K. R. Mavani

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...

Effects of cation disorder and size on metamagnetism in A-site substituted Pr0.5Ca0.5MnO3 system

The effects of A-site cation disorder and size on metamagnetism of ABO3 type charge and orbital ordered Pr0.5Ca0.5MnO3 system have been studied by substituting Ba+2 for Ca+2 or La+3 for Pr+3. Substitution of 5% Ba+2 or 5% La+3 drastically reduces the critical magnetic field (Hc) for metamagnetism and induces successive steplike metamagnetic transitions at low temperatures. Interestingly, with further increase in substitution, Hc rises. We find that there is a sharp decrease in electrical resistivity corresponding to the metamagnetic transitions, which is indicative of strongly correlated magnetic and electronic transitions in these manganites.

Implications of phase-segregation on structure, terahertz emission and magnetization of Bi(Fe1-xMnx)O3 (0⩽x⩽0.5) thin films

Structural, magnetic and terahertz emission properties of Bi(Fe1-xMnx)O3 (0≤x≤0.5) thin films of various thicknesses were studied. A transition from coherently strained structure to relaxed structure at a film thickness (t) of ~80–90 nm occurs only for x<0.2. It is shown that terahertz-emission efficiency is not deteriorated with increasing Mn-doping (x). The magnetic moment of thin films (t≤85 nm) exhibits only a weak enhancement with increasing x —a feature suggesting that Mn-doping is ineffective in inducing ferromagnetism in BiFeO3. The thicker films (t≥150 nm), on the contrary, possess larger magnetic moment which evidently arises from the segregated magnetic MnFe2O4 phase.

Effects of disorder and scaling of optical conductivity in Nd0.5Ca0.5−xBaxMnO3 (x=0 and 0.02) thin films as observed by terahertz time-domain spectroscopy

We have explored the low energy charge dynamics of charge-ordered Nd0.5Ca0.5MnO3 and Ba-doped Nd0.5Ca0.48Ba0.02MnO3 manganite thin films by recording the complex optical spectra using terahertz time-domain spectroscopy. The extracted frequency- and temperature-dependent optical conductivity (σ) and dielectric constant (e1) show distinct features at Neel’s ordering and the charge-ordering temperatures (TN and TCO). We found that the Ba-doping induced cation disorder effects on σ and e1 are more pronounced above TCO, but these effects are subtle in a large temperature region below TCO. Both e1 and σ increase as a function of temperature and form a scaling relationship e1∝σ.

Visualization of photoassisted polarization switching and its consequences in BiFeO3 thin films probed by terahertz radiation

The principle of terahertz radiation emission from multiferroic BiFeO3 thin films was employed to acquire high-resolution images of their photoassisted ferroelectric polarization switching. These images are evidence that poling in “electric-field assisted by illumination of band gap light” enhances the terahertz-emission amplitude by creating additional switchable ferroelectric dipoles.

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.

Effect of Sr-substitution on the restitution of superconductivity in Pr-substituted at rare earth and Ba-site in EuBa2Cu3Oz

Abstract We report the effect of Sr-substitution in restoring the superconductivity of Pr-doped (Eu 1− x Pr x )Ba 2 Cu 3 O z [A] and Eu(Ba 2− x Pr x )Cu 3 O z [B] samples. The structural and superconducting properties of these A and B samples have been investigated using X-ray diffraction, a.c. susceptibility, electrical resistivity, d.c. magnetization and iodometric measurements. It is observed that the superconductivity gets suppressed at the rate of 1.6 K per at.% of Pr substitution at Eu-site due to the localization of mobile holes, while the sample with 15% Pr at Ba-site [B(0.3)] becomes non-superconducting mainly due to the hole filling and localization of holes. However, the increasing substitution of Sr at Ba-site in both A(0.4) and B(0.3) samples upto 25% resulted in the restoration of superconductivity (T c ∼33 K ) due to the delocalization of holes. Interestingly, the increasing substitution of Sr at Eu-site in the non-superconducting B(0.4) sample upto 30%, increases T c from 0 to 52 K mainly due to the hole doping mechanism which is much faster and larger restoration of superconductivity than the Ba-site doping of Sr in the same sample.

Structural and superconducting properties of La2−xRxBa2CayCu4+yO10+δ (R=Nd, Gd; y=2x) system

A nonsuperconducting La2Ba2Cu4O9+δ system has been made superconducting by the addition of CaCuO2, along with rare earth doping at the La site, with the stoichiometric composition La2−xRxCayBa2Cu4+yO10+δ (R=Nd, Gd, and y=2x). All the samples with 0.0<x<0.5; y=2x were characterized by x-ray diffraction and neutron diffraction measurements for structural properties and by ac susceptibility, dc resistivity, and iodometric analysis for superconducting properties. The x=0.0 sample displays semiconducting behavior. Increasing x increases Tc from less than 15 K for Nd/Gd samples with x=0.1 to ∼78 K for x=0.5 samples. It is interesting to note that the hole concentration and Tc exhibit a strong correlation with increasing dopant concentration. Neutron diffraction studies confirm the occupancy of Ca and Nd ions at the La site with a concomitant displacement of La on to the Ba site.