Site-specific local structure of Mn in artificial manganese ferrite films

Diffraction anomalous fine structure (DAFS) spectroscopy has been applied to resolve site-specific Mn local structure in manganese ferrite films grown under nonequilibrium conditions. The DAFS spectra were measured at a number of Bragg reflections in the vicinity of the Mn absorption $K$ edge. The DAFS data analysis done with an iterative Kramers-Krönig algorithm made it possible to solve separately the local structure around crystallographically inequivalent Mn sites in the unit cell with nominal octahedral and tetrahedral coordination. The strong preference for Mn to be tetrahedrally coordinated in this compound is not only manifested in the relative site occupancies but also in a strong reduction in coordination number for Mn ions at nominal octahedral sites.

Figure 1

Fluorescence EXAFS spectrum from $\mathrm{Mn}{\mathrm{Fe}}_2{\mathrm{O}}_4$ artificial film normalized using tabulated absorption cross sections to provide the proper ratio of resonant and nonresonant contributions to the absorption coefficient. These data were used for absorption correction in the refinement of DAFS spectra as discussed in the text.

Figure 2

Raw DAFS spectra of $\mathrm{Mn}{\mathrm{Fe}}_2{\mathrm{O}}_4$ film obtained from Bragg reflections probing different site contributions: (a) purely A-site contribution measured at (422) reflection, (b) purely B-site contribution taken at (222) reflection, and (c) mixed contribution from both A and B sites measured at (111) reflections. The mixing ratio is discussed in the text.

Figure 3

(a) $f^{″}\left(E\right)E\propto \mu \left(E\right)$ for Mn atoms located at A and B sites obtained from DAFS data refinement and (b) the magnitude of its complex Fourier transform. (c) $f^{″}\left(E\right)E\propto \mu \left(E\right)$ for A sites calculated in two independent ways: directly from (422) DAFS spectrum (line) and by combining refinement results for (111) and (222) reflections (dashed line), and (d) the magnitude of the corresponding complex Fourier transform.

Figure 4

(a) Magnitude of the complex Fourier transform of refined absorption data and its fit with FEFF6 theory for Mn absorbing atoms located at A sites. Vertical lines indicate fitting range. (b) Back Fourier transform of the fitted regions of $R$ space and corresponding fit for Mn absorbing atoms located at A sites.

Figure 5

(a) Magnitude of the complex Fourier transform of refined absorption data and its fit with FEFF6 theory for Mn absorbing atoms located at B sites. Vertical lines indicate fitting range. (b) Back Fourier transform of the fitted regions of $R$ space and corresponding fit for Mn absorbing atoms located at B sites.

Figure 6

(Color online) (a) Magnitude of the complex Fourier transform of experimental fluorescence EXAFS data and its fit with FEFF6 theory and site-specific structural parameters taken from DAFS analysis. The electron amplitude reduction factor obtained from the fit is $S_0^2=0.76$. (b) Refined site-specific absorption spectra together with experimental fluorescence EXAFS spectrum (points), reduced to photoelectron wave-vector space. In the solid line is shown the average of the site-specific spectra taken with corresponding site occupancies of 0.82 for the A site and 0.18 for the B site. The curves are shifted in the vertical scale for convenience.