6.11.4.2 Properties of Hard Ferrites

Hard ferrites or hard magnets exhibit high coercive fields, typically above 150 kA.m^-1 and are often called permanent magnets. This is because these materials are able to withstand any demagnetizing effects that may arise either internally or externally.

In addition to magnetization or remanence (B_r) and coercive field, a permanent magnet is often characterized by product B.H i.e. area under the magnetic hysteresis curve.

Remanence in materials like hexaferrites which are strongly anisotropic can be affected by processing. Since the effect of magnetic anisotropy in a polycrystalline ceramic is small, the material can be synthesized under application of a magnetic field thereby aligning moments along c-axis in the grains giving larger remanence. Similarly, coercivity is a strong function of grain size and it is found for hexaferrites that it is maximum for grain sizes of about 1 μm.