A magnetic coupling occurring in partial self‐reversal of magnetism and its association with increased magnetic viscosity in basalts

Summary. Samples of Rauher Kulm basalt containing titanomagnetite were heat treated to exhibit partial self‐reversal caused by a CRM acquired by an exsolved or ‘daughter’ phase reversely coupled with the original or ‘mother’ phase. Alternation of thermal demagnetization with alternating field (AF) demagnetization from progressively higher values of peak AF showed that the two phases were mainly demagnetized together. The coercivity of the coupled. phases exceeded that of the natural sample. Screening of the daughter phase by a shell of mother phase was demonstrated using large pieces of synthetic titanomagnetite with segregated magnetite and applying a field during the process of self‐reversal. Saturation magnetization was measured to indicate the relative proportion of the phases. Measurements of viscous magnetization of Rauher Kulm samples confirmed that heat treatment caused increased viscosity, but with anomalous anisotropy. After a certain time the viscosity coefficient changed sharply. Explained in terms of grains present such viscous growth initially involved growth of the two phases in opposition. Owing to its greater volume the mother phase growth, which involved low‐energy domain wall barriers, was assumed to be the greater. When there were no more daughter phase nuclei of sub‐critical volume available, the mother phase growth continued unopposed. Analysis of available growth curves showed that the magnetic viscosity coefficient of the daughter phase was related to the viscosity coefficient for this unopposed growth, which depended on the sum of the external field and the change of the back field produced by the daughter phase. Small granules within grains could be a factor in increasing viscosity due to domain wall movement.