Mechano-chemical synthesis and characterization of microstructure and magnetic properties of nanocrystalline Mn1−xZnxFe2O4

Single-phase nanocrystalline Mn1−xZnxFe2O4 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 Mössbauer 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 Mössbauer spectroscopy reveals a ferromagnetic-to-paramagnetic transition with decrease in Mn content. Any superparamagnetic component was completely absent in the Mössbauer 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 (TB) is above room temperature for Mn-rich ferrites. In general, TB 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 TB is attributed to the high anisotropy present in the milled ferrite. Annealing at lower temperature decreases TB and ensures superparamagnetic behaviour at lower temperature.