Influence of environment and grain size on magnetic properties of nanocrystalline Mn–Zn ferrite

Nanocrystalline Mn_1−xZn_xFe₂O₄ (0.2?x?0.9) was prepared by mechanical alloying of the concerned oxide precursors and subsequent annealing in air and Ar atmosphere, respectively. Milling and annealing in air produces Zn-ferrites (ZnFe₂O₄) instead of Mn–Zn ferrites as MnO converts to higher oxides at higher oxygen partial pressure and fails to dissolve in the spinel phase. This is confirmed by careful quantitative X-ray diffraction analysis using Rietvelt profile matching and also by the non-saturating paramagnetic nature of the magnetization response with very low saturation level of these spinels milled and annealed in air. On the other hand, single-phase Mn–Zn ferrite results from the identical precursor oxide blend when milling and annealing are carried out under controlled (Ar) atmosphere. The average grain size of the as-milled and annealed powders, measured by Rietvelt refinement, varies between 6–8 and 14–18 nm, respectively. Further investigations performed with Mn_0.6Zn_0.4Fe₂O₄ reveal that a careful selection of annealing parameters may lead to an early superparamagnetic relaxation. Therefore, the blocking temperature can be significantly reduced through proper heat treatment schedule to ensure superparamagnetism and negligible hysteresis at low temperature.