Magnetic Ordering in Tetragonal 3d Metal Arsenides M₂As (M = Cr, Mn, Fe): An Ab Initio Investigation

Abstract

The electronic and magnetic structures of the tetragonal Cu₂Sb-type 3d metal arsenides (M₂As, M = Cr, Mn, Fe) were examined using density functional theory to identify chemical influences on their respective patterns of magnetic order. Each compound adopts a different antiferromagnetic (AFM) ordering of local moments associated with the 3d metal sites, but every one involves a doubled crystallographic <i>c</i>-axis. These AFM ordering patterns are rationalized by the results of VASP calculations on several magnetically ordered models using <i>a</i> × <i>a</i> × 2<i>c</i> supercell. Effective exchange parameters obtained from SPRKKR calculations indicate that both direct and indirect exchange couplings play essential roles in understanding the different magnetic orderings observed. The nature of nearest-neighbor direct exchange couplings, that is, either ferromagnetic (FM) or AFM, were predicted by analysis of the corresponding crystal orbital Hamilton population (COHP) curves obtained by TB-LMTO calculations. Interestingly, the magnetic structures of Fe₂As and Mn₂As show tetragonal symmetry, but a magnetostrictive tetragonal-to-orthorhombic distortion could occur in Cr₂As through AFM Cr1–Cr2 coupling between symmetry inequivalent Cr atoms along the <i>a</i>-axis, but FM coupling along the <i>b</i>-axis. A LSDA+U approach is required to achieve magnetic moment values for Mn₂As in better agreement with experimental values, although computations always predict the moment at the M1 site to be lower than that at the M2 site. Finally, a rigid-band model applied to the calculated DOS curve of Mn₂As correctly assesses the magnetic ordering patterns in Cr₂As and Fe₂As