MgO-based magnetic tunnel junction (MTJ) device includes in essence a ferromagnetic reference layer, a MgO tunnel barrier and a ferromagnetic free layer. The microstructure of MgO tunnel barrier, which is prepared by the metallic Mg deposition followed by the oxidation process or reactive sputtering, is amorphous or microcrystalline with poor (001) out-of-plane texture. In the present invention at least only the ferromagnetic reference layer or both of the ferromagnetic reference and free layer is proposed to be bi-layer structure having a crystalline preferred grain growth promotion (PGGP) seed layer adjacent to the tunnel barrier. This crystalline PGGP seed layer induces the crystallization and the preferred grain growth of the MgO tunnel barrier upon post-deposition annealing.