In this paper we have investigated the effect of nanometric grain size modulation on the behavior of magnetoimpedance (MI), magnetoresistance, and magnetic properties of a series of single-phase nanocrystalline colossal magnetoresistance La0.7Sr0.3MnO3 (LSMO) manganites all synthesized through chemical route “pyrophoric reaction process.” MI measurements were carried out at room temperature (T=300K) in the ac signal frequency (f) range of 40kHz-110MHz and in dc magnetic field (H) range of −2.5kOe<H<+2.5kOe, respectively. The experimental results show that MI% increases with the increase in nanometric grain size (ϕ) for this series of samples. This behavior can be explained in the light of classical electrodynamics, which relates this effect with the dependency of skin depth on external magnetic field and applied ac signal frequency. Interestingly, MI of about 92% was obtained for the 28nm LSMO sample at a frequency (f) of 10MHz and at H=2.5kOe. The same sample also showed a maximum MI sensitivity of about 1%/Oe near 60Oe dc magnetic field at f=10MHz. Magnetization measurements were carried out in the applied magnetic field range of −5kOe<H<+5kOe and the experimental results reveal that spontaneous magnetization decreases with decrease in particle size. These nanometric LSMO samples also showed a paramagnetic-ferromagnetic transition near TC=360K. For comparison, we have also presented room temperature low-field magnetoresistance (LFMR) study of this series of LSMO nanoparticles. This study reveals that within this particle size range, 14–28nm, LFMR decreases with decrease in particle size, which in fact mimics the variation of MI with particle size.

We have performed magnetic and transport measurements on La0.67Ca0.33MnO3 polycrystalline and partially melted samples. The magnetization and resistance decrease as sintering temperature is increased. The insulator-to-metal transition temperature is remarkably enhanced in the partially melted sample, enabling its possible applications at room temperature. The microscopic studies in melt samples show an excellent connectivity between grains, suggesting the enhancement of percolative transport by opening new conduction channels and the disappearance of magnetic phase boundary at elevated temperature by the ordering of Mn spins blocked at the grain boundary of sintered samples.

We report significant enhancement of magnetoresistance (MR) in melt-processed La0.67Ca0.33MnO3 (LCM) and in Ho-doped LCM samples. The LCM system exhibits surprising enhancement of metal–insulator transition (TIM) that coincides with the Curie transition temperature (Tc), TIM≈Tc, illustrating the enhanced percolative transport and spin-polarization through grain boundaries. Surprisingly, 1 mol % of addition of Ho enhances Tc of ∼70 K with remarkable colossal MR. The nanoscale Ho distribution is consistent with the magnetic inhomogeneity-induced MR due to phase segregation in Ho-doped sample whereas charge-segregation picture is valid for undoped LCM.

The effects of nanometric grain size (ϕ=14–28nm) modulation on the behavior of magnetoimpedance (MI), magnetoresistance, and magnetic properties of chemically synthesized a series of single-phase soft ferromagnetic nanocrystalline colossal magnetoresistance La0.7Sr0.3MnO3 (LSMO) manganites at room temperature have been reported. From the frequency and magnetic filed dependent MI measurement it is found that the MI percent increases with the increase in nanometric grain size (ϕ) for this series of samples. This behavior has been explained in the light of classical electrodynamics, which relates this effect with the dependency of skin depth on external magnetic field and applied ac signal frequency. Room temperature low field magnetoresistance (LFMR) study of this series of LSMO nanoparticles has also been carried out for comparison.

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