Future Advances in Areal Density

Getting to 1 Terabit per Square Inch - The Near Term 

To reach ultimate densities of 500-1000 Gbit/in2 in PMR the average grain diameter needs to be reduced to about 5 nm and dispersions need to be 

Regardless, however, of whether intergranular interactions are lowered or the physical grain size is reduced, magnetically harder media materials and larger write fields will be required in the future, in order to meet thermal stability requirements [3, 4, 7, 10]. 

Thermal stability is a function of the grain size and grain size distribution [24], To estimate the minimal thermally stable grain core size Dp, one needs to make assumptions about the magnetization reversal and thermal decay mechanisms. Thermal decay is described by an exponential Arrhenius law, which relates the time constant z for storage to the ratio of a reversal energy barrier EB and thermal energy kBT (T = absolute temperature in Kelvin) (Neel-Brown model [25-27])  

To maintain thermal stability, hence a condition EB/kBT= In (for) needs to be fulfilled. For z = 10 years storage, 109-10u Hz [28] and ignoring dispersions, i.e. assuming monodisperse particles, this becomes Es/kBT= 40-45. Reversal for isolated, well-decoupled grains to first order can be described by coherent rotation over EB. This simple model, as first discussed by Stoner and Wohlfarth in 1948 [29], considers only intrinsic anisotropy and external field (Zeeman) energy terms. For perpendicular geometry one obtains the following expression  

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