Magnetic order in insulators Weiss molecular-field(Mean-field) theory(1907 ∑Ss=∑S)+∑SS)-∑<S〉 =2ZM)S2-ZNM where M= s)is the order parameter H=yM is the molecular field r=2zls(s+)is the molecular field constant• Weiss Molecular-field (Mean-field) theory (1907) Magnetic order in insulators