Structural, microstructural, magnetic and hyperfine characterization of nanosized Ni0.5Zn0.5Fe2O4 synthesized by high energy ball-milling method

The XRD, HRTEM, SQUID magnetometer and Mössbauer spectroscopy are applied to follow the structural (by Rietveld refinement method), microstructural and magnetic properties of nanosized (∼15.3 nm) Ni0.5Zn0.5Fe2O4 (MM) synthesized by high energy ball milling method. MM possesses cubic spinel structure of image symmetry. The values of saturation magnetization and coercivity at 15 K are 53.7 emu g−1 and 900 Oe respectively. MM is superparamagnetic in nature and behaves ferrimagnetically below 220 K. The inherent superparamagnetic relaxation has significantly reduced in MM and it exhibits enhancement in magnetization, magnetic ordering temperature, magnetic hyperfine field and coercivity compared to its counterparts prepared by chemical routes. In contrast the magnetic ordering temperature of MM is less than that of one having same stoichiometry of MM prepared under mild milling condition. Thus the magnetic properties of nanometric ferrites strongly depend not only on the synthesis techniques but also the physico-chemical parameters of the synthesis process. Further the particles of MM possess core–shell structure, where a ferrimagnetic core is surrounded by magnetically dead surface layer. The infield Mössbauer study strongly corroborates this. Results are explained within the purview of core–shell model. This leads us to propose that surface spin disorder reduces the magnetic ordering temperature of mechanically milled ferrites.