Time decay of magnetization in particulate media

Intrinsic coercivities, Hc, measured in several particulate recording media are observed to fall as the sweep speed of the field through Hc decreases. Signal amplitudes in magnetic tape are also observed to decay with time; this decay is a function of writing density, increasing with wavelength to 7.3% in 1 hr. at 4 kfc/cm in one case. These phenomena are explained by time decays of magnetization, M, which are attributed to a thermal-fluctuation after effect proposed by Néel [1], involving the irreversible switching of interacting particles. In an external field, several types of magnetic recording media exhibit a decay of magnetization, with time constants from 20 to 1500 seconds. Particulate magnetic materials, such as gamma-iron oxide, Co-modified gamma-iron oxide, isotropic iron, acicular chromium dioxide, and barium ferrite show somewhat different decay characteristics. Longitudinally oriented chromium dioxide media exhibit a faster and greater decay in the direction of orientation than in the transverse direction, both measured at Hc. In general, however, the fall of magnetization is maximum at Hc, with little decay at H=O and at fields greater than twice Hc.