  Thermal stability factor (delta) of recording layer was studied in
perpendicular anisotropy CoFeB/MgO magnetic tunnel junctions (p-MTJs) with
various CoFeB recording layer thicknesses and junction sizes. In all series of
p-MTJs with different thicknesses, delta is virtually independent of the
junction sizes of 48-81 nm in diameter. The values of delta increase linearly
with increasing the recording layer thickness. The slope of the linear fit is
explained well by a model based on nucleation type magnetization reversal.
