H-K DISTRIBUTIONS AND H-C CALCULATIONS FOR MAGNETIC RECORDING PARTICLES

Magnetization vs applied magnetic field data for several different classes of particulate magnetic recording media have been analyzed by fitting the first quadrants to log-normal distributions in anisotropy fields using the Stoner-Wohlfarth assumptions. Materials examined included small and large particle gamma-Fe2O3 and partially reduced gamma-Fe2O3 (precursor), Co-adsorbed precursor, CrO2, Fe metal, pure Ba ferrite, and Co-Ti substituted Ba ferrite. Measurement temperatures ranged from 4.2 to 296 K. Of the two distribution parameters, [H-K](G) (geometric mean anisotropy held) and a (standard deviation), a generally shows little thermal variation. [H-K](G) generally exhibits the expected temperature response, but shows an anomalous linear temperature dependence for the small spinel particles (which is related to spin canting), and a modest jump at low temperatures for the Fe particles (which is due to interaction with the Fe oxide outer layer). Substituted Ba ferrite shows strong temperature dependence of both parameters, due to a large K-2 anisotropy constant, which the Stoner-Wohlfarth analysis does not properly account for. Moments additive to the distribution calculations were found for CrO2 and the small spinels. From the observed temperature dependence, the additional moment for the former arises from superparamagnetism, and for the latter from spin canting. Extension of the modeled first quadrant moments into the second quadrant yields coercivities much closer to experimental values than those predicted by mean anisotropy fields or by curling models. Thus, coherent rotation is close to, if not the easiest, reversal mode in these particles.