Gyromagnetism.

Gyromagnetism
For gyromagnetic media (see Faraday rotation)
the magnetic permeability response to an
alternating electromagnetic field in the
microwave frequency domain is treated as a
non-diagonal tensor expressed by: [5]
Values for some common
materials
The following table should be used with caution
as the permeability of ferromagnetic materials
varies greatly with field strength. For example
4% Si steel has an initial relative permeability
(at or near 0T) of 2,000 and a maximum of
35,000 [6] and, indeed, the relative permeability
of any material at a sufficiently high field
strength trends toward 1.
Magnetic susceptibility and permeability data for
selected materials