Structural and magnetic phase evolution study on needle-shaped nanoparticles of magnesium ferrite

摘要

Nanoparticles of magnesium ferrite (MgFe_2O_4) have been synthesized by chemical co-precipitation route. The microstructure, infrared spectral and magnetic properties have been studied by means of X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, infrared spectroscopy, high field magnetization, low field ac susceptibility and Mossbauer spectroscopic measurements. Transmission electron microscopy (TEM) observation showed that the nanocrystals present a needle-like shape with an aspect ratio of around 3.5 and long axis of 37 nm. The kinetics of needle-shaped nanocrystals formation has been discussed. The Mg~(2+) seems to play key role in governing the rate of growth of growing planes of nanocrystals. The magnetic behaviour is explained by invoking the concept of super-paramagnetism assuming core-shell structure of the nanoparticles. Mossbauer spectral and susceptibility studies showed that the as-prepared nano-sized ferrite is super paramagnetic at room temperature and magnetic ordering evolves on baking and annealing the nanopowdered sample due to increase in the crystalline size. The high temperature annealing transforms the nanostructured ferrite to ordered magnetic structure of ceramic ferrite having long range ferrimagnetic ordering. Infrared (IR) spectral analysis reveals the weak high frequency shoulder due to disparity in the masses of cations present at tetrahedral sites, while splitting of low and high frequency absorption bands has been explained on the basis of Jahn-Teller effect in Fe~(2+)-ions which lead to a non-cubic component of the crystal field potential.