Interacting single-domain properties of magnetite intergrowths

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Recognition that magnetite intergrowths are a common carrier of stable remanence in igneous rocks has prompted a detailed investigation of their magnetic properties. A Tertiary basalt containing homogeneous titanomagnetite grains was oxidized in air to form magnetite/ilmenite intergrowths, the morphology of which has been studied by electron microscopy. The intergrown grains are shown to contain arrays of interacting single-domained magnetites with uniaxial shape anisotropy. Interactions between single domains cause reduction of coercivity from the very large values associated with isolated single domains to values which are, however, many times larger than those measured in multidomained grains. The intergrown grains have an observed IRS/IS value of 0.30, whereas the value of this ratio obtained experimentally under nondemagnetizing conditions is found to be 0.51. The reduction is shown to be entirely explicable in terms of localized array demagnetizing fields arising from surface poles. A similar effect is seen in magnetite/ulvospinel intergrowths. Qualitative evidence that this reduction proceeds mainly by rotation of the spontaneous magnetization from easy axes is seen in a measured anisotropy of low field axial susceptibility having a value 11% higher in a direction perpendicular to the saturation remanence than that parallel to it. This phenomenon may be used to discriminate between rocks containing interacting single domains and those containing either multidomains or noninteracting single domains. These results also apply to magnetic powder experiments reported in the literature, in which reduction in IRS/IS horn 0.5 will occur if single-domained particles of uniaxial anisotropy are clumped together, a situation which is difficult to avoid. Magnetic intergrowth structures commonly observed in subaerial basalts should be regarded as arrays of interacting single-domain magnetite particles capable of carrying a strong remanent magnetization stable over geological intervals of time.