Spin-wave theory for finite classical magnets and superparamagnetic   relation
  Analytical calculations based on finite-size spin-wave theory and Monte Carlo (MC) simulations are performed to investigate the validity of the well-known relation m(H,T)=M(H,T)B_D[M(H,T)NH/T] between the induced magnetization m of the magnetic particle and its intrinsic magnetization M for the Ising and isotropic classical models [B_D(x) is the Langevin function, D is the number of spin components, N is the number of atoms in the particle]. It follows from general arguments and from our analytical results for the Heisenberg model at T << T_c that this relation is not exact for any finite D and nonzero temperature. Nevertheless, corrections to this formula remain very small practically in the whole range T<T_c if N >> 1, as confirmed by our Monte Carlo calculations. At T <~ T_c/4 there is a good agreement between the   MC and finite-size spin-wave calculations for the field dependence of m and M for the Heisenberg model with free boundary conditions. 
H. Kachkachi and D. A. Garanin