G. Rangarajan

Direct current magnetron sputtered In2O3 films as tunnel barriers

Insulating films of In2O3 were prepared by sputtering indium in the presence of pure oxygen using dc magnetron sputtering. Transmission electron microscopic investigations showed the films to be single phase and polycrystalline. Analysis of the optical transmittance data showed the films to have an optical band gap of 3.71±0.01 eV. Tunnel junctions were made with high Tc superconductors Bi2Sr2Ca1Cu2Oy and NdBa2Cu3O7−δ using indium oxide as the barrier layer and Pb0.5In0.5 as the counter electrode. The conductance spectra displayed prominent structures attributable to energy gap. The reduced gap parameters for Bi2Sr2Ca1Cu2Oy and NdBa2Cu3O7−δ were found to be 4.0±0.5 and 5.2±0.6, respectively.

Complex ferromagnetic state and magnetocaloric effect in single crystalline Nd0.7Sr0.3MnO3

The magnetocaloric effect in single crystalline Nd_{0.7}Sr_{0.3}MnO_{3} is investigated by measuring the field-induced adiabatic change in temperature which reveals a single negative peak around 130 K well below the Curie temperature (T_C=203 K). In order to understand this unusual magnetocaloric effect, we invoke the reported {55}^Mn spin-echo nuclear magnetic resonance, electron magnetic resonance and polarized Raman scattering measurements on Nd_{0.7}Sr_{0.3}MnO_{3}. We show that this effect is a manifestation of a competition between the double exchange mechanism and correlations arising from coupled spin and lattice degrees of freedom which results in a complex ferromagnetic state. The critical behavior of Nd_{0.7}Sr_{0.3}MnO_{3} near Curie temperature is investigated to study the influence of the coupled degrees of freedom. We find a complicated behavior at low fields in which the order of the transition could not be fixed and a second-order-like behavior at high fields.

Tricritical point and magnetocaloric effect of Nd1−xSrxMnO3

A tricritical point is observed in the Nd1−xSrxMnO3 (NSMO) (x=0.3, 0.33, and 0.4) manganites at x=0.33 which separates the first-order transition in NSMO-0.3 and second order transition in NSMO-0.4. The ferromagnetic transition of these compounds is further investigated by measuring magnetocaloric effect (MCE) and by applying a theoretical model based on Landau theory of phase transitions. Results indicate that the contributions to the free energy from the presence of correlated clusters are strongly influencing the MCE by coupling with the order parameter around the Curie temperature.

Enhanced magnetocaloric effect in single crystalline Nd0.5Sr0.5MnO3

The magnetocaloric effect in single crystalline Nd0.5Sr0.5MnO3 (NSMO 0.5) is investigated by computing the field dependent entropy change (ΔS) and adiabatic temperature change (ΔTad). At the charge ordering temperature (TCO), the value of ΔSmax is found to be much higher than ΔSmax reported in polycrystalline samples. This “giant” entropy change is attributed to interplay (stronger in single crystals) among spin, charge, lattice, and orbital degrees of freedom resulting in a field induced transition at TCO. In contrast, the change in entropy associated with Curie temperature (TC) is very low. The direct measurements of the field induced temperature change (ΔT) are in agreement with the computed value of ΔS. The presence of short-range correlations with charge/orbital order (COO) above and below TC may be responsible for the suppression of the negative MCE at TC. A critical exponent analysis of the paramagnetic (PM) to ferromagnetic (FM) transition using magnetization data yields mean-field-like values, wh...

Specific heat of praseodymium doped yttrium barium copper oxide

Abstract The specific heat of samples of Y 1−x Pr x Ba 2 Cu 3 O 7−y for x = 0.1 to 0.5 has been measured from 4.5 to 65 K using a quasi-adiabatic calorimeter. A plot of C/T vs T 2 shows a linear behavior below 20 K. The excess specific heat due to praseodymium in all the compounds is obtained by subtracting the specific heat of YBa 2 Cu 3 O 7−x from the that of doped compound. The excess specific heat shows a peak near 19 K and its height decreases with increase in x. The excess specific heat could be interpreted on the basis of Schottky anomalies due to Pr 3+ and Pr 4+ ions, the peak at 19 K arising from the former. The fraction of Pr 3+ ion obtained from a fit to the specific heat curves decreases from 0.30 for x = 0.1 to 0.05 for x = 0.5.

