Selection rules for bulk piezomagnetic coefficients of isotropic titanomagnetite dispersions

A thermodynamic approach is developed for piezomagnetic phenomena, linked to reversible variations of magnetic susceptivity in isotropic stressed media. In spite of the problematic interpretation of rock piezomagnetism at the microscopic level, a simple macroscopic parameterization of antiferromagnetic media is obtained, based on invariance arguments. In intrinsically isotropic media, linearity in the stress-strain relation is shown to imply linearity of the relation between induced magnetization and stress. The macroscopic piezomagnetic behaviour of a sample in the linear stress-strain regime is described by a fourth rank tensor with 2 independent coefficients only. These coefficients can be measured in the laboratory by simple uniaxial compression tests. Comparisons with experimental data on titanomagnetite bearing rocks and synthetic dispersions show that the linear approach is valid in so far as the applied stress does not exceed a few 100 bars.