Thermodynamics, Statistical Physics, and Quantum Mechanics

THERMODYNAMICS AND STATISTICAL PHYSICS^199

For J = 1,

c) For largeHthe magnetization saturates

It is convenientto define theso-calledBrillouinfunction [Brillouin,

Journal de Physique 8 , 74 (1927)] in such a way that

So,

For smallH, we can expandcoth

So,

The saturationvalue(S.4.81.10) corresponds to aclassicaldipole per
atom, where all the dipoles are aligned along the direction of H, whereas the
value at small magnetic field H (S.4.81.11) reflects a competition between
order(H)and disorder