The origin of signal-to-noise ratio improvements in laminated recording media

We report an experimental and theoretical study of laminated recording media made of two CoPtCrB magnetic layers separated by a Ru spacer layer. We find that a 4A Ru spacer layer is sufficient to decorrelate the noise of the magnetic transitions in the two layers, thus producing improved media signal-to-noise ratios. This result is surprising given that the layers remain structurally correlated due to their epitaxial growth, and therefore the noise in the layers would be expected to be correlated as well. Experiments suggest that the reduced transition noise is produced by a slight shift between the transition locations in the layers. This shift is on the order of the grain size and allows the transitions to sample a different microstructure causing the noise associated with the transitions to be uncorrelated.