Magnetic cluster size and activation volume in perpendicular recording media.

The magnetic cluster size D[SUBcluster] 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[SUBcluster] (observed by magnetic force microscopy in the ac demagnetized state) are ∼55 nm for CoPtCr-SiO[SUB2] media of 24 nm thickness, and are several times larger than activation diameter D[SUBact] (D[SUBact] = V[SUBact]/ δ, V[SUBact]; the activation volume). D[SUBcluste] decreases as the film thickness δ decreases, particularly in a series of media with small intergranular exchange coupling, while D[SUBact] is nearly constant at ∼10 nm. It was experimentally shown that the slope of the magnetization curve α decreases as the value of D[SUBcluster]/δdecreases, and reaches a value of approximately 1 at D[SUBcluster]/ δ of around 2. The results suggest that reduction of film thickness d is important in order to realize both small D[SUBcluster] and large a values in granular-type perpendicular recording media. [ABSTRACT FROM AUTHOR]