Veeturi Srinivas

The effect of heat-treatment temperature on evolution of microstructures, mechanical and soft magnetic properties and magnetoimpedance (MI) effect in rapidly solidified Fe68.5Si18.5Cu1Nb3B 9 ribbons, has been investigated. The as-quenched ribbons were subjected to heat-treatment at different temperatures between 400 and 600°C for 1 h under high vacuum. Detailed structural studies on the ribbons heat-treated at and above 525°C revealed the presence of nanocrystalline Fe3Si phases embedded in a residual amorphous matrix. The ribbon heat-treated at 550°C temperature exhibits maximum ductility, maximum relative permeability of 4.8 × 104, minimum coercivity of 0.1 Oe, and maximum MI value of 62%. The enhanced MI effect is believed to be related to the magnetic softening of 550°C heat-treated ribbons. However, the magnetic properties and MI effect deteriorated in the samples heat-treated above 550°C due to the coarsening of grain sizes. The soft magnetic behavior of the nanocrystalline ribbons are discussed in the light of random anisotropy model, whereas the MI effect is discussed through standard skin effect in electrodynamics. © 2013 Elsevier B.V. All rights reserved.

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.

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.

Here, we report microstructure and magnetic properties of MnxZn1-xFe2O4 (x = 0–1) nanoparticles synthesized by sol-gel method. From microstructure studies, it is confirmed that all compositions of the present study exhibited a pure cubic spinel ferrite phase with narrow particle size distribution with an average particle size between 13 and 20 nm. From the temperature and magnetic field dependence magnetization data, a superparamagnetic (SPM)-like behavior is seen with a large magnetic moment (~104 μB) in particles. From the analysis of the difference in field-cooled and zero-field-cooled magnetization data, which presented a distribution of blocking temperatures which in turn supported the microstructural data. Further, the analysis of the thermoremanence magnetization data reveals the activation resulting from multiple anisotropy energy barriers due to the size distribution of the particles. The results obtained from the memory effect studies on temperature dependent magnetization in field-cooled and zero-field-cooled protocols confirm the SPM behavior of the particles.