Magnetic domain structures and domain walls in iron fine particles

Abstract

In order to estimate the critical particle size lc for a single domain structure, the magnetic energy densities of an iron fine particle are calculated as a function of the particle size for various configurations of magnetization. It is assumed in this calculation that in a small particle the energy of a domain wall consists of the exchange, anisotropy, and magnetic stray field energies. It is found that lc is 20 nm for a particle with an axial ratio of n = 1, increases with an increase in n, and is 60 nm for a particle with n = 10. The wall energy of a fine particle is found to be much larger than that of bulk material owing to the stray field energy. Due to this wall energy, the magnetization configuration is the circular configuration of spins for the particle with a size larger than lc , and it becomes the flux closure configuration by four domains for the particle with a size larger than 900 nm if n = 1 and 600 nm if n = 10.