Abstract: M-type barium ferrite (BaFe₁₂O₁₉, BaM) is a kind of uniaxial magnetocrystalline anisotropic hexagonal crystal system hard magnetic material. Because of strong magnetocrystalline anisotropy, it has broad application prospects in the field of self-biased microwave devices. In this paper, the normal temperature RF magnetron sputtering method was used to deposit a BaFe₁₂O₁₉ amorphous film with a thickness of about 130 nm on a sapphire (000l) substrate. Then they were annealed at 850 ℃, 900 ℃, 950 ℃, and 1 000 ℃ for 3 h to obtain BaM crystal film samples. X-ray diffraction method is used to identify the phase and crystal growth orientation of the film sample. The roughness and surface morphology of the film sample are measured and observed using a scanning probe microscope and a scanning electron microscope. X-ray diffraction results show that the main crystal phases of BaM are grown along the (000l) orientation, which is contributed by two factors: (1) suitable RF sputtering power and pressure; (2) BaM and sapphire have the same hexagonal. The crystal structure and the matching degree of unit cell parameters are relatively high, which can induce the crystal grain orientation of the film. When the annealing temperature reaches 950 ℃, the film begins to show the diffraction peaks of non-BaM crystal phase, indicating that excessive annealing temperature will cause the generation of impurity phases. When the annealing temperature is 900 ℃, the surface microstructure shows uniform grain size and clear interface. At 850 ℃, there are a small number of slender needle-like structures on the surface of the film. At 950 ℃, the morphology of the crystal grains transforms into large flaky particles. A sharp uplifted structure appears at the grain boundary. The flaky grain structure is the crystal grains with the C axis perpendicular to the film surface, and the needle-like structure is the random orientation of the C axis in the BaM plane, the sharp warped structure may be due to the miscellaneous phases caused by grain boundary segregation. The needle-like and sharp warped structure will cause the surface roughness of the film to increase. The results of the hysteresis loop test show that the magnetic properties of the thin film annealed at 900 ℃ are the best, with a surface roughness of 2.8 nm, a squareness ratio of 0.84, a saturation magnetization of 247 emu/cm³, and a coercivity of 1 528 Oe.

Key words: barium ferrite, thin film, radio frequency magnetron sputtering, annealing, magnetism