Hi Graeme,
These plots as you stated represent the inductance and loss terms for the ferrite material. Dependent on the construct for the sample to be tested, the leakage inductance can vary and hence the loss term. So as a consequence the crossover point can move dependent on the sample build, for the same ferrite. Consider, if you have Identical ferrite forms, say 2 - hole cores, same physical size. Wind 2 turns on all units whose perm values vary from say 40-10,000 (type 67, 65, 64, 43, 73, 75, 76) you should see a plot of initial permeability (perm values that are well below the crossover point), that increases with larger perm. In all cases, if the measurement frequency is kept low, say below 1 MHz, the L value obtained from a S11 measure would simply increase. That would be a good place to start in an effort to be able to categorize the ferrite material.
Alan
