Nonuniform switching of single domain particles at finite temperatures

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Abstract
It has recently been shown that nonuniform thermal fluctuations are able to significantly reduce the coercivity of elongated ferromagnetic particles compared to the uniform rotation model of Néel and Brown. In particular, this theory revealed that even for particles that are single domain in the remanent state, a nonuniform energy barrier exists which is proportional to the product of the particle cross-sectional area and the domain wall energy. For sufficiently long particles, this energy barrier is therefore always lower than that of uniform reversal. Here, several implications of this theory are discussed. It is shown that the coercivity of a particle with fixed volume decreases with increasing aspect ratio. For a fixed particle shape, the coercivity is lower with decreasing exchange constant. For small particle aspect ratios the theory is shown to merge continuously into the Néel–Brown theory. The angular dependence of the coercivity is evaluated explicitly. The reduction from the Stoner–Wohlfarth value is maximal for external fields parallel and perpendicular to the particle axis and the coercivity shows an asymmetric angular dependence. Both features are in accordance with recent experiments on an elongated single domain particle.