  A recently-developed molecular field theory (MFT) has been used to fit
single-crystal magnetic susceptibility chi versus temperature T data below the
respective antiferromagnetic ordering temperatures TN for a variety of
collinear and coplanar noncollinear Heisenberg antiferromagnets. The spins in
the system are assumed to interact by Heisenberg exchange and to be identical
and crystallographically equivalent. The fitting parameters for chi(T) of
collinear antiferromagnets are measurable quantities: the Weiss temperature
theta_p in the Curie-Weiss law, TN, chi(TN), and the spin S. For coplanar
noncollinear helix and cycloid structures, an additional fitting parameter is
the turn angle between layers of ferromagnetically-aligned spins. Here MFT fits
to anisotropic chi(T) data from the literature for single crystals of the
collinear antiferromagnets GdCu2Si2 and CuO and the noncollinear
antiferromagnets LiCr2As2 with a 120 degree cycloidal structure and
alpha-CaCr2O4 with a 120 degree helical structure below their respective Neel
temperatures are presented. The MFT fit to the anisotropic chi(T < TN) data for
CuO is poor, whereas the fits to the data for GdCu2Si2, LiCrO2, and
alpha-CaCr2O4 are quite good. The poor fit for CuO is attributed to the
influence of strong quantum fluctuations associated with the small Cu spin and
the quasi-one-dimensional magnetism that are not taken into account by the MFT.
The magnetic contribution to the zero-field heat capacity of the collinear
antiferromagnet GdNiGe3 at T < TN is also fitted by the MFT.
