Mixed hyperfine interactions and defect structure in magnesiowuestite Fe-Mg-O

An analysis of the 4.2 K spectra of FexO (x∝0.91) and (Fe0.4Mg0.6)xO is presented in which it is considered that because of the large electric field gradient at Fe2+ defect sites, the spectra cannot be described by Lorentzian sextets. It is assumed that the magnetic hyperfine field vector is oriented at random in the coordinate system defined by the EFG main axis, and that the EFG coordinate system is also distributed randomly. The simplifying assumption of the asymmetry parameter η=0 allows an analytical formula to be used to describe the complex spectra. Distributions of both magnetic hyperfine field and quadrupole interaction were progressively refined resulting in reasonable fits to the spectra with the main features being well reproduced. The magnetic hyperfine field distribution is rather broad with several features present while distinct values were obtained in the distribution of quadrupole interactions. These latter values are considered to correspond to the defect configurations around the Fe2+ sites. The distribution of hyperfine fields is considered to reflect the varying strengths of superexchange to to the high defect concentration as well as the effects of magnetic relaxation. (orig.)