Optical properties of ferrimagnetic NiFe$_{2}$O$_{4}$ thin films

Magnetic insulators like NiFe$_{2}$O$_{4 }$are attracting attention due to the high Curie temperature (850 K), which is rare among oxides . We recently demonstrated the growth of high-quality NiFe$_{2}$O$_{4}$ thin films practically down to room temperature, which permits investigation of the optical properties over wide growth temperatures. Our spectroscopic work reveals that NiFe$_{2}$O$_{4}$ displays both direct and indirect band gaps. A plot of ($\alpha $E)$^{2}$ vs. energy places the 300 K direct gaps for the highest quality films at 2.77 and 2.36 eV for the majority and minority channels, respectively whereas a plot of ($\alpha $E)$^{0.5}$ vs. energy places the indirect band gap at 1.64 eV. For the indirect case, we extract a coupling phonon energy of $\sim $50 meV (400 cm$^{-1})$, which corresponds an infrared active O-Fe-O bending mode. The difference between the direct and indirect gap energies reveals an opportunity to obtain spin-polarized carriers via optical excitation. These features have strong overlap with the solar spectrum.