The magnetic susceptibility is introduced as a thermodynamic quantity
which requires a partial differentiation of the magnetization accordin
g to the applied field. This differential (isothermal) magnetic suscep
tibility differs from the frequent definition of the mean magnetic sus
ceptibility if the behaviour the magnetic material is non-linear (gene
rally it is at low temperature and high magnetic fields). (C) 1998 Els
evier Science S.A.