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 material becomes anisotropic in the presence of a magnetizing field. The tensor is described mathematically as:

Neglecting the effects of damping, the components of the tensor are given by

where

 (rad / s) / (A / m) is the effective gyromagnetic ratio and , the so-called effective g-factor (physics), is a ferrite material constant typically in the range of 1.5 - 2.6, depending on the particular ferrite material. is the frequency of the RF/microwave signal propagating through the ferrite,  is the internal magnetic bias field,  is the magnetization of the ferrite material and  is the magnetic permeability of free space. To simplify computations, the radian frequencies of  and  can be replaced with frequencies (Hz) in the equations for  and  because the  factor cancels. In this case,  Hz / (A / m)  MHz / Oe. If CGS units are used, computations can be further simplified because the  factor can be dropped. References

Ferrites
Tensor physical quantities
Ferromagnetic materials
Magnetic ordering