Probing optical band gaps at the nanoscale in NiFe2O4 and CoFe2O4 epitaxial films by high resolution electron energy loss spectroscopy

Abstract

Nanoscale optical band gap variations in epitaxial thin films of two different spinel ferrites, i.e., NiFe2O4 (NFO) and CoFe2O4 (CFO), have been investigated by spatially resolved high resolution electron energy loss spectroscopy. Experimentally, both NFO and CFO show indirect/direct band gaps around 1.52 eV/2.74 and 2.3 eV, and 1.3 eV/2.31 eV, respectively, for the ideal inverse spinel configuration with considerable standard deviation in the band gap values for CFO due to various levels of deviation from the ideal inverse spinel structure. Direct probing of the regions in both the systems with tetrahedral A site cation vacancy, which is distinct from the ideal inverse spinel configuration, shows significantly smaller band gap values. The experimental results are supported by the density functional theory based modified Becke-Johnson exchange correlation potential calculated band gap values for the different cation configurations.

Effect of Bloch wave electron propagation and momentum-resolved signal detection on the quantitative and site-specific electron magnetic chiral dichroism of magnetic spinel oxide thin films

“…According to the experimental results, we observed consistent dichroic signal for NFO from nanoscale probing of different regions while for CFO variation in signal is observed. This is consistent with the earlier observation of robust inverse spinel configuration for NFO and thermodynamically favored cation mixing in the case of CFO [20]. The experimentally observed moments results are supported by first principle based calculations.…”

“…7 (b), C = -5.593±1.5%) and these correspond to A site cation vacancy regions (see Ref. 20,30,39), where in the absence of A site cations, the net moment contributions increases from the B site Fe [20]. Similar observation has been made for g = 440 diffracting condition.…”

“…39,40. The previously performed spatially resolved optical study by HREELS revealed that the films do not show any cation mixing and the stability of the inverse spinel structure is robust [20]. However, some A site cation vacancy regions are observed in this system [41,42].…”

“…So the recent method development by F. Tran and P. Blaha [28,29] to calculate the exact excited state properties, known as the 'modified Becke-Johnson' exchange correlation potential (mBJLDA), has been used to correct the band gap in these systems,. This method is more accurate and comparatively less compute-intensive [20,[30][31][32]. XMCD spectra are calculated on the resultant system by calculating the matrix elements corresponding to photon absorption cross section with respect to the polarization of light and the sample magnetization within the dipole approximation [26].…”

“…In addition to this, the 'bw code' developed for calculating percentage dichroic signal is not capable of providing spectral features necessary for the density of states (DOS)-based evaluation of the spectra [27]. Ferrimagnetic NiFe 2 O 4 and CoFe 2 O 4 inverse spinel structures were considered for calculating the density of states and the resultant total magnetic moment [20]. Spin-polarized and spin-orbit coupled system with orbital polarization are considered to enable the calculation of both spin and orbital moments.…”