Molecular Oxygen-Induced Ferromagnetism and Half-Metallicity in α-BaNaO4: A First-Principles Study

Abstract

Molecule oxygen resembles 3d and 4f metals in exhibiting long-range spin ordering and electron strong correlated behaviors in compounds. The ferromagnetic spin ordering and half-metallicity, however, are quite elusive and have not been well acknowledged. In this article, we address this issue to study how spins will interact each other if the oxygen dimers are arranged in a different way from that in the known super- and per-oxides by first principles calculations. Based on the results of structure search, thermodynamic study and lattice dynamics, we show that tetragonal α-BaNaO₄ is a stable half-metal with a Curie temperature at 120 K, a first example in this class of compounds. Like 3d and 4f metals, the O₂ dimer carries a local magnetic moment 0.5 μ_B due to the unpaired electrons in its π^* orbitals. This compound can be regarded as forming from the O₂ dimer layers stacking in a head to head way. Different from that in AO₂ (A=K, Rb, Cs), the spins are both ferromagnetically coupled within and between the layers. Spin polarization occurs in π^* orbitals with spin-up electrons fully occupying the valence band and spin-down electrons partially the conduction band, forming the semiconducting and metallic channels, respectively. Our results highlight the importance of geometric arrangement of O₂ dimers in inducing ferromagnetism and other novel properties in O₂ dimer containing compounds.