Particle-size effects on the switching behavior of uniaxial and multiaxial magnetic recording materials

The coercivity of a magnetic material is a well-defined parameter only if the time-scale of interest is specified. A measurement that produces rapid changes of magnetization will yield a higher value of the coercivity than one that operates on a longer time-scale. The difference is due to the thermally-assisted nature of the magnetic switching process. Thus, for a magnetic recording material, the coercivity relevant to high-frequency writing will be greater than that relevant to demagnetization processes (such as transition broadening) during long-term storage. The discrepancy between the two values will become more pronounced as the volume of the magnetic switching unit becomes smaller; this effect is relevant to the question of the ultimate information densities achievable by magnetic media. The "writing coercivity" and the "storage coercivity" can be estimated from typical magnetic measurements and simple kinetic theory, for both uniaxial and multiaxial ("isotropic") recording materials.