  BaMn2As2 is unique among BaT2As2 compounds crystallizing in the
body-centered-tetragonal ThCr2Si2 structure, which contain stacked square
lattices of 3d transition metal T atoms, since it has an insulating
large-moment (3.9 muB/Mn) G-type (checkerboard) antiferromagnetic AF ground
state. We report measurements of the anisotropic magnetic susceptibility chi
versus temperature T from 300 to 1000 K of single crystals of BaMn2As2, and
magnetic inelastic neutron scattering measurements at 8 K and 75As NMR
measurements from 4 to 300 K of polycrystalline samples. The Neel temperature
determined from the chi(T) measurements is TN = 618(3) K. The measurements are
analyzed using the J1-J2-Jc Heisenberg model. Linear spin wave theory for
G-type AF ordering and classical and quantum Monte Carlo simulations and
molecular field theory calculations of chi(T) and of the magnetic heat capacity
Cmag(T) are presented versus J1, J2 and Jc. We also obtain band theoretical
estimates of the exchange couplings in BaMn2As2. From analyses of our chi(T),
NMR, neutron scattering, and previously published heat capacity data for
BaMn2As2 on the basis of the above theories for the J1-J2-Jc Heisenberg model
and our band-theoretical results, our best estimates of the exchange constants
in BaMn2As2 are J1 = 13 meV, J2/J1 = 0.3 and Jc/J1 = 0.1, which are all
antiferromagnetic. From our classical Monte Carlo simulations of the G-type AF
ordering transition, these exchange parameters predict TN = 640 K for spin S =
5/2, in close agreement with experiment. Using spin wave theory, we also
utilize these exchange constants to estimate the suppression of the ordered
moment due to quantum fluctuations for comparison with the observed value and
again obtain S = 5/2 for the Mn spin.
