Mechano-chemical synthesis and characterization of microstructure and magnetic properties of nanocrystalline Mn_1-xZn_xFe₂O₄

Abstract

Single-phase nanocrystalline Mn_1-xZn_xFe₂O₄ powders (x = 0.2, 0.4, 0.6, 0.8 and 0.9) were prepared by mechanical alloying and subsequent annealing in Ar atmosphere. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), magnetization measurements and Mossbauer spectroscopy. The average grain size as determined by Rietvelt refinement method of the XRD patterns varies between 6 and 8 nm for the as-milled powders and increases to 14-18 nm after annealing. A good agreement between the measured lattice parameters and the standard values from the ICDD database was found for all the samples with different stoichiometry. Room temperature Mossbauer spectroscopy reveals a ferromagnetic-to-paramagnetic transition with decrease in Mn content. Any superparamagnetic component was completely absent in the Mossbauer spectra of Mn-rich alloys. The saturation magnetization values of the present nanocrystalline alloys are nearly the same as for identical coarse-grained ferrites. The blocking temperature (T_B) is above room temperature for Mn-rich ferrites. In general, T_B gradually increases with increase in Mn content. Consequently, the coercivity of Mn-rich nanocrystalline Mn-Zn ferrites is significantly higher than that of their coarse-grained counterparts due to blocking of superparamagnetic grains. This high T_B is attributed to the high anisotropy present in the milled ferrite. Annealing at lower temperature decreases T_B and ensures superparamagnetic behaviour at lower temperature.