Magnetic exchange interactions in BaMn2As2: A case study of the J1-J2-Jc Heisenberg model

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 mu B/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 As-75 nuclear magnetic resonance (NMR) measurements from 4 to 300 K of polycrystalline samples. The Neel temperature determined from the chi(T) measurements is T-N = 618(3) K. The measurements are analyzed using the J(1)-J(2)-J(c) Heisenberg model for the stacked square lattice, where J(1) and J(2) are, respectively, the nearest-neighbor (NN) and next-nearest-neighbor intraplane exchange interactions and J(c) is the NN interplane interaction. 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 C-mag(T) are presented versus J(1), J(2), and J(c). 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 J(1)-J(2)-J(c) Heisenberg model and our band-theoretical results, our best estimates of the exchange constants in BaMn2As2 are J(1) approximate to 13 meV, J(2)/J(1) approximate to 0.3, and J(c)/J(1) approximate to 0.1, which are all antiferromagnetic. From our classical Monte Carlo simulations of the G-type AF ordering transition, these exchange parameters predict T-N approximate to 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.