Thermally Stable Perpendicular Magnetic Anisotropy in MgO-Free CoFeB/Heavy Metal Bilayers Grown by Pulsed Laser Deposition

Perpendicular magnetic anisotropy (PMA) in CoFeB-based thin films is a key phenomenon for the development of next-generation spintronic devices. The most common realizations of perpendicular magnetization are based on ferromagnet/oxide (FM/Ox) and ferromagnet/heavy metal (FM/HM) interfaces in canonical MgO/CoFeB/HM structures. In this work, we investigate the nature of PMA in purely metallic CoFeB/Mo and CoFeB/W bilayer structures grown by pulsed laser deposition (PLD) without an MgO underlayer. The optimal thickness ranges of the CoFeB and heavy-metal layers required for PMA formation were determined. A systematic investigation of the effect of vacuum post-annealing in the temperature range of 300–500°C on the magnetic properties revealed that increasing the annealing temperature enhances the effective perpendicular magnetic anisotropy energy (Keff). CoFeB/Mo and CoFeB/W structures exhibited Keff values of 1.1 × 104 and 2.3 × 104 J/m3 after annealing at 500°C, respectively. The results also showed that tungsten-capped structures exhibit higher saturation magnetization and thermal stability than molybdenum-capped CoFeB films. A comparative investigation of CoFeB/Mo structures grown on Si/SiO2 and MgO substrates further revealed improved PMA properties at intermediate annealing temperatures. However, at 500°C, the thermal stability deteriorated completely. These results demonstrate that PLD-grown metallic thin-film structures are capable of developing PMA at the FM/HM interface with high thermal stability, expanding the prospects for designing spintronic devices without oxide layers.