MAGNETIC REVERSAL IN SMALL STRUCTURES

In this paper we establish a model explaining the experimentally obser ved anomalous enhanced coercivity of granular magnetic solids and thin films or multilayers that have pinned surface magnetic moments. We al so predict a normalized critical length for the material, above which the magnetic reversal mechanism is coercive and below which it occurs by a switching mode which is non-energy dissipative. The model used as sumes a dependence of the magnetization along the small dimension of t he granule or the depth of the layer. The contribution of the exchange term is obtained using the Landau semi-classical, formalism, and thus we do away with the mean field approximation. The coercivity is defin ed in such a manner as to retain its nonreversible nature. Results are presented in dimensionless units. When applied to cobalt and iron, th ey show coercivities larger than the theoretical maximum obtainable wi thout pinning in accord with experimental data.