Kramers’s rate theory, broken symmetries, and magnetization reversal (invited)

The theory of thermally induced magnetization reversal in small particles is reviewed. The conventional Néel–Brown theory for uniform magnetization reversal and its derivation from Kramers’s rate theory are first discussed. For sufficiently elongated particles, however, a nonuniform energy barrier (‘‘nucleus’’) has lower energy than the uniform barrier and thus yields a lower coercivity. This coercivity reduction is shown to occur also for vanishing hard-axis anisotropy when the nucleus breaks the rotational symmetry around the easy axis. The prefactor of the Arrhenius factor is calculated for uniform and nonuniform barriers.