Exchange Anisotropy in Rock Magnetism

Some igneous rocks are magnetized in a direction opposite to that expected, if they had been cooled in the earth's magnetic field. These rocks are said to have a reverse thermo-remanent magnetization (reverse TRM). Uyeda has shown that the reverse TRM of the Haruna deposit in Japan is due to an ilmenite-hematite solid solution and has synthesized a solid solution that has a reverse TRM when cooled in fields as high as 16000 oe. He has put forth a theory which contains a mechanism similar to that found in the cobalt-cobaltous oxide system. Based upon Uyeda's explanation of the reverse TRM, we postulated that the material should have a shifted hysteresis loop and that it should be shifted in the opposite direction to that found in the Co-CoO system. We found that the solid solution—0.6 FeTiO3 0.4 Fe2O3—was shifted by 350 oe in the opposite direction to Co-CoO when cooled in a field. The loop was symmetrical when cooled in zero field. Moreover,we have shown that when the material is cooled in a magnetic field through the Morin transition the loop is shifted in the same direction as Co-CoO. These results confirm the general features of Uyeda'a model, although the detailed mechanism is still being studied. We believe that Uyeda's work definitely establishes the reverse TRM of the Haruna deposit as due to a magnetic phenomenon and not due to the reversal of the earth's magnetic field. We believe that the type of magnetic investigation reported in this paper may be applied to other deposits that have a reverse TRM to establish if they are also due to a magnetic phenomenon.