Thermoremanent Magnetization of Fine Powders

This is a brief explanation of the magnetic properties of fine ferromagnetic grains of single elementary domains, especially in reference to the influence of time on the magnetization at a given temperature. Application is made to the interpretation of the thermoremanent magnetization of lavas and clays and to the determination of the direction and intensity of the terrestrial magnetic field in the past. It is shown that, when the substance contains two kinds of fine ferromagnetic grains with clearly different Curie points, it is possible, under certain conditions explained in the text, that the constituent having the lowest Curie point becomes magnetized in a direction opposite to the exterior field: there arise, by this phenomenon, interactions between the grains and the demagnetizing field created by the constituent having the highest Curie point. Also, in certain cases, the total thermoremanent magnetization is directed inversely to the acting field. Recently Nagata found samples of lava in which the thermo-remanent magnetization obeys the mechanism thus described. In the geophysical interpretation, there will be, then, reason to keep in mind the possibility of this curious phenomenon, in considering the possible inversions of the terrestrial magnetic field.