Estimation of structural composition of the inverse spinel ferrites using 57Fe-Zero Field Nuclear Magnetic Resonance

We have demonstrated 57Fe Zero Field Nuclear Magnetic Resonance (ZFNMR) as a powerful tool in determining the structural composition of nickel-cadmium spinel ferrites of various compositions of Ni1-x Cdx Fe2O4 from x = 0 to1, which are synthesized via one-step auto combustion technique. The XRD measurements confirm the phase purity of all the samples. Vibrating Sample Magnetometry (VSM) measurements show that saturation magnetization (MS) increases initially (up to x = 0.3) and then decreases for higher concentrations of cadmium. The Fe3+ ions in the inverted spinel ferrite distribute equally among two possible sites (tetrahedral A and octahedral B) with different hyperfine fields. Therefore for x = 0 under the assumption that Ni enters B sites, 57Fe NMR of Fe3+ ions yield two signals of equal integral intensities in spectral lines corresponding to these sites. Thus, for the sample series Ni1-x Cdx Fe2O4, the contribution of Fe3+ nuclei varies for A and B sub-spectra with the substitution of a non-magnetic Cd2+ ion. By measuring the Fe3+ distribution on A and B sites which is determined from relative spectral areas of A and B NMR sub-spectra the cation distribution is estimated and has been verified by the binomial distribution. Further, XRD Rietveld refinement results are also in good agreement with the composition estimated by NMR technique and the ideal composition. We have demonstrated the usefulness of NMR technique to quantify the accurate composition of the mixed spinel ferrite systems using Ni-Cd ferrite as a test case. Further, the estimated inversion parameter (at around x = 0.4), for the studied system, obtained from ZFNMR, XRD, and VSM techniques are in excellent agreement with each other.