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LCMP, Department of Condensed Matter Physics & Material Sciences S.N. Bose National Centre for Basic Sciences, Salt Lake, Kolkata 700 098, India The present work reports the synthesis of polyvinylchloride encapsulated superparamagnetic magnetite micro-agglomerates. These were characterized with scanning as well as transmission electron micrographs, and X-ray diffraction. Magneto-rheological fluids were produced by dispersing these particles in various concentrations in castor oil, which is the carrier fluid here. The viscoelastic characterization of these fluids were made at room temperature, under different shear rates, both with and without magnetic field. The behaviour of the fluids was modelled as a Herschel-Bulkley fluid. Overall, these fluids have been found to have desirable and usable properties. [ABSTRACT FROM AUTHOR]

Ferromagnetic shape memory alloys (FSMAs) have been in active research nowadays for their unique properties of responses to both temperature changes and external magnetic fields. Recently, one such material, a CoNiAl based system, showed a photoinduced microactuation (PIMA) effect in which an incident spot of a tiny laser beam could induce actuation in these systems. This is unparalleled in ordinary materials, and its real cause is yet to be found. However, we tried to still use it in real world engineering applications, and for that, the limitations on its usability should be found. With this idea, we studied here in detail the effect of continuous degradation of the PIMA effect on a ribbon sample through TEM, as it was exposed to higher temperatures from room temperature. While the as-spun alloy had microstructured B2 ordered (Ni, Co)-Al micrometer sized grains embedded in a continuous L12 ordered cobalt-rich matrix channel, at 400 °C where the PIMA effect was fully destroyed, Co-rich nanosized precipitates were found to have formed inside the B2 grains, the actual FSMA component in the sample, and these are also found to have been oxidized. Concurrent oxidation studies showed the formation of an irregular shaped composite oxide layer with a thickness of about several hundred nanometers at the top surface. These are possibly the reasons behind the hindrance in the actuating motion. This was corroborated by magnetic studies also. [ABSTRACT FROM AUTHOR]

The magnetoresistance (MR) of Ni2+xMn1-xGa (-1≤x≤0.35) ferromagnetic shape memory alloy shows a large increase in magnitude at room temperature (RT) with increasing x. For Mn2NiGa (x=-1), MR at 8 T is -0.2%, while for Ni2.35Mn0.65Ga (x=0.35), it is -7.3%. Thus, MR of Ni2+xMn1-xGa can be varied over one order of magnitude by changing composition (x). Considering that the Curie temperature (TC) varies with x, the MR behavior in the austenitic phase is explained on the basis of the s-d scattering model. By fitting the MR at 8 T in the austenitic phase for different x and T, a (T/TC)6 power law dependence is obtained. In contrast to the monotonic MR variation with x, the magnetization at RT is highest for Ni2MnGa (x=0) and decreases for both Ni and Mn excess compositions. [ABSTRACT FROM AUTHOR]

We have investigated magnetotransport behaviors of a series of single-phase, nanocrystalline La0.67Sr0.33MnO3 (LSMO) samples having grain sizes in the nanometric regime (14, 22, and 26 nm), all synthesized through chemical route “pyrophoric reaction process.” The motivation behind the present investigation is to study the effects of nanometric grain size on magnetoresistance (MR), specially its temperature and magnetic-field dependences. Magnetoresistance measurements show that in all samples there is a large negative MR at very low fields (LFMR), followed by a slower varying negative MR at comparatively high fields (HFMR), in the ferromagnetic regime. Surprisingly, we observed that at both low- and high-field regimes, the magnitude of MR remains constant up to sufficiently high temperature and then drops sharply with temperature. This temperature-dependent MR behavior gets enhanced with the decrease in particle size. Most interestingly, we found a considerable low-field MR (14%) persisting even at 200 K, which is an appreciable improvement on the results of previous workers. In order to explore the basic physics behind this unusual temperature dependence of MR, we analyzed our data in the light of a phenomenological model [P. Raychaudhuri et al., J. Appl. Phys. 84, 2048 (1998)], based on spin-polarized transport of conduction electrons at the grain boundaries, with major attention being paid to the gradual slippage of domain walls across the grain-boundary pinning centers in an applied magnetic field. Finally, we have attributed this feature of LFMR to the surface magnetization of our nanosize granular LSMO samples, which is crucial for nanodimensional systems. [ABSTRACT FROM AUTHOR]

Large magnetocaloric effect is observed in Ni1.8Pt0.2MnGa close to room temperature. The entropy change shows a crossover from positive to negative sign at the martensite transition. It is negative above 1.6 T and its magnitude increases linearly with magnetic field. An increase in the saturation magnetic moment is observed with Pt doping in Ni2MnGa. Ab initio theoretical calculations show that the increase in magnetic moment with Pt doping in Ni2MnGa is associated with increase in the Mn and Pt local moments in the ferromagnetic ground state. The Curie temperature calculated from the exchange interaction parameters is in good agreement with experiment, showing the absence of any antiferromagnetic correlation due to Pt doping. [ABSTRACT FROM AUTHOR]