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In this paper we report the structural, magnetic, and transport properties of nanoparticles of Pr0.5Ca0.5MnO3(PCMO). On comparing our results with that of bulk PCMO, we find that there is a likely destabilization of charge ordering in nanoparticles of PCMO. The investigation has been done with particle sizes as small as 15nm synthesized by polyol route. The size reduction (by keeping the chemical composition unchanged) reduces the orthorhombic c axis preferentially and thus reduces the orthorhombic distortion. The size reduction to 15nm enhances the ferromagnetic moment at low temperatures and strongly suppresses the activated charge transport which is seen in the bulk samples of charge ordered PCMO.

A negative-positive-negative switching behavior of magnetoresistance ͑MR͒ with temperature is observed in a ferromagnetic shape memory alloy Ni 1.75 Mn 1.25 Ga. In the austenitic phase between 300 and 120 K, MR is negative due to s-d scattering. Curiously, below 120 K MR is positive, while at still lower temperatures in the martensitic phase, MR is negative again. The positive MR cannot be explained by Lorentz contribution and is related to a magnetic transition. Evidence for this is obtained from ab initio density-functional theory, a decrease in magnetization and resistivity upturn at 120 K. Theory shows that a ferrimagnetic state with antiferromagnetic alignment between the local magnetic moments of the Mn atoms is the energetically favored ground state. In the martensitic phase, there are two competing factors that govern the MR behavior: a dominant negative trend up to the saturation field due to the decrease in electron scattering at twin and domain boundaries and a weaker positive trend due to the ferrimagnetic nature of the magnetic state. MR exhibits a hysteresis between heating and cooling that is related to the first-order nature of the martensitic phase transition.

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Nanoparticles of Ni0.2Zn0.6Cu0.2Fe2O4 were prepared by the standard co-precipitation method. The formation of nanocrystalline mixed spinel phase has been confirmed by x-ray diffractograms. The sizes of the nanoparticles were estimated in the range 7–30 nm, which was confirmed by transmission electron microscopy. Thermal variations of the real part of AC magnetic susceptibilities measured from 450 K down to 80 K and Mössbauer effect measurements at room temperature and down to 20 K clearly indicate the presence of superparamagnetic particles in all the samples. Specific saturation magnetizations measured by VSM are found to increase steadily with the increase of average particle size. The coercive field obtained from low frequency measurements shows that in all the samples a small fraction of particles is not relaxed within the measuring time. For samples showing a less dominating superparamagnetic behaviour, AC magnetic susceptibility data showed the expected increase of blocking temperature with increase in particle size. Magnetic anisotropy energy constants of the nanoparticles were estimated from the blocking temperature and the values cannot be directly correlated with their particle sizes.