Ultrathin percolated high-Ku magnetic films with thicknesses of 2 nm are realized simply by using sputter deposition and post annealing. L1₁ CoPt:MgO, with Ku on the order of 10(7) erg cm(-3), was deposited on a MgO(111) substrate at 350 °C, followed by post annealing to induce complete segregation of the added MgO dopants. The optimized film shows significant enhancement of the out-of-plane coercivity, approximately an order of magnitude greater than that of the CoPt binary film, a remanence ratio close to unity, considerably reduced in-plane magnetization, sharp perpendicular magnetic reversal, and reduced surface roughness in the range of a few angstroms. Microstructure results indicate that MgO precipitates into grains within the interconnected L1₁ grains after appropriate post annealing. The MgO grains, with sizes in the range 2-7 nm, form coherent interfaces to the CoPt matrix and penetrate through the whole depth of the film. The development of ideal non-magnetic domain wall pinning sites explains the optimization of the perpendicular magnetic properties. The advantages of a simple fabrication process, a thin film layer structure, and remarkable enhancement of the magnetic characteristics demanded by ultrahigh-density recording reveal its potential for practical applications.