We have studied the role of ${alpha}-Al_{2}O_{3}$ buffer layer as a diffusion barrier in the Ba-ferrite/$SiO_{2}$ magnetic thin films for high-density recording media. In the interface of amorphous Ba-ferrite $(1900-{AA}-thick)/SiO_{2}$ thin film during annealing, the interfacial diffusion started to occur at ${sim}700^{circ}C$. As the annealing temperature increased up to $800^{circ}C$, the interfacial diffusion abruptly proceeded resulting in the high interface roughness and the deterioration of the magnetic properties. In order to control the interfacial diffusion at the high temperature, we introduced ${alpha}-Al_{2}O_{3}$ buffer layer ($110-{AA}-thick$) in the interface of Ba-ferrite/$SiO_{2}$ thin film. During the annealing of Ba-ferrite/${alpha}-Al_{2}O_{3}/SiO_{2}$ thin film even at ${sim}800^{circ}C$, the interface was very smooth. The magnetic properties, such as saturation magnetization and intrinsic coercivity, were also enhanced, due to the inhibition of interfacial diffusion by the ${alpha}-Al_{2}O_{3}$ buffer layer. Our study suggests that the ${alpha}-Al_{2}O_{3}$ buffer layer act as a useful interfacial diffusion barrier in the Ba-ferrite/$SiO_{2}$ magnetic thin films.