Half-metallic ferromagnetism in rocksalt NaN and KN from first principles

Abstract

The first-principles full-potential linearized augmented plane wave method based on the density functional theory is used to study the structural and magnetic properties of alkali-metal nitrides (LiN, NaN and KN) in wrutzite, rocksalt, zinc blende, CsCl-, and NiAs-type structures. The results show that LiN prefer ground-state with CsCl structure and behave as a paramagnet. In contrast, the ground state phases of NaN and KN are rocksalt structure, and both of them exhibit excellent half-metallic ferromagnets with magnetic moment of 2μB per formula unit. It is found that the majority of the magnetic moments of both compounds originate from the nitrogen sites since the p states of nitrogen are spin polarized. The half-metallic ferromagnetism is found to be rather robust and the moments remain integer with the lattice constants varying in a wide range. The calculated electronic structures characterized by a flat band show that both NaN and KN have large half-metallic gaps (up to 1.83eV). We argue that the flat band is essential for the origin of the half-metallic ferromagnetism of these materials. The Curie temperature for rocksalt-type NaN and KN are estimated and are predicted to be higher than room temperature. Furthermore, the calculated formation energies for both compounds indicate that they are stable once they are synthesized. Wide half-metallic gaps, high Curie temperature, and the absence of the transition-metal ions make rocksalt NaN and KN attractive as materials for possible spintronic devices.