Underlayer Effect on Perpendicular Magnetic Anisotropy in Co₂₀Fe₆₀B₂₀\MgO Films

Perpendicular Magnetic Tunneling Junctions (pMTJs) 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 a underlayer\Co₂₀Fe₆₀B₂₀\MgO stack. Our goals were to survey more elements which could conceivably create a PMA in CoFeB and thereby to explore the mechanisms enabling these underlayers to enhance or create the PMA. We found underlayer elements having both an outer shell of 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 Co₂₀Fe₆₀B₂₀. Hybridization between the 3d electrons of the Fe or Co (in the Co₂₀Fe₆₀B₂₀) at the interface with the 4d or 5d electrons of the underlayer is thought to be the cause of the PMA development.