Abstract
At least four types of sub-domain magnetic moments, on a scale smaller than the main domain structure, could contribute to pseudo-single-domain intensities of TRM (thermoremanent magnetization) in small multidomain grains. Of these, moments pinned by the stress fields of dislocations, surface moments, and moments due to the Barkhausen discreteness of domain wall positions are either strongly shielded by the magnetically soft matrix, subject to the internal demagnetizing field during magnetization changes, or so coupled to the domain structure that they cannot change magnetization independently. Only the net moments of domain walls themselves qualify as ‘psarks’—subdomain moments with truly single-domain behaviour. A new reversal mode, domain wall inversion or curling, is postulated to explain the incoherent reversal of domain wall moments. It amounts to nucleating and propagating a Bloch line across a 180° domain wall.
The experimental evidence for the existence of single-domain-like moments within multidomain grains is reviewed. Both initial susceptibility and weak-field TRM have distinctively single-domain components in magnetite grains just above single-domain size. The volume activated in magnetization changes of these same grains, whether determined from TRM acquisition curves, from blocking temperatures or from coercive forces at high temperature, is about equal to the volume of a domain wall, as expected. However, there is no present evidence for psarks in >1μm grains. Psarks are, therefore, a partial explanation at best of the pseudo-single-domain question.
Psarks, if they exist, could have curious behaviour when cooled from above their blocking temperatures, including the possibility of a ‘self-rotated TRM’ at 90°to the direction of the field applied at high temperature.
