Coherent vs. incoherent switching for elongated CoPt-O grains in high-anisotropy medium

In most micro-magnetic models, a coherent switching mode is assumed for magnetic grains in the media. In this paper, we use state-of-art perpendicular recording media to study the magnetic switching behavior at different temperatures. The temperature dependence of the coercive field is measured and used as an indication of the strength of the thermal activation. For the thinner samples, the normalized coercive field can be described by an Arrhenius-Néel Brown (ANB) model. This model suggests that coherent switching dominates in these samples. For the thicker films, the normalized coercive field and, hence, the switching volume does not increase with increasing thickness, indicating the existence of incoherent switching. For the particular CoPt-O alloy studied, the crossover from coherent to incoherent switching occurs at a thickness of about 21 nm. For an average grain diameter of 7 nm, this corresponds to a grain aspect ratio of 3.