Perpendicular Magnetic Tunnel Junctions Using Co-based Multilayers

CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) free and reference layers composed of Co/M (where M=Pd or Ni) multilayers have been optimized for high PMA and high tunneling magnetoresistance (TMR). The effects of Co thickness, Pd thickness, and the number of Co/Pd bilayers on the anisotropy and coercivity of the [Co/Pd]n multilayer films have been studied for both free and reference layers. The damping parameter α of CoFeB capped multilayers was determined using broadband ferromagnetic resonance. The transport properties of the patterned MTJ stacks were measured from 10 to 400 K. A maximum TMR of 10% at 10 K (5%–10% at 300 K) was obtained for these perpendicular MTJs, regardless of whether or not they were magnetically annealed for MgO–CoFeB crystallization. This indicates that the fcc-bcc-fcc transitions from the fcc multilayers to the bcc CoFeB/MgO/CoFeB do not promote the “giant MgO TMR effect” caused by symmetry filtering.