抄録
Magnetic viscosity of ensemble of medium-sized grains of natural magnetites was examined at various temperatures. It was found that magnetic viscosity coefficient S, defined as I-I0=S(Q+logt) where I and I0 are intensity of magnetization at time t and t=0, is proportional to external magnetic field, and S is a linear function of temperature, except at temperatures close to the Curie point and -160°C, so far as the Rayleigh region of magnetization is concerned. It seems also that S tends to a finite value according as grain size tends to the order of a single domain.
For ensemble of magnetite grains, S in expression of ΔI=S(Q+logt) is very small, and the limit of half life time τ for change in thermoremanent magnetization, expressed by τ=expItr/2S amounts to 10120 years.
Further, the half life time for sedimentary rocks, in which directions of grain magnetization are scattered, with reduction factor w, is about 1010 years for w=1/5. These results may show that remanent magnetization of igneous and sedimentary rocks have been stable against thermal viscosity during a geologically long time.
