High temperature annealing stability of magnetic properties in MgO-based perpendicular magnetic tunnel junction stacks with CoFeB polarizing layer

Abstract

We studied Co/Pd multilayers (MLs) and CoFeB spin polarizing layer for MgO-based perpendicular magnetic tunnel junctions (MTJs) by engineering the exchange coupling between CoFeB and Co/Pd MLs and the interface morphology of Co and Pd in Co/Pd MLs. One of the key challenges of maintaining perpendicular anisotropy in both the fixed and free layers was achieved by tuning the energy balance between the Co/Pd MLs and the CoFeB layers. A perpendicular squareness ratio of near unity in M–H loops of full stack structures clearly indicated excellent perpendicular anisotropy even after annealing at 350 °C for 1 h in vacuum. Very low intermixing with sharp Co/Pd interfaces confirmed by the small angle x-ray reflectivity measurements was believed to be the key to high temperature annealing stability of magnetic properties. Our results shed light on the mechanisms resulting in low TMR (tunneling magnetoresistance) for Co/M (M=Pd, Pt, Ni) multilayer-based MTJs in this study as well as in previously published reports.