We have analyzed the crystallization processes of sputtered thin films of Co ₂₀ Fe ₆₀ B ₂₀ and Co ₂₀ Fe ₆₀ B ₂₀ -X alloys (X = Ta, Hf) using the increase in saturation magnetization that accompanies the formation of α-FeCo. Constant heating rate analysis shows a decrease in crystallization temperature for MgO/CoFeB/Ta multilayers with decreasing CoFeB thickness (from 350 °C for a 20 nm film to 275 °C for a 2 nm film) that we attribute to interface effects on critical nucleus formation. Alloying CoFeB with Ta or Hf is found to decrease Curie temperature and inhibit crystallization. For very thin (<;2 nm) CoFeB films, such as those found in modern perpendicular devices, interdiffusion of adjacent Ta layers in the absence of a low-energy nucleation surface, such as MgO, can prevent crystallization altogether. CoFeB films with adjacent TaN layers are found to crystallize regardless of film thickness, and we suggest that these alternate capping layers have lower interdiffusivity.