Noise Mechanisms in Small Grain Size Perpendicular Thin Film Media

In this paper, we present a set of systematic experimental investigations on possible noise mechanisms for current perpendicular thin film media of small grain sizes. In particular, we focus on intergranular exchange coupling and grain boundary surface anisotropy in the granular layer of the present continuous-granular-composite film structure. Micromagnetic modeling studies are conducted to study the impact of the observed experimental phenomenon. Modeled experiments show that significant intergranular exchange coupling may occur when oxide grain boundary thickness becomes less than 1 nm. If the grain boundary thickness has significant distribution below this critical value, the exponential dependence of the coupling strength on the oxide thickness would yield significant degradation of the medium signal-to-noise ratio. Carefully designed experiments have also been conducted to study possible grain boundary interfacial anisotropy. Co/Cr, CoPt/Cr, Co/SiO₂, Co/Cr₂O₃, and Co/TiO ₂ interfaces are investigated and the corresponding interfacial anisotropy strengths are quantitatively measured. Although Co/SiO₂ interfacial anisotropy appears to be the weakest among them, the measured interfacial anisotropy energy strengths for all of them are significant fractions of the crystalline perpendicular anisotropy of the grains at present grain sizes. Finally, we investigated the impact of stacking faults in hcp Co-alloy grains. It is found that when the anisotropy strength of a small segment of a grain substantially reduces due to the existence of stacking faults, it will yield a switching field reduction disproportional to the volume ratio of the segment.