Coexistence of ferromagnetic and antiferromagnetic phases in Nd0.5CaxSr0.5−xMnO3 manganites

We have investigated the spontaneous magnetization exhibited in Nd0.5CaxSr0.5−xMnO3 (x = 0.1, 0.2, 0.3 and 0.4) (NCSMO) samples by measurements of magnetization and electron spin resonance. As like end members, the samples crystallize in an orthorhombic structure with Imma (x = 0.1) and Pnma (0.2, 0.3 and 0.4) space groups. Transport studies reveal that Mott small polaron hopping type conduction occurs in the insulating regime of the samples. The drop in the zero field cooled dc magnetization, a large thermal irreversibility of the field cooled and zero field cooled data and the presence of two resonance fields below the temperature T* strongly suggest the coexistence of phases in NCSMO. The occurrence of coexisting phases may be strongly influenced by the random distribution of A-site cations such as Sr and Ca, which in turn leads to delocalization of eg electrons and real space ordering of Mn3+ and Mn4+ ions.

Structural and crystal field studies on a single crystal of PrBa2Cu3O7−δ

Abstract The crystal structure, and magnetic susceptibilities of single crystals of PrBa 2 Cu 3 O 7−δ are reported here. The crystals are tetragonal with space group P4/mmm. A crystal field (CF) Hamiltonian appropriate to tetragonal symmetry at the Pr 3+ site was diagonalised considering J-mixing. A set of CF parameters is being reported which fits the susceptibility data closely and explains the reported results of inelastic neutron scattering measurements also reasonably well.

ESR study of spin–lattice correlated clusters in single crystalline Nd0.7Sr0.3MnO3

We report electron spin resonance measurements in single crystalline Nd0.7Sr0.3MnO3 that provide evidence for the existence of spin–lattice correlated clusters above and below TC (= 205 K). The linewidth of the paramagnetic spectrum indicates the presence of strong electron–phonon interaction rather than the spin only interaction seen in other manganites. The gradual increase observed in the g value above TC is attributed to the presence of orbital correlations. The observation of ferromagnetic resonance (FMR) spectra only below 185 K (TC = 205 K) and noisy features in the FMR spectra above 140 K are ascribed to the strong competition between localization due to lattice distortions and delocalization of charge carriers. The magnetocrystalline anisotropy of the sample splits the FMR line into two below 185 K. An additional splitting observed in the FMR line is attributed to the existence of spin–lattice correlated (insulating) clusters within the ferromagnetic (metallic) phase. The influence of these clusters on the spin dynamics above and below TC is discussed.

Influence of Al substitution in PrBa2Cu3O7−δ single crystals

Abstract The role of Al substitution in single crystals of PrBa 2 Cu 3 O 7− δ (PBCO) has been investigated using low temperature magnetic susceptibility and heat capacity measurements. The crystal composition and the amount of impurity have been determined using electron probe microanalysis (EPMA) and the lattice parameters have been determined using X-ray diffraction. Heat capacity measurements on single crystals of PrBa 2 Cu 3− y Al y O 7− δ reveal a sharp peak at 16.6 K for y =0.0 originating from an antiferromagnetic ordering of the Prion moments ( T N ) and broader transitions at ≈10 K for y =0.25 and ≈5.3 K for y =0.4. Subtle anomalies are also found at these temperatures in the magnetic susceptibilities. The present results along with the neutron diffraction studies demonstrate the deleterious role of Al on the magnetic properties in this sample.

Single particle tunneling studies on Bi2Sr2Ca1Cu2Oy

Abstract Single particle tunneling studies on Bi2Sr2Ca1Cu2Oy using a sandwich configuration with sputtered indium oxide film as artificial barrier and Pb0.5In0.5 film as counter electrode, at 4.2 K are reported. The conductance spectra displayed well defined structure near bias voltages of 13 mV. The results were fitted to a life-time broadened BCS density of states function. The energy gap parameter, Δ and the damping factor, Γ, were found to be 13 meV and 1.5 meV repectively. This gives a value of 3.8 (± 0.3) for the coupling strength parameter, 2Δ/KBTC.