Nickel-Zinc Ferrite from Reverse Micelle Process: Structural and Magnetic Properties, Mossbauer Spectroscopy Characterization

Nickel-zinc ferrite (Ni0.58Zn0.42Fe2O4) nanoparticles with an average crystallite size of about 8.4 nm were synthesized by reverse micelle technique. Bulk sample was prepared by annealing nickel-zinc ferrite (NZFO) nanoparticles at 1473 K. Room temperature Mossbauer spectra of NZFO nanoparticles exhibit collective magnetic excitations, while annealed (bulk) NZFO particles have the ferrimagnetic phase. At 5 K, the broad shape of Mossbauer spectral lines for nanoparticles in comparison to bulk particles provide clear evidence of a wide distribution of magnetic fields acting at the Fe3+ nuclei in the nanoparticles. Bulk NZFO particles and inner core of nanoparticles exhibit a fully inverse spinel structure with a Neel type collinear spin arrangement, whereas the major feature of the ionic and spin configuration in the grain boundary (surface) region are a nonequilibrium cation distribution and a canted spin arrangement. The cation distribution of nano and bulk particles has been studied by using in-field Mossbauer spectroscopy. The dependence of Mossbauer parameters viz isomer shift, quadrupole splitting, line width, and hyperfine magnetic field on bulk and nano samples has been studied. As a consequence of spin canting and site exchange of cations in the surface shell, the NZFO nanoparticles exhibit a reduced nonsaturating magnetization compared to bulk particles. The thickness of the surface shell of about 1.3 nm estimated from Mossbauer measurement is found to be in agreement with that obtained from magnetization measurements. Finite size effects have implications on the temperature dependence of the saturation magnetization. The fit of the saturation magnetization to the Bloch T-3/2 law for nanoparticles yields a Bloch constant larger than the bulk particles. It was found that a better fit is obtained if the exponent of the temperature is in the range of 1.43 to 1.5. The larger value of Bloch constant (b) Suggests the possibility of interactions among the nanoparticles. The dynamic ac susceptibility measurement shows the relaxation time T-0 as 1.77 x 10(-13) s. This value is in good agreement with the theoretical value. Such an agreement is possibly as a result of interparticle interaction in nanoparticle sample.