A strong perpendicular magnetic anisotropy (PMA) and a high thermal stability are essential for long‐term stable storage of data in PMA‐based magnetic tunnel junctions (p‐MTJs). This work investigates the magnetic anisotropy of the MgO/CoFeB/X/CoFeB/MgO double‐interface free layer stacks, where X represents the spacer material. After annealing at 350 °C for an hour, interfacial magnetic anisotropy (Ki) as high as 4.06 mJ m−2 is obtained in the MgO/CoFeB/Mo (0.4 nm)/CoFeB/MgO stacks, much higher than those for Ta‐ and W‐based films. Experimental and first‐principle calculation results demonstrate that bulk PMA plays a great role in the Mo‐based structure, which is often negligible in Ta‐ and W‐based structures. Moreover, a strong PMA is achieved after annealing at 500 °C, which is attributed to the weak interdiffusion and good uniformity as shown in high‐resolution transmission electron microscopy and energy‐dispersive X‐ray spectroscopy results. These findings help to understand the origin of strong PMA in Mo‐based structures and show the promising prospect of using this structure for high‐packing‐density p‐MTJs and other spintronic device applications down to 10 nm scale.

Spin transfer torque magnetic random-access memory is widely considered as a promising candidate for a next generation cache, with a core device of magnetic tunnel junction. The MgO/CoFeB/W/CoFeB/MgO structure as recording layer for magnetic tunnel junctions is of great interest due to features such as strong perpendicular magnetic anisotropy (PMA) and thermal stability. However, the origin of correlation between PMA and interlayer exchange coupling (IEC) in this structure remains unclear. In this paper, we investigate the PMA and IEC in the MgO/CoFe/W/CoFe/MgO structure with first-principles calculation. IEC with long and short period oscillations is observed, which results in much different magnetic anisotropy energies at CoFe/W interface in MgO/CoFe/W/CoFe/MgO structure compared to MgO/CoFe/W structure. Furthermore, we find an oscillation of magnetic anisotropy energies in ferromagnetically coupled MgO/CoFe/W/CoFe/MgO structure. According to k-space and band-decomposed charge-density analyses, this oscillation can be elucidated with quantum well states on spin-up states near Fermi energy within W layers. Thus, the correlation between PMA and IEC is explained by quantum well states. These findings provide comprehensive understanding of PMA and IEC in double-interface structures and pave a new way to modulate PMA with quantum well states in heavy metal layers.