Competition between Direct and Indirect Exchange Couplings in MnFeAs: A First-Principles Investigation

Abstract:

The electronic and magnetic structures of the tetragonal and hexagonal MnFeAs were examined using density functional theory to understand the reported magnetic orderings and structural change induced by high-pressure synthesis. The reported magnetic ground states were confirmed using VASP total energy calculations. Effective exchange parameters for metal−metal contacts obtained from SPRKKR calculations indicate indirect exchange couplings are dominant in tetragonal MnFeAs. Weak direct exchange couplings for adjacent Fe−Fe and Fe−Mn contacts cause the coexistence of several low-energy magnetic structures in tetragonal MnFeAs and result in a near zero magnetic moment on the Fe atoms. On the other hand, the nearest-neighbor Fe−Fe and Fe−Mn interactions in hexagonal MnFeAs are a combination of direct and indirect exchange couplings. In addition, indirect exchange couplings in tetragonal MnFeAs are rationalized by both RKKY and superexchange mechanisms. Finally, to probe the high-pressure-induced phase transition, total energy changes with the change of volume was studied on both tetragonal and hexagonal MnFeAs.

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