Anomalous permeability in Polder-Smit region of ferrites

A study of the magneto-microwave Kerr effect on two samples of ferrites lying in the Polder-Smit region gives a negative value of μ', the real part of the transverse diagonal term of permeability tensor. There is a negative peak for μ' at a d.c. magnetization approximately equal to $\text{3}\text{4}$ of the saturation magnetization.