Magnetic susceptibility is defined as the ratio of the intensity of magnetisation (M) at any point within a given material to the magnetic field(H).
Xm = M/H
Xm is a property which determines how easily a specimen can be magnetised.
Explanation. When a magnetic substance is placed in a magnetic field, then a number of tiny dipoles of the magnetic substance start orienting in the direction of the field and thus contribute towards intensity of magnetisation.
The intensity of magnetisation is proportional to the strength of the external field H.
M µ H
or M = Xm H
Where, Xm =M/H is the proportionality constant called magnetic susceptibility.