An object may be magnetized by placing it in magnetic field of strength H. The magnetic susceptibity χm is defined as-
The relative permeability is related to the magnetic susceptibilty via
The magnetic flux density B
is then due to the external magnetic field H
and the magnetized field of the object M
In a diamagnetic substance the χm is small and negative (~ -10-5). In a paramagnetic substance it is small (~ 10-3) and positive while in a ferromagnetic substance it is much larger (~ 106) and positive.
Note, the magnetization M does not always respond linearly to the applied magnetic field H. In such cases, the magnetic susceptiblity can be defined by the slope dM/dH or by some other approximation.
Note, the magnetic susceptibility does not have units.