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 350C 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 (MPd, Pt, Ni) multilayer-based MTJs in this study as well as in previously published reports.
Bibliographical noteFunding Information:
J.P.W. and W.J.G. acknowledge support from the National Science Foundation GOAL program under Award No. ECCS 0702264. A.L. is grateful for financial support from the National Science Foundation MRSEC program under Award No. DMR-0819885. T.R. is grateful for partial support from DARPA STT-RAM program after July 2009. FIG. 1. (Color online) Out-of-plane and in-plane magnetic hysteresis loop of full stack PMTJ structure with CoFeB thickness (a) 2 nm and (b) 1.5 nm. FIG. 2. (Color online) Major and minor magnetic hysteresis loop of full stack PMTJ structure (a) as deposited, (b) annealed at 350 °C for 1 h, and (c) variation of fixed layer and free layer H c as a function of annealing temperature. FIG. 3. (Color online) Low angle x-ray reflectivity pattern of Sub/Ta3/Pd8/(Co0.3/Pd1)5/Ta3 (the units are in nanometers) film in as-deposited and annealed state (350 °C for 1 h).