Theoretical Coercive Field for an Interacting Anisotropic Dipole Pair of Arbitrary Bond Angle

The coercive force for a pair of identical interacting uniaxially anisotropic dipoles of arbitrary bond angle is calculated for the case of the applied field coincident with the common anisotropy axis. The technique used is to examine the shape of the total energy surface about the equilibrium state as the applied field is lowered from an arbitrarily large initial value and subsequently reversed. At the nucleating field, the energy surface curvature disappears in some eigendirection which can be related to the initial mode of switching. An examination of the stability of available states to which the system may switch shows that for this simple model the nucleating field is identical to the coercive field. The results, which coincide with a planar solution, are expressed as coercive field versus pair bond angle for various pair separations. They differ significantly from the usual ``local field'' approach in which the magnetization is presumed to remain parallel to the applied field until nucleation. This exact collective treatment leads to a nonlinear reduction of coercive force with packing fraction for an ensemble of independent randomly oriented pairs, whereas the ``local field'' approach yields a coercivity independent of packing.