Magnetostrictive control of coercive force in multidomain magnetite

Abstract

Although the natural remanent magnetization of terrestrial igneous rock is often due to multidomain grains of magnetite or titanomagnetite, it is not certain what controls the coercivity of the multidomain grains and hence the stability of their remanence¹. Even the related but simpler problem of what controls the bulk coercive force H_c of multidomain magnetite is still not solved. I consider this simpler problem here by noting^2,3 that the coercive force of multidomain magnetite powders⁴ varies with low temperature approximately in proportion to the magnetostriction⁵ coefficient λ. I find the same approximate proportionality between H_c and λ on cooling six igneous rocks bearing multidomain magnetite. Hence, the coercive force of these powders and rocks seems mainly to be magnetostrictively controlled–presumably through internal stresses. This is even true of the three rocks with high enough H_c (∼150 Oe) to be classed as pseudo-single-domain, including two with the fine magnetite–ilmenite intergrowths often observed in terrestrial igneous rocks with stable remanence⁶.