Density Functional Studies of the Functional Magnetic Materials

Abstract

Spin polarized calculations based on FP-(L)APW+lo method were performed on non-magnetic, ferromagnetic and antiferromagnetic phase of Bi2Fe4O9. Antiferromagnetic phase was energetically favourable. All exchange correlation functionals accurately describe structural properties of Bi2Fe4O9 but failed to calculate correct band structure. All exchange correlation functionals with U gave bandgap values (2.05 - 2.2eV) closer to experimental values (1.9 - 2.1 eV). The calculated DOS showed hybridization between Fe 3d and O 2p states along with minor overlap between Bi 6p and O 2p states in upper valance band. Lower conduction band was composed of Fe 3d states. Magnetic properties of Bi2Fe4O9 were due to Fe3+ ions having magnetic moment ~4μB. Induced magnetic moments at other atomic sites depend on their local environment. Charge density plots reveal slight covalent character and strong ionic character between Fe-O atoms and Bi-O atoms. In absorption spectrum peak at 1.5 eV was attributed to d-d transition of Fe3+ ions. While peak at 2.05 eV was ascribed to transitions from O 2p valence band to conduction band Fe 3d levels. Static dielectric constant and refractive index were 17.61 and 4.4 respectively. Reflectivity has peak at 15.75 and 18.37 eV (λ = 78.66 and 67.44 nm). Peak in energy loss spectrum at 21.99 eV corresponds to abrupt decrease of reflectivity. Spin polarized calculations of Fe doped ferromagnetic EuO (Eu0.75Fe0.25O, Eu0.50Fe0.50O and Eu0.25Fe0.75O) using mBJ+U method show that lattice constant linearly decrease with increasing doping concentration of Fe atoms. Semiconductor EuO shows metallic character when doped with Fe atoms. Doped EuO exhibit extra peaks of Fe atom 3d states between -10 to 3.5 eV in valence band. Decrease in bandgap was due to increased spd exchange interaction between band electrons and localized 3d electrons of transition atom. Effect of doping was found to be dependent on the location of dopant ions. Electronic and magnetic properties of ferromagnetic EuFeO3 were calculated using mBJ+U exchange correlation functional that better describe energy bandgap values of strongly correlated 4f system. The calculated bandgap 2.7 eV agrees with experimental value. EuFeO3 was found to be charge transfer insulator due to existence of bandgap between O 2p and Eu 4f states.