Hysteretic properties of synthetic and natural monodomain grains

Abstract

To examine the importance of monodomain grains or monodomain-like regions in multi-domain grains as possible carriers of high coercivity isothermal remanence in some rocks, comparative hysteretic properties were measured for four samples containing synthetic monodomain grains and for two fine-grained rocks, a baked laterite containing maghemite and an oxidized basalt containing magnetite. For all samples, remanence coercivities were high and values of the ratio of remanent to induced magnetization near saturation were 60 to 75% of those expected for exclusively monodomain samples. Experimental susceptibilities were within 30% of theoretical values calculated for coherently reversing monodomain particles. For the laterite, the basalt, and a synthetic magnetite, the temperature dependence of remanence coercivity was similar to that of saturation magnetization, as expected for monodomain grains dominated by shape anisotropy. These results suggest that a major fraction of the magnetic material in some baked earths and oxidized basalts is in a monodomain state.