New method to calculate Moessbauer recoilless f-factors in NiFe_2O_4. Magnetic, morphological and structural properties

摘要

Mossbauer spectroscopy has long been used to calculate the degree of inversion (lambda) in ferrites. To perform this calculation in NiFe2O4, it is assumed that the recoilless f-factors ratio of the Fe3+ cations at octahedral [B] and tetrahedral (A) sites of the spinel structure, f(FeB3+)/f(FeA3+), is equal to 1 at very low temperatures and equal to 0.94 at room temperature (RT). However, these values were reported for ideal Fe3O4 but not for NiFe2O4, and the physical properties of both materials are very different. In this work, we use a simple method and propose equations for directly determining the f(FeA3+) and f(FeB3+) values at RT relative to the recoilless f-factor of standard metallic iron powder (f(Fe)). NiFe2O4 was synthesized from a stoichiometric mixture of nickel oxide and hematite by combining milling processes and heat treatments. The sample was characterized using X-ray diffraction, Mossbauer spectroscopy, infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and magnetic measurements. The results of all the characterization techniques confirm the synthesis of NiFe2O4 and the formation of the spinel structure. The lattice parameter of the cubic unit cell of NiFe2O4 was calculated using a crystallographic model and the Rietveld refinement. By comparing the lattice parameters, it was possible to determine the degree of inversion (lambda = 0.96) of the spinel structure of NiFe2O4, and it could be established that the sample obtained has a mixed spinel structure of the type (Ni0.042+Fe0.963+)(A)[Ni0.962+Fe1.043+](B)O-4(2-). Finally, a value of f(FeB3+)/f(FeA3+) = 1.09 +/- 0.01 was calculated for NiFe2O4 at RT.