Hi Andrew,
Irina has provided great information about overall approach for this simulation. Specific to permanent magnet, to define the ferrite as permanent magnet in HFSS, the magnetic bias we use is a link from Maxwell (where you can define magnetic coersivity). The Circulator example I mentioned in my previous response includes that as well. Please review the example as well as the help document that comes with it, especially go through the Non-Uniform Bias. Design 02_hmag and Design 03_FCNonUniform are related to that.
Note that in Maxwell, besides this B-H curve, there is option of Magnetic Coercivity vector (which you define it along z). Check the material definitions that are used in the example.
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