arXiv subjects

H. K. Singh

Publications and source records attributed to H. K. Singh.

22 records · Page 2Linked to original sources

arXiv2005

Colossal Magnetoresistance Study in Nanophasic La_0.7Ca_0.3MnO_3 Manganite

In this study we report the effect of sintering temperature on the low field magnetotransport properties of La_0.7Ca_0.3Mn_O3 manganite synthesized through polymeric precursor route. The La_0.7Ca_0.3MnO_3 has been sintered at 600^oC (S6), 700^oC (S7), 800^oC (S8), 900^oC (S9) and 1000^oC (S10). XRD confirms that phase formation starts at 600^oC. All the samples are single phasic having orthorhombic unit cell. The lattice parameters decrease on lowering the sintering temperature (T_S). The crystallite as well as particle size also show strong dependence on the sintering temperature. All the samples possess insulator-metal (T_IM) as well as paramagnetic-ferromagnetic (T_C) transitions. The T_C shows a small variation [274K (S10) to 256K (S6)] as a function of sintering temperature whereas the T_IM goes down from 267K (S10) to 138K (S6), a strong decrease of 129K. This T_C-T_IM discrepancy is due to the fact that whereas the former is an intrinsic characteristic, the latter depends strongly on the extrinsic factors e.g. synthesis conditions grain boundaries and associated disorders. For all the samples MR shows strong dependence on T_S. The MR increases on lowering the temperature as well as on increasing the field with the occurrence of an intrinsic contribution around T_C. These variations of MR for all the samples have been explained in terms of the microstructural variations and spin-polarized tunneling at low temperatures.

cond-mat.mtrl-sci↗

arXiv2005

Low field magneto-transport in La_0.7Ca_0.3MnO_3-PMMA composites synthesized by polymeric precursor route

A detailed investigation of the effect of PMMA on the structure, microstructure and magneto-transport properties of manganite La_0.7Ca_0.3MnO_3 (LCMO) is presented. LCMO-PMMA nanostructured composites have been synthesized by a unique polymeric sol-gel route, which leads to improved solubility of PMMA in the LCMO matrix. The LCMO phase is grown in the presence of varying PMMA concentration at ~500 ^0 C. This route yields single phase material and the grain size is observed to decrease slightly with increasing PMMA concentration. On increasing the PMMA concentration, T_C undergoes a small decrease, resistivity is observed to increase by two orders of magnitude, with a concomitant large decrease in T_IM, e.g., from 218 K for virgin LCMO to 108 K for 50 wt% PMMA admixed LCMO. Low field magneto-resistance measured in the temperature range 77-300 K shows considerable enhancement as a function of the PMMA concentration. These phenomena are explained by taking into account the increased intergranular disorder as a consequence of PMMA admixture.

cond-mat.mtrl-sci↗

arXiv2005

Effect of Strain Relaxation on Magnetotransport properties of epitaxial La_0.7Ca_0.3MnO_3 films

In this paper, we have studied the effect of strain relaxation on magneto-transport properties of La_0.7Ca_0.3MnO_3 epitaxial films (200 nm thick), which were deposited by pulsed laser deposition technique under identical conditions. All the films are epitaxial and have cubic unit cell. The amount of strain relaxation has been varied by taking three different single crystal substrates of SrTiO_3, LaAlO_3 and MgO. It has been found that for thicker films the strain gets relaxed and produces variable amount of disorder depending on the strength of strain relaxation. The magnitude of lattice relaxation has been found to be 0.384, 3.057 and 6.411 percent for film deposited on SrTiO_3, LaAlO_3 and MgO respectively. The films on LaAlO_3 and SrTiO_3 show higher T_{IM} of 243 K and 217 K respectively as compared to T_{IM} of 191 K for the film on MgO. Similarly T_C of the films on SrTiO_3 and LaAlO_3 is sharper and has value of 245 K and 220 K respectively whereas the TC of the film on MgO is 175 K. Higher degree of relaxation creates more defects and hence TIM (T_C) of the film on MgO is significantly lower than of SrTiO_3 and LaAlO_3. We have adopted a different approach to correlate the effect of strain relaxation on magneto-transport properties of LCMO films by evaluating the resistivity variation through Mott's VRH model. The variable presence of disorder in these thick films due to lattice relaxation which have been analyzed through Mott's VRH model provides a strong additional evidence that the strength of lattice relaxation produces disorder dominantly by increase in density of defects such as stacking faults, dislocations, etc. which affect the magneto-transport properties of thick epitaxial La_0.7Ca_0.3MnO_3 films.

cond-mat.mtrl-sci↗

arXiv2004

Effect of Ca doping for Y on structural / microstructural and superconducting properties of YBa2Cu 3O7-d

We report systematic studies of structural, microstructural and transport properties of Y1-xCaxBa2Cu3O7-d bulk samples with 0.20 x 0.0. The partial substitution of Ca2+ at Y3+ site in YBa2Cu3O7-d leads towards slightly overdoped regime, which along with disorder in CuO2 planes is responsible for decrease in superconducting transition temperature (Tc) of the doped system. The microstructural variants being studied by transmission electron microscopy (TEM) technique in imaging and selected area diffraction modes revealed an increase in the density of twins with increase in Ca concentration despite the fact that there is slight decrease in the orthorhombicity. This is against the general conviction of decrease in twin density with decreasing orthorhombicity. An increase in twin density results in sharpening of twin boundaries with increasing Ca concentration. These sharpened twin boundaries may work as effective pinning centers. A possible correlation between microstructural features and superconducting properties has been put forward and discussed in the present investigation.

cond-mat.supr-con↗