Curie law: χ = C/T, where C = nμ₀g²μ_B²J(J+1)/3k_B. The Brillouin function B_J(x) = (2J+1)/(2J) coth((2J+1)x/2J) − 1/(2J)coth(x/2J) describes magnetization at arbitrary field and temperature. For J=1/2, B_J reduces to tanh. At high T/low B: linear in B/T (Curie regime). At low T/high B: saturates at M = ngJμ_B.