Add AFM exchange coupling energy term to the SB model

[LemurPwned]

$$E_\mathrm{AFM/FM} = -\mu_0M_st_\mathrm{FM}(H_\mathrm{EB} + H_\mathrm{w})\cos\phi_M$$
or from Meiklejohn-Bean model.

W. H. Meiklejohn and C. P. Bean. New Magnetic Anisotropy. Physical Review, 102(5):1413–1414, June
1956. Publisher: American Physical Society.

E. C. Stoner and E. P. Wohlfarth. Interpretation of High Coercivity in Ferromagnetic Materials. Na-
ture, 160(4071):650–651, November 1947. Number: 4071 Publisher: Nature Publishing Group.