Polder tensor
The Polder tensor is a tensor introduced by Dirk Polder for the description of magnetic permeability of ferrites. The tensor notation needs to be used because ferrimagnetic materials become anisotropic in the presence of magnetizing field.

The tensor is described mathematically as:

where:

and  [kHz/(A/m)] is a gyromagnetic ratio and g is a factor between 1.9-2.4 depending on ferrite material. Magnetizing frequency (f) is expressed as , H0 is a bias field, M is magnetization and  is magnetic permeability of free space.

References

Ferrites
Tensor physical quantities
Ferromagnetic materials
Magnetic ordering