C. M. Thaker

Magnetic and transport properties of (La0.7−2xEux)(Ca0.3Srx)MnO3: Effect of simultaneous size disorder and carrier density

Structural, magnetic and transport studies have been carried out on (La0.7−2xEux)(Ca0.3Srx)MnO3 (0.05⩽x⩽0.25) compounds forming in a distorted orthorhombic structure (space group Pnma, No. 62). The Eu and Sr substitutions avoid any average A-site cation size disparity throughout the series. However, increasing both the cation size mismatch at the A-site and carrier concentration induces interesting changes in structural, transport and magnetic properties. Both insulator–metal transition temperature (Tp) and Curie temperature (TC) decrease with increasing x. The resistivity of all the samples in the semiconducting regime fits to the Variable Range Hopping (VRH) of Mott type model. Carrier localization length, L, obtained from VRH plots, decreases from 4.6 A for x=0.05 to 3.9 A for the x=0.20 sample. In the metallic region, the n term in the resistivity fits to the Zener-Double exchange polynomial law (ρ=ρ0+ρ2T2+ρnTn) increases from n=5.5 for x=0.05 to n=7.5 for x=0.15. From magnetic susceptiblity measureme...

Disorder effects in (LaTb)0.5(CaSr)0.5MnO3 compounds

Electrical, magnetotransport, and magnetization measurements have been carried out on half-doped (La0.5−xTbx)(Ca0.38−ySr0.12+y)MnO3 (0.025⩽x⩽0.125; y=0.8x) compounds. The increase in Tb3+ and Sr2+ contents keeps the average A-site ionic radius constant at ∼1.215 A but introduces increasing size disorder. The insulator-metal transition temperature (Tp) falls from 191 to 107 K and the Curie temperature (TC) falls from 230 to 106 K as x increases from 0.025 to 0.125. Interestingly, with increasing size disorder, the disparity between TC and Tp disappears and magnetoresistance increases around Tp but decreases at low temperatures. This is discussed in the light of phase segregation.

Structural investigations of La-2125 mixed oxide superconducting system

Crystallographic studies on the (La2 − xNdx)CayBa2Cu4 + yOz′ [y = 2x; 0.0 ≤ x ≤ 0.5] samples are carried out using neutron diffraction technique. The analysis of the neutron diffraction patterns for the series of samples with 0.0 ≤ x ≤ 0.5 was done using Rietveld method. The La2Ba2Cu4Oz′ (La-224, x = 0.0) system exhibits tetragonal phase and is a nonsuperconductor, which on addition of a “CaCuO2” rock salt like layer becomes superconducting. It is important to note the significant role played by Ca-doping in improvising the superconducting properties. Analysis of the structural data reveals that, as Ca concentration increases, the unit cell volume decreases while Tc increases with a maximum value of 78 K for y = 1.0. The detailed crystal structure for LaNdCaBCO series of samples has been studied in the light of changes in bond lengths with increase in Ca concentration and its role in “turning on” the superconductivity.

Studies on La2-xPrxCayBa2Cu4+yOz (x = 0.1 - 0.5, y = 2x) type mixed oxide superconductors

The La2-xPrxCayBa2Cu4+yOz (LaPrCaBCO) mixed oxides have been studied for their structural and superconducting properties using X-ray diffraction (XRD), d. c. resistivity, d. c. susceptibility and iodometric double titration. All the LaPrCaBCO samples for x = 0.1 - 0.5, exhibit tetragonal crystalline structure with P 4/mmm space group as determined by Rietveld analysis of the X-ray diffraction data. With increasing x, enhancement in Tc is observed, which is quite interesting for Pr substituted high Tc oxides. Maximum Tc ~ 58 K has been observed for x = 0.5(La-2125 stoichiometry). The results of structural studies and superconducting property measurements are presented in light of increase in Tc in LaPrCaBCO system with increasing Pr concentration.Abstract The La 2− x Pr x Ca y Ba 2 Cu 4+ y O z (LaPrCaBCO) mixed oxides have been studied for their structural and superconducting properties using X-ray diffraction (XRD), d.c. resistivity, d.c. susceptibility and iodometric double titration techniques. The Rietveld analysis of the XRD data of all the LaPrCaBCO samples ( x =0.1–0.5) exhibits that all the compounds crystallize in tetragonal R-123 type structure (space group P 4/ mmm ). With the increase in x , T c increases up to a maximum value of 58 K ( x =0.5, La-2125 stoichiometry), which is interesting for Pr substituted high T c oxides. The results of structural studies and superconducting property measurements are presented in light of increase in T c in LaPrCaBCO system with increasing Pr concentration.