteristic influencing the stress response is the domain state, which is itself strongly dependent upon the grain size. Three types of response are defined. The single domain response consists of an increase in susceptibility in the direction perpendicular to stress and an equal decrease in the direction parallel to stress. The magnetoelastic energy evidently modifies the dominant anisotropy energy against which the magnetization must be rotated by the applied field. In the pseudo-single domain range, wall nucleation appears to produce an isotropic increase in susceptibility. This effect combined with the induced anisotropy characteristic of multidomain material accounts for the observations. In multidomain material, the principal effect seems to be due to loss of wall area parallel to stress, accompanied by some gains in walls perpendicular to stress.