The perpendicular magnetic anisotropies of CoFeB/MgO films with Nb buffer layers

The effects of Nb buffer layers on the magnetic properties of Nb/CoFeB/MgO/Ta films have been studied. The films exhibited perpendicular magnetic anisotropy (PMA) when the Nb buffer layer thickness approached or exceeded 3 nm. PMA was retained at annealing temperatures up to 350 °C for the films containing 5-nm Nb buffer layer. The Ta, CoFeB, MgO, and Nb layers were easily distinguished by energy-dispersive X-ray spectroscopy. High-resolution transmission electron microscopy showed that the Nb, MgO, CoFeB, and Ta layers were crystalline. X-ray photoelectron spectroscopy indicated that the oxygen reduced from MgO combined with Fe and Co to form FeO_x and CoO_x after annealing, strengthening the PMA. The Nb(5)/CoFeB(1)/MgO(2)/Ta(3) film exhibited a smooth CoFeB/MgO interface after annealing, as confirmed by X-ray reflectivity. The crystallized MgO and CoFeB, formation of Fe and Co oxides, and smooth CoFeB/MgO interface all contributed to the strong PMA.