Susceptibility of an Antiferromagnet below

Below 7V, the two sublattices of a collinear antiferromagnet have their magnetic moments aligned antiparallel. Because of the axial 

For H i. Mi, M2 below TV, the sublattice moments are each turned by an angle 0 from the original axis so that the magnitude of the net moment parallel to H is M = (Mi + M2) sin 0 for small 0. 

Formation of the angle 20 between Mi and M2 is resisted by the Weiss field. At equilibrium, the magnitude of the component of the Weiss field that opposes H is equal to H, so that for small 0 

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