The perpendicular anisotropy and thermal stability of CoFeB/Pt 5 multilayers are investigated with various CoFeB thicknesses (t CoFeB) and a fixed Pt thickness of 10.0 Å. Magnetic hysteresis loops exhibit a square shape for 2.2-4.0 t CoFeB and suggest that multilayers with appropriate CoFeB and Pt thicknesses display perpendicular magnetic anisotropy (PMA). After post-deposition annealing at temperatures of T a 300°C, no obvious change occurs in the loop shape for t CoFeB 2.2 Å, while the perpendicular coercivity increases with increasing T a for the sample with t CoFeB 4.5 Å due to the enhanced (111) texture. At T a ∼ 300°C, the original square loops for both samples start to tilt, showing that the occurrence of the PMA degradation is independent of the CoFeB thickness. The XRD results indicate that the observed decay of the PMA in CoFeB/Pt multilayers upon postannealing is associated with the interdiffusion and alloying effects at the CoFeB/Pt interfaces.