Veeturi Srinivas

Fine particles of Mn 0.6 Zn 0.4 Fe 2 O 4 (MZF) are prepared by sol-gel method and investigated the structural and magnetic properties by coating the particles with passive shell. Among different samples coated on MZF we have observed in ZnO coated sample the fraction of spinel phase is increased by diminishing Fe 2 O 3 phase compared to the all other samples. The saturation Magnetization of the MZF sample is 2.67 emu/g decreased to 1.87emu/g for the sample coated with ZnO and annealed at 600 °C. From the present investigation we conclude that the reduction of impurity phase is due surface layer reaction that results in formation of new impurity phase.

A comparative study of structural and magnetic properties of MnZn spinel ferrite (SF) and ZnO coated MnZn ferrite (ZF) nanoparticles (NPs) has been carried out. The as-prepared NPs show a single phase cubic spinel structure, with lattice parameter ~8.432 Å. However, α-Fe2O3 impurity phase emerge from SF particles when subjected to annealing at 600 °C in air. The weight fraction of α-Fe2O3 phase increases with increasing Mn concentration (9% for x=0.2 and 53% for x=0.6). On the other hand in ZF (x=0.2 and 0.4) NPs no trace of impurity phase is observed when annealed at 600 °C. The magnetic measurements as a function of field and temperature revealed superparamagnetic like behavior with cluster moment ~104 μB in as-prepared particles. The cluster size obtained from the magnetic data corroborates well with that estimated from structural analysis. Present results on ZnO coated MnZn ferrite particles suggest that an interfacial (ZnO@SF) reaction takes place during annealing, which results in formation of Zn-rich ferrite phase in the interface region. This leads to deterioration of magnetic properties even in the absence of α-Fe2O3 impurity phase.

MnZn ferrite nanoparticles (NPs) with MnxZn1-xFe2O4 ( x=0 and 0.6) compositions have been synthesized by sol-gel process. As prepared (AP) particles were annealed at 1200 °C in air to compare the physical properties of NPs with their bulk counterparts. The crystallite sizes of AP samples were estimated to be 18 nm for x=0 and 11 nm for x=0.6. The cationic disorder was evaluated from the Rietveld analysis of X-ray diffraction (XRD) data, which decreases with increasing annealing temperature. The average particle size of the AP samples is observed to be below 20 nm and transform into single-phase spinel structure of ∼ 500 nm long cylindrical rods of 60 nm-diameter for x=0.6 , when annealed at 1200 °C in air. The magnetic properties of AP NPs show ferrimagnetic behavior below the blocking temperature ( T) and superparamagnetic behavior above the T. The observed magnetic characteristics corroborate well with the structural parameters obtained from the Rietveld analysis of the XRD data.

We report a detailed study of the structural and magnetic properties of Ni80Cr20 fine particles prepared by physical methods, such as mechanical alloying, wire explosion process (WEP), and chemical methods like sodium borohydride reduction, polyol method. From the analysis of structural and microstructural data, phase formation with the desired composition is confirmed in the particles prepared by the WEP. A comparative study of magnetization data of bulk and nanoparticles of Ni80Cr20 suggests that a weak magnetic moment develops in nanoparticles due to finite-size effects. Analysis of magnetization data suggests a superparamagnetic (SPM) behavior in the smallest sized (34 nm) particles prepared by WEP, while nanoparticles prepared by other methods show ferromagnetic ground state. Further, it is shown that the origin of weak magnetic moment and development of SPM state in the nanoparticles is due to uncompensated surface spins with a negligible interaction effect.