Underlayer Effect on Perpendicular Magnetic Anisotropy in Co20Fe60B20/MgO Films

Perpendicular magnetic tunneling junctions with Ta\CoFeB\MgO have been extensively studied in recent years. However, the effects of the underlayer on the formation of the CoFeB perpendicular magnetic anisotropy (PMA) are still not well understood. Here, we report the results of our systematic use of a wide range of elements (Ti, V, Cr, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt, and Au) encompassed by columns IVA, VA, VIA, VIIA, and VIIIA of the periodic table as the underlayer in an underlayer\Co
₂₀
Fe
₆₀
B
₂₀
\MgO stack. Our goals were to survey more elements that could conceivably create a PMA in CoFeB and thereby to explore the mechanisms enabling these underlayers to enhance or create the PMA. We found that underlayer elements having an outer shell of both 4d electrons (Zr, Nb Mo, and Pd) and 5d electrons (Hf, Ta, W, Re, Ir, and Pt) resulted in the development of a PMA in the MgO-capped Co
₂₀
Fe
₆₀
B
₂₀
. Hybridization between the 3d electrons of the Fe or Co (in the Co
₂₀
Fe
₆₀
B
₂₀
) at the interface with the 4d or 5d electrons of the underlayer is thought to be the cause of the PMA development.