Ultrathin (Gd, Tb)-FeCo Films With Perpendicular Magnetic Anisotropy

Perpendicular ultrathin RE-TM films with thickness 3 nm were investigated for potential use as free layers of perpendicular magnetic tunnel junctions. The free layer thickness should be as thin as possible to reduce spin-transfer switching current density while maintaining adequate thermal stability. Using Ti or Al as under- and over-layers, we found that both 3 nm GdFeCo and TbFeCo films had perpendicular magnetic anisotropy (PMA), but DyFeCo had PMA only in cases where thickness exceeded 5 nm. The effective PMA constants of TbFeCo films were of order of 10⁵ erg/cm³, which provided sufficient thermal stability. By annealing at various temperatures, we found that the coercivity of TbFeCo films can be ranged from 120 to 1000 Oe. Therefore, insertions of under/overlayers together with suitable annealing treatment provide a way to fabricate RE-TM ultrathin layer with desired magnetic properties.