R. G. Kulkarni

Metal-insulator type transition in aluminium and chromium co-substituted nickel ferrites

Abstract Polycrystalline Al and Cr co-substituted disordered spinel series NiAl x Cr x Fe 2−2 x O 4 with varying Al–Cr content (0.1≤ x ≤0.9) have been prepared in the pellet form and studied by X-ray diffraction, magnetization and electrical resistivity measurements. The lattice constants are determined and the applicability of Vegards law has been tested. Samples with x ≥0.5 exhibit a relatively sharp cusp in the electrical resistivity vs. temperature curve at T = T max identified as metal-insulator transition showing metallic behaviour for T T max and insulating behaviour for T > T max . Metallic conductivity and ferrimagnetism occur in these x =0.5 to 0.9 samples mainly due to Cr spins and 3d transition metal electron correlation.

Study of cation distribution and macro-magnetic properties of magnesium and aluminum co-substituted lithium ferrite

Abstract The structural and bulk magnetic properties of mixed spinel ferrite system Mg x Al 2 x Li 0.5(1− x ) Fe 2.5(1− x ) O 4 ( x =0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) have been investigated by means of X-ray diffraction, magnetization and a.c. susceptibility measurements. The cation distribution determined through X-ray data are supported by the magnetization and susceptibility measurements. The study has revealed the strong octahedral site preference of Li + and Al 3+ as compared to Mg 2+ . It is also found that the magnetic ordering in the system is of collinear type and the variation of magnetization can be explained on the basis of Neels collinear spin model. The Neel temperatures calculated theoretically are in good agreement to those deduced experimentally.

Influence of hole filling by Co and hole doping by Ca on the superconductivity of ErBa2Cu3O7−δ

Abstract The structural and superconducting properties of (Er1−yCay)Ba2 (Cu1−xCox)3Oz samples are investigated using X-ray diffraction, resistivity and oxygen content measurements. The effect of increasing Co concentration in ErBa2(Cu1−xCox)3Oz lowers the carrier concentration and decreases Tc, which is attributed to hole filling by Co. This suppression of Tc has been compensated by appropriate hole doping with Ca, for instance, in the oxide (Er1−yCay)Ba2(Cu0.94Co0.06)3Oz, the Tc increases from 34 K (y = 0.0) to 85 K (y = 0.18, compensated oxide) as the hole filling by Co is completely balanced by hole doping from Ca.

Structural, magnetization and susceptibility studies on cobalt–ferri-aluminates synthesized by wet-chemical method

Abstract Spinel ferrites having the general formula CoAl x Fe 2− x O 4 ( x =0.0–1.0) were prepared using the wet-chemical method. The samples were sintered at 200°C, 500°C and 800°C temperature and were studied by means of X-ray diffraction, magnetization and low-field AC susceptibility measurements. The observed features for the wet samples, such as the intense peak in χ AC – T curve, the co-existence of paramagnetic doublet and magnetic sextets in Mossbauer spectra and lower saturation magnetization values confirm fine particle ferrite behaviour. The variation of saturation magnetization with Al 3+ substitution is explained on the basis of the Neels collinear spin model. The cation distribution is estimated from the X-ray intensity calculations and magnetization measurements.

Cation distribution of the system CuAlxFe2-xO4 by X-rays and Mossbauer studies

The structural and Mossbauer spectroscopy studies have been performed on the spinel solid solution series CuAlxFe2−xO4 (0.0 ≤ x ≤ 1.0). All the compounds with 0.0 ≤ x ≤ 1.0 crystallised with cubic spinel structure. Lattice constant values calculated from XRD analysis were found to decrease on increasing x, linearly obeying Vegards law. The X-ray intensity calculations indicated that Cu2+ prefers to occupy octahedral (B) site, where as Al3+ ions replace Fe3+ ions from both tetrahedral (A) and octahedral (B) sites. Mossbauer spectra at room temperature display magnetic sextets corresponding to A and B-sites superimposed on each other. The data shows that Al-possesses greater preference for B-site compared to A-site, and iron exists in high spin ferric Fe3+ state. The hyperfine fields for both A and B-sites decrease with increasing x. The cation distribution calculated from X-ray intensity data agrees with the Mossbauer results.

