FERROMAGNETIC COERCIVITY AND APPLIED-FIELD ORIENTATION

We analyze the relationship between the coercive force of a ferromagnetic material and the angle of the applied magnetic field. The material is assumed to contain a ferromagnetic domain wall as well as a planar defect parallel to the wall and it is further assumed that the dominant mechanism determing the coercivity is that of pinning of the wall by the defect. Our formulation takes into account the spatial dependence of the direction of magnetization along the normal to the plane of the defect. Numerical solutions are obtained for the resulting nonlinear differential equations and analysis is done on the roles of the anisotropic, magnetostatic, and exchange energies in determining the behavior of the coercivity as the direction of the applied magnetic field is varied. Our results show that, in contradiction to previous thought, the inverse cosine of the applied field angle is not a good approximation to the coercivity dependence unless the coercivity is about two orders of magnitude smaller than the anisotropy field. Also, there exist ranges of parameter values for which the domain wall pinning coercivity decreases as the angle between the applied magnetic field and the anisotropy field increases - a behavior previously assumed to occur only when the coercivity is dominated by nucleation rather than pinning of domain walls. Thus, caution must be exercised when using the angular dependence of the applied field to determine the mechanism of magnetic reversal of a given material.