Determination of Magnetocrystalline Anisotropy Fields of Magnetically Isotropic Polycrystalline Hexaferrites by the FMR Method

The parameters that determine practical applications of oxide ferrimagnets are the values of saturation magnetizations and the magnitudes and signs of the magnetocrystalline anisotropy fields. Measuring magnetocrystalline anisotropy fields using ferromagnetic resonance (FMR) on single-crystal or textured polycrystalline hexaferrite samples is straightforward. However, these methods are not applicable to hexaferrite, which are typically synthesized as macroscopically magnetically isotropic polycrystalline or powder samples. This paper discusses two methods for calculating the components of the permeability tensor of polycrystalline (or powder) hexaferrites within the independent grain approach. The methods are described for experimentally determining the magnitudes and signs of magnetocrystalline anisotropy fields and magnetomechanical ratios for such materials, based on FMR curves measured at various frequencies. The proposed method is illustrated by the results of an FMR study of a Y-type polycrystalline Ba2Ni2−xCuxFe12O22 (0.0 ≤ x ≤ 2.0) system, synthesized using a standard ceramic method. The measurements were performed on spherical samples in the frequency range of 36–50 GHz. The method allows one to evaluate the magnetocrystalline anisotropy fields and magnetomechanical ratios of both the target Y-phase and impurity phases. The phase composition estimates obtained from FMR data were compared with the results of X-ray phase analysis. It has been shown that up to x = 1.8, these materials are ferroxplana-type hexaferrites, while the hexaferrite Ba2Cu2Fe12O22 exhibits the easy-axis type of magnetocrystalline anisotropy.