Strongly bound Wannier-Mott exciton in pristine (LaO)MnAs and origin of ferrimagnetism in F-doped (LaO)MnAs

We study the electronic, magnetic, and optical properties of (LaO1-xFx)MnAs (x = 0, 0.0625, 0.125, 0.25) systems, calculated using the generalized gradient approximation (GGA) corrected by Hubbard energy (U) = 1 eV. For x = 0, this system shows equal bandgap (E-g) values for spin-up and spin-down of 0.826 eV, with antiferromagnetic (AFM) properties and local magnetic moment in the Mn site of 3.86 mu(B) per Mn. By doping F with x = 0.0625, the spin-up and spin-down E-g values decrease to 0.778 and 0.798 eV, respectively. This system, along with antiferromagnetic properties, also has a local magnetic moment in the Mn site of 3.83 mu(B) per Mn. Increasing doping F to x = 0.125 induces increases of E-g to 0.827 and 0.839 eV for spin-up and spin-down. However, the AFM remains, where mu(Mn) slightly decreases to 3.81 mu(B) per Mn. Furthermore, the excess electron from the F ion induces the Fermi level to move toward the conduction band and changes the bandgap type from indirect bandgap (Gamma -> M) to direct bandgap (Gamma -> Gamma). Increasing x to 25% induces the decrease of spin-up and spin-down E-g to 0.488 and 0.465 eV, respectively. This system shows that the AFM changes to ferrimagnetism (FIM) for x = 25%, with a total magnetic moment of 0.78 mu(B) per cell, which is mostly contributed by Mn 3d and As 4p local magnetic moments. The change from AFM to FIM behavior results from competition between superexchange AFM ordering and Stoner's exchange ferromagnetic ordering. Pristine (LaO)MnAs exhibits high excitonic binding energy (similar to 146.5 meV) due to a flat band structure. Our study shows that doping F in the (LaO)MnAs system significantly modifies the electronic, magnetic, and optical properties for novel advanced device applications.