Ferromagnetism in Ge–SiO2 co-sputtered films

Room-temperature ferromagnetism (FM) has been observed in the Ge-SiO2 co-sputtered films. The as-deposited and annealed films with 5 nm thickness were all ferromagnetic at room temperature. With the increase of annealing temperature, the saturation magnetization of the samples decreased. Seen from the energy dispersive x-ray spectrometry results, there exist only Ge, Si and O elements in all the co-sputtering films, which can rule out the role of any magnetic impurities and indicates that the FM is intrinsic. The FM of the films was attributed to both the quantum size effect and the couplings among Ge nanostructures distributed in SiO2 matrix. These couplings tend to align spins in both the junctions as well as the Ge quantum dots in a ferromagnetic manner. The magnetization of thinner films is larger than that of thicker films, showing that the magnetic coupling must be located mainly at the surface of the film.