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BaMn2As2 is unique among BaT2As2 compounds crystallizing in thebody-centered-tetragonal ThCr2Si2 structure, which contain stacked squarelattices of 3d transition metal T atoms, since it has an insulatinglarge-moment (3.9 muB/Mn) G-type (checkerboard) antiferromagnetic AF groundstate. We report measurements of the anisotropic magnetic susceptibility chiversus temperature T from 300 to 1000 K of single crystals of BaMn2As2, andmagnetic inelastic neutron scattering measurements at 8 K and 75As NMRmeasurements from 4 to 300 K of polycrystalline samples. The Neel temperaturedetermined from the chi(T) measurements is TN = 618(3) K. The measurements areanalyzed using the J1-J2-Jc Heisenberg model. Linear spin wave theory forG-type AF ordering and classical and quantum Monte Carlo simulations andmolecular field theory calculations of chi(T) and of the magnetic heat capacityCmag(T) are presented versus J1, J2 and Jc. We also obtain band theoreticalestimates of the exchange couplings in BaMn2As2. From analyses of our chi(T),NMR, neutron scattering, and previously published heat capacity data forBaMn2As2 on the basis of the above theories for the J1-J2-Jc Heisenberg modeland our band-theoretical results, our best estimates of the exchange constantsin BaMn2As2 are J1 = 13 meV, J2/J1 = 0.3 and Jc/J1 = 0.1, which are allantiferromagnetic. From our classical Monte Carlo simulations of the G-type AFordering transition, these exchange parameters predict TN = 640 K for spin S =5/2, in close agreement with experiment. Using spin wave theory, we alsoutilize these exchange constants to estimate the suppression of the orderedmoment due to quantum fluctuations for comparison with the observed value andagain obtain S = 5/2 for the Mn spin.

Magnetic Exchange Interactions in BaMn2As2: A Case Study of the J1-J2-Jc Heisenberg Model