Shape anisotropy
المؤلف:
J. M. D. COEY
المصدر:
Magnetism and Magnetic Materials
الجزء والصفحة:
169
16-2-2021
1571
Shape anisotropy
The magnetostatic energy of a ferromagnetic ellipsoid with magnetization Ms is
Other simple shapes can be approximated to ellipsoids. The anisotropy energy is related to the difference in energy Δε when the ellipsoid is magnetized along its hard and easy directions. N is the demagnetizing factor for the easy direction; N' = 1/2(1 − N) is the demagnetizing factor for the perpendicular, hard, directions. Hence Δεm = 1/2 μ0VM2s [1/2 (1 − N) − N], which gives for a prolate ellipsoid
This is zero for a sphere (N = 1/3), as expected. Shape anisotropy is only fully effective in samples which are so small that they do not break up into domains. Non-ellipsoidal shapes are approximately described by an effective demagnetizing factor. In a multidomain state, each domain creates its own demagnetizing field, and is subject to the stray field of the other domains. The order of magnitude of shape anisotropy for a ferromagnet with μ0Ms ≈ 1 T is 200 kJ m−3 when N = 0.
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