Magnetic susceptibility of collinear and noncollinear heisenberg antiferromagnets.

Predictions of the anisotropic magnetic susceptibility χ below the antiferromagnetic (AFM) ordering temperatures T(N) of local moment Heisenberg AFMs have been made previously using molecular field theory (MFT) but are very limited in their applicability. Here a MFT calculation of χ(T ≤ T(N)) is presented for a wide variety of collinear and noncollinear Heisenberg AFMs containing identical crystallographically equivalent spins without recourse to magnetic sublattices. The results are expressed in terms of directly measurable experimental parameters and are fitted with no adjustable parameters to experimental χ(T ≤ T(N)) data from the literature for several collinear and noncollinear AFMs. The influence of spin correlations and fluctuations beyond MFT is quantified by the deviation of the theory from the data. The origin of the universal χ(T ≤ T(N)) observed for triangular lattice AFMs exhibiting coplanar noncollinear 120° AFM ordering is clarified.