KRAMERSS RATE THEORY, BROKEN SYMMETRIES, AND MAGNETIZATION REVERSAL

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.