Exchange, Heisenberg field, anisotropic

In 2-D and 1-D systems even in non-uniaxial materials, the Neel model can still be used since the anisotropic field in the plane where the moments can rotate is small compared with the exchange field in the perpendicular direction. After first considering the particular case of CsNiF3, we will described several 2-D and 1-D Heisenberg fluorides showing spin-flop behavior. 

The general conclusion which we can draw is that exchange forces in Tb pnictides are very anisotropic, in spite of the fact that the observed magnetization axes correspond to the predictions of the crystal-field theory. These predictions are, as is stated clearly by Trammell (1963), valid only for isotropic Heisenberg-type exchange. 

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