Magnetic exchange interactions and band gap bowing in NixMg1−xO (0.0 ≤ x ≤ 1.0): A GGA+U density functional study

Abstract

We report variations in magnetic exchange interactions and energy bandgap of the NixMg1−xO (0.0≤x≤1.0) system with a change in x using the first principle density functional theory calculations employing the generalized gradient approximation + Coulomb interaction approach. We consider large supercell fcc structures (2×2×2) for different Ni molar fractions (x). Our results demonstrate that the type-II antiferromagnetic ordering is energetically more favorable for all the compositions, which exhibits a small local moment (about 2μB) due to high-spin divalent Ni. The nearest neighbor (NN) and the next nearest neighbor (NNN) exchange interactions J1 and J2, respectively, are evaluated by considering different Hund's coupling parameters (0≤J≤4) for a constant Columbic interaction (U=6.3eV). Below a critical composition, xC<0.25, NN interaction dominates over NNN resulting in |J2J1|<1 and for moderate compositions (0.25≤x≤0.75), J2 decides the global magnetic ordering of NixMg1−xO, making the system strongly antiferromagnetic for |J2J1|>1. For a higher concentration (x>0.75), the ratio (|J2J1|) remains constant suggesting that both J1 and J2 play important roles. For x=1, calculated values of J1(=1.52meV) and J2(=−17.14meV) are consistent with the experimental results (J1=1.4meV and J2=−19.1meV) obtained from spin-wave dispersion measurements. Our calculations and analysis of the electronic structure reveal the evidence for energy bandgap (Eg) bowing in NixMg1−xO with two different charge transfer transitions: (i) O−2p→Ni−4s/Mg−3s (Eg↓) and (ii) O−2p→Ni−3d−eg states (Eg↑). For x>xc, additional O−2p states are generated near the Fermi level in the valence band (∼−0.87eV), which contributes to the bowing of Eg.