Regarding ferromagnetic resonance frequency as a function of the perpendicular external magnetic field, the ferromagnetic resonance phenomenon is investigated. The results show that for the case where the external magnetic field is parallel to the its plane and the magnetic thin film has a perpendicular uniaxial magnetiocrystalline anisotropy, magnetic resonance frequency is divided into two categories, i.e., low frequency branch and high frequency branch, their specific relation depends on the magnetic anisotropy of the magnetic film; when external magnetic field is perpendicular to the systemic plane, the magnetic resonance frequency displays only a branch with the change of external magnetic field. In general, the magnetic resonance frequency decreases with the increase of the external magnetic field nonlinearly and monotonically. However, when the ratio between the cubic magnetocrystalline anisotropic field Hk1 and uniaxial magnetocrystalline anisotropic field Ha is about 2/3 Hk1/Ha < 1, the magnetic resonance frequency increases with the increase of external magnetic field, which is in good agreement with the experimental results. The results show that through magnetic resonance spectrum, the vertical magnetic anisotropy in magnetic thin film can be distinguished.

The effective magnetic field caused by exchange coupling between a ferromagnetic and an antiferromagnetic bilayer and its magnetic anisotropy have been investigated by means of ferromagnetic resonance. It was shown that the exchange bias phenomenon occurrence and its positive or negative characteristic for ferromagnetic/antiferromagnetic bilayer can be identified from its magnetic resonance spectrum. In addition, when the field was applied along different crystalline directions, the behavior of magnetic resonance field will be changed, and is closely dependent on the magnetocrystalline anisotropy and unidirectional anisotropy induced by the interface coupling in ferromagnetic/antiferromagnetic bilayer. Anomalies in the ferromagnetic resonance frequency studied as functions of applied field, including its orientation and strength, are interpreted as evidence for the four fold anisotropy and unidirectional anisotropy. Our conclusions are in good agreement with the related experiments.