Stoner-Wohlfarth Calculation on Particle with Both Magnetocrystalline and Shape Anisotropy

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Abstract
Previously, it had been assumed that the presence of a minimum of magnetocrystalline anisotropy energy along the long axis of an acicular single‐domain particle would increase the coercive force. A theoretical calculation using the Stoner‐Wohlfarth model with a magnetocrystalline energy term indicates this is not always true. For an acicular polycrystalline single‐domain particle with cubic magnetocrystalline anisotropy, it is assumed that the crystallites are ordered with the [111] direction parallel to the long axis, and the other directions randomly oriented around this [111] direction. Carrying out the Stoner‐Wohlfarth calculation with various ratios of magnetocrystalline anisotropy to shape anisotropy shows the presence of certain stable states of magnetization which are inaccessible by merely traversing the hysteresis loop. Also, the coercive force of a random agglomerate is related in a complex way to the anisotropy ratio (ratio of crystalline to shape anistropy constants), becoming zero for large values of the anistropy ratio. “Butterfly” hysteresis loops also appear for a wide range of anisotropy ratios.

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