Critical sizes of cobalt fine particles with uniaxial magnetic anisotropy

Assuming that the wall energy of a fine ferromagnetic particle consists of the exchange, magnetocrystalline anisotropy, and stray field energies, the critical particle size lc of a single‐domain structure is calculated for an elongated cobalt particle as a function of the axial ratio n. It is found that lc is 55 nm for a particle with an axial ratio of n=1, increases with an increase in n, and is 300 nm for a particle with n=10. The particle with a size larger than lc begins to contain two domains divided by a 180° wall. Using the wall energy of bulk material, which consists of exchange and magnetocrystalline anisotropy energies, the critical particle size lcb is also calculated, and lcb is found to be smaller than lc by about 10%.