Micromagnetism

Abstract

The theory of magnetic domain is based on the assumption that a magnetically ordered object constitutes of domains and domain walls. The domains are uniformly magnetized to saturation and non-uniform magnetization is distributed only in the walls. In some cases, however, $\vec {M}_\mathrm{ s}$ distribution is out of domain structure category. The $\vec {M}_\mathrm{ s}$ distribution in a small object of transition dimension from single domain size to multi-domain size and its variation during magnetization processes should be nonuniform which cannot be treated by the domain model. Micromagnetism discards the assumption of the domains, and intends to slove such problems from general minimum condition of the thermal potential of the system. Even in a bulk object, when an applied magnetic field is decreased from a sufficiently large value, the magnetization decreases from saturation along the upper hysteresis curve. At some critical field there should form a nucleus of reverse domain before it grows up to an observable spike-like domain of opposite magnetization. The nucleus is a small local region of non-uniform magnetization which is neither a domain nor a domain wall.