Below the two sublattices of an antiferromagnet spontaneously magnetise in the same way as a ferromagnet but the net magnetisation is zero due to the opposing orientation of the sublattice's magnetisation. If an external field, , is applied a small net magnetisation can be detected. The resultant magnetisation depends upon the orientation of the field with respect to the magnetisation or spin axis.
If the field is applied parallel to the spin axis the zero-field value of magnetisation of one sublattice, A, is increased by whilst the other, B, is decreased by . The net magnetisation in the direction of the field is then
If the field is applied perpendicular to the spin axis each sublattice magnetisation is turned from the spin axis by a small angle, , as shown in Figure 3.6. The spins reorient by angle given by
Dr John Bland, 15/03/2003