Role of Calcium in the Evolution of Superconductivity in a (La2−xRx)Ba2(CayCu4+y)Oz (R = Y, Er, Gd) System

It has been reported that, by adding equal amounts of CaO and CuO to non superconducting La3Ba3Cu6Oz (La-336), a series of superconductors with nominal compositions La3CayBa3Cu6+yOz were prepared with maximum Tonc ∼ 80K. Similar studies on addition of CaO and CuO in nonsuperconducting LaBaCu2Oz (La-112) resulted into superconducting LaCaBaCu3Oz (La-1113). To date no attempt has been made to synthesize La2CaBa2Cu5Oz (La-2125) superconducting phase by addition of CaO and CuO to non superconducting La2Ba2Cu4Oz (La-224) system. Also no reports are published to study the effect of replacing larger La3+-ions (1.01Å) by smaller rare earth ions viz Y3+(0.89Å), Er3+(0.91Å), Gd3+(0.91Å) on the structural and superconducting properties of (La2−xRx)Ba2(CayCu4+y)Oz (LRBCaC); 0.0 ≤ x ≤ 0.5; y=2x system. In this paper, we report the method of synthesis, structural and superconducting property characterization using X-ray diffraction, oxygen content measurements using iodometry, resistivity measurements using d.c. four probe technique and a.c. susceptibility measurements in the temperature range RT to 15K. Also a comparative study, on the evolution of superconducting phase with Ca-concentration for different rare earth substitutions for LRBCaC system in the context of hole doping mechanism, is carried out.

Influence of hole-filling by La and hole-doping by Ca on the superconductivity of NdBa2Cu3O7-δ

Abstract The structural and superconducting properties of La- and Ca-substituted (Nd 1− x Ca x )(Ba 2− y La y )Cu 3 O z samples prepared under identical conditions have been investigated by X-ray diffraction (XRD), resistivity, ac susceptibility and oxygen content measurements. The effect of increasing La-concentration in Nd(Ba 2− y La y )Cu 3 O z is in lowering the carrier concentration and decreasing T c , which is attributed to hole-filling by La. This suppression of T c has been compensated by appropriate hole-doping with Ca in (Nd 1− x Ca x )(Ba 2− y La y )Cu 3 O z , which shows that for a fixed La content at Ba-site, T c increases dramatically as the Ca content increases. For instance, in the oxide (Nd 1− x Ca x )(Ba 1.6 La 0.4 )Cu 3 O z , the T c increases from 0.0 K ( x =0.0, non-superconductor) to 84.0 K ( x =0.4, compensated oxide) as the hole-filling by La is completely balanced by hole-doping from Ca.

Superconductivity of the La3.5−x–yRyCa2xBa3.5−xCu7Oz system (R = Gd, Nd and Dy)

Abstract The structural and superconducting properties of single phase La 3.5− x – y R y Ca 2 x Ba 3.5− x Cu 7 O z (0 ≤ x = y ≤ 0.70) (LRCB with R = Gd, Nd and Dy compounds with tetragonal triple-perovskite structure are investigated using X-ray diffraction, resistivity, AC susceptibility and oxygen-content measurements. Increasing Ca and Gd substitution for La and Ca for Ba resulted in a decrease in unit cell axes and an increase in oxygen stoichiometry. Samples with 0.2 ≤ x = y ≤ 0.70 are superconducting and T c R = 0 is between 31 and 79 K. With increasing x = y , both the hole concentration in the Cu2-O layers and T c R = 0 increase to a maximum value of 0.313 and 79 K at x = y = 0.6, respectively. A correlation between the T c and the hole concentration is suggested. Similar superconducting behaviour has been observed for R = Nd and Dy in LRCB compounds.

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.

Enhanced flux pinning and critical currents by Hf and Hf-Ca substitution in YBa2Cu3O7 − δ

Abstract Hf and Ca substituted samples with stoichiometric compositions (Y1 − xHfx)Ba2Cu3Oz with x = 0.0, 0.1, 0.2 and (Y0.85 − yCayHf0.15)Ba2Cu3Oz with y = 0.1, 0.2, 0.4, respectively, prepared under identical conditions have been characterized by d.c. magnetization and thermogravimetric analysis measurements. Detailed magnetization measurements on randomly orientated powdered samples as a function of temperature and field exhibit pronounced M(H) hysteresis loops, indicative of large flux pinning, even as T approaches Tc. It is believed that the flux pinning arises due to the preferential substitution of Hf or Hf-Ca on the Y site, which acts as a pinning centre. The critical current density Jc has been deduced from hysteresis data which is discussed in the light of the flux creep model.