Magnetic cluster size and activation volume in perpendicular recording media

The magnetic cluster size D-cluster of perpendicular recording media, which is a main factor in media noise, is discussed in terms of activation volume, intergranular exchange coupling, and the slope of magnetization curves. It was experimentally shown that the values of D-cluster (observed by magnetic force microscopy in the ac demagnetized state) are similar to55 nm for CoPtCr-SiO2 media of 24 nm thickness, and are several times larger than activation diameter D-act (D-act=V-act/delta, V-act; the activation volume). D-cluster decreases as the film thickness delta decreases, particularly in a series of media with small intergranular exchange coupling, while D-act is nearly constant at similar to10 nm. It was experimentally shown that the slope of the magnetization curve alpha decreases as the value of D-cluster/delta decreases, and reaches a value of approximately 1 at D-cluster/delta of around 2. The results suggest that reduction of film thickness delta is important in order to realize both small D-cluster and large alpha values in granular-type perpendicular recording media. (C) 2003 American Institute of Physics.