Effect of magnetic viscosity on the angular dependence of coercive field in particulate recording media

In order to demonstrate the effect of magnetic viscosity on the angular dependence of a coercive field, H_c, the authors measured H_c as a function of the angle of the applied field for IBM 3480 CrO₂ tape, JVC Super-VHS Co-γFe₂O₃ tape, and a 0.3 μm Co-Cr film with perpendicular anisotropy, at 4.2 K, 77 K, and 300 K. The data are compared to the numerical simulation of a modified Stoner-Wohlfarth model which includes thermally activated reversals of the magnetic moments of an ideal assembly of particles. The model suggests that, as temperature increases, H_c is reduced by a larger factor when the angle between the applied field and the alignment direction is small. Experimental evidence for this behavior is found in the CrO₂ and Co-Cr samples. It is concluded that, for some common magnetic recording media magnetic viscosity effects must be evaluated before one can infer a magnetization reversal mechanism from H_c vs. angle data