A magnetic tunnel junction (MTJ) is disclosed wherein a free layer (FL) interfaces with a metal oxide (Mox) layer and a tunnel barrier layer to produce interfacial perpendicular magnetic anisotropy (PMA). The Mox layer has a non-stoichiometric oxidation state to minimize parasitic resistance, and comprises a dopant to fill vacant lattice sites thereby blocking oxygen diffusion through the Mox layer to preserve interfacial PMA and high thermal stability at process temperatures up to 400° C. Various methods of forming the doped Mox layer include deposition of the M layer in a reactive environment of O2 and dopant species in gas form, exposing a metal oxide layer to dopant species in gas form, and ion implanting the dopant. In another embodiment, where the dopant is N, a metal nitride layer is formed on a metal oxide layer, and then an anneal step drives nitrogen into vacant sites in the metal oxide lattice.