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 Cu 2 Sb-type 3d metal arsenides (M 2 As, 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 c-axis. These AFM ordering patterns are rationalized by the results of VASP calculations on several magnetically ordered models using a × a × 2c 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 Fe 2 As and Mn 2 As show tetragonal symmetry, but a magnetostrictive tetragonal-to-orthorhombic distortion could occur in Cr 2 As through AFM Cr1-Cr2 coupling between symmetry inequivalent Cr atoms along the a-axis, but FM coupling along the b-axis. A LSDA+U approach is required to achieve magnetic moment values for Mn 2 As 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 Mn 2 As correctly assesses the magnetic ordering patterns in Cr 2 As and Fe 2 As. ABSTRACT: The electronic and magnetic structures of the tetragonal Cu 2 Sb-type 3d metal arsenides (M 2 As, 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 c-axis. These AFM ordering patterns are rationalized by the results of VASP calculations on several magnetically ordered models using a × a × 2c 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 Fe 2 As and Mn 2 As show tetragonal symmetry, but a magnetostrictive tetragonal-to-orthorhombic distortion could occur in Cr 2 As through AFM Cr1−Cr2 coupling between symmetry inequivalent Cr atoms along the a-axis, but FM coupling along the b-axis. A LSDA+U approach is required to achieve magnetic moment values for Mn 2 As in better agree...

Electronic structure studies of complex intermetallic arsenides and borides

“…The similarity of these orderings, as well as the robustness with respect to U of their energetic closeness, suggest that these antiferromagnetic orderings are likely to coexist within the CrMnAs crystal structure at finite temperatures. It is suggestive that the same four magnetic orderings were previously found to be energetically competitive in Cr 2 As[9].The coloring I non-spin-polarized density of states shows a pseudogap at the Fermi energy for U from 1 eV to 3 eV, which is consistent with the values of U that have proven to be useful in manganese oxides. U is an empirical parameter, and similar values of U should not necessarily be optimal for different systems.…”

“…Therefore, the DFT + U method with empirically fitted values of U is often useful for examining the electronic structures of highly correlated electron systems such as those containing rare-earth metals. In a previous computational study on 3d dimetal arsenides by Zhang et al, including a Hubbard U term resulted in better agreement between calculated and experimental magnetic moments for Mn 2 As than were obtained by DFT alone [9]. In manganese oxides, the DFT+U method has proven superior to DFT alone in calculating oxidation [53].…”

“…Valence electron count has continued to be an important chemical feature in the search for other systematic rules for predicting the existence and properties of intermetallic compounds [7]. The rigid band approximation, which assumes that band structure does not vary with small changes in valence electron count as a result of chemical substitution, has proven useful in studying many intermetallic systems [7][8][9][10]. This approximation allows small compositional changes to be modeled as a shift in the Fermi energy relative to a known density of states.…”

Magnetic ordering and metal‐atom site preference in tetragonal CrMnAs: Electronic correlation effects

“…The values of effective U parameters for Mn used in these studies ranged from 1.6 eV to 4 eV . A previous computational study on 3 d di‐metal arsenides by Zhang et al showed that including a Hubbard U ‐term with a value ranging from 1 to 4 eV for Mn in Mn 2 As resulted in better agreement between calculated and experimental local magnetic moments. Also in this study the calculated magnetic ground state switched from antiferromagnetic (the experimentally determined ground state) to ferrimagnetic for U values between 0.5 eV and 2 eV, but then returned to antiferromagnetic for values of U above 2 eV, results which indicate significant sensitivity to the ground state magnetic ordering and the U parameter assigned to Mn.…”

“…According to these diagrams, the direct M1M1 and M1M2 interactions of CrMnAs resemble those of Cr 2 As and not Mn 2 As, whereas the nearest‐neighbor M2M2 interactions resemble those of Mn 2 As and not Cr 2 As. As a result, the indirect M2M2 exchange interactions across the M1 planes are antiferromagnetic in CrMnAs, as in Cr 2 As but not in Mn 2 As.…”