Rashmi Nigam

Coexistence of ferromagnetism and cluster glass state in superconducting ferromagnet RuSr2Eu1.5Ce0.5Cu2O10−δ

Systematic dc and ac magnetic susceptibility measurements performed on RuSr2Eu1.5Ce0.5Cu2O10−δ (Ru1222) demonstrate a paramagnetic to ferromagnetic transition around 95 K. The third harmonic of the ac susceptibility reveals that the system undergoes a spin glass transition below 75 K. The features of the zero-field cooled and field cooled dc magnetization curves of Ru1222 material resemble those of a cluster glass state, rather than those of a canonical spin glass state. The magnetization versus applied field loops do not saturate, even at very high applied fields, resulting in the short range magnetic order in the system, which facilitates the formation of clusters that freeze at low temperature. The temperature dependence of the second and third harmonic ac susceptibilities further confirms the coexistence of a cluster glass state and ferromagnetic order in the Ru1222 system.

Impact of sintering temperature on the physical properties of the superconducting ferromagnet: RuSr2Eu1.5Ce0.5Cu2O10

We report the influence of the sintering temperature on the electromagnetic behavior for the RuSr2Eu1.5Ce0.5Cu2O10 material, in which superconductivity and ferromagnetism coexist. The inadequate heat treatment results in the coexistence of the secondary Ru-1212 phase along with the dominant Ru-1222 phase. The presence of two phases leads to the magnetic superposition of the signals from both phases, which results in the observation of a small peak around 120–130K. In the pure Ru-1222 no such magnetic anomaly is observed. In addition, the impure samples exhibit a double step superconductivity transition. In the normal state, these impure samples exhibit a semiconductinglike behavior of the resistivity. In contrast, the pure Ru-1222 sample with much larger, well connected grains has a single step resistivity transition, as well as a metalliclike behavior in the normal state. Our work sends a strong message that only high quality, phase pure samples can be chosen for the investigation of the complex behavior in the Ru system.We report the influence of the sintering temperature on the electromagnetic behavior for the RuSr2Eu1.5Ce0.5Cu2O10 material, in which superconductivity and ferromagnetism coexist. The inadequate heat treatment results in the coexistence of the secondary Ru-1212 phase along with the dominant Ru-1222 phase. The presence of two phases leads to the magnetic superposition of the signals from both phases, which results in the observation of a small peak around 120–130K. In the pure Ru-1222 no such magnetic anomaly is observed. In addition, the impure samples exhibit a double step superconductivity transition. In the normal state, these impure samples exhibit a semiconductinglike behavior of the resistivity. In contrast, the pure Ru-1222 sample with much larger, well connected grains has a single step resistivity transition, as well as a metalliclike behavior in the normal state. Our work sends a strong message that only high quality, phase pure samples can be chosen for the investigation of the complex behavior...

Magnetic anisotropy in doped and undoped LiFePO4 single crystals

We report on the magnetic anisotropic properties of pure and Mg doped LiFePO4 single crystals grown by the traveling solvent floating zone technique. Our results reveal a considerable anisotropy in the susceptibilities along H‖a, H‖b, and H‖c for both crystals. LiFePO4 and Li0.99Mg0.01FePO4 crystals show antiferromagnetic long-range ordering below the Neel temperature of about 52K, with the b axis being the magnetic easy axis. The effective moments have been determined to be 6.18μB and 6.07μB for the pure and the doped crystals, respectively. These values are higher than what is expected on the basis of the spin-only value of 6.70μB with a high-spin d6 configuration for the Fe2+ ion.

Band structures, and magnetic and transport properties of la doped two dimensional Sr2CoO4

In this work, we report on our studies on the band structure calculations, structures, transport, and magnetic properties in two dimensional layer structured perovskite compounds Sr2−xLaxCoO4. Structure refinement results reveal that these compounds crystallized in K2NiF4-type structures with space group I4∕mmm. The temperature dependence of resistivity shows a semiconductorlike behavior over a wide range of temperatures, a metal-insulator transition at 240K, and an upturn at 160K for the x=1, 1.25, and 0.75 samples. A coercive field is about 1T for the sample with x=0.75, while it is about 0.05T for x=0.75 and 0.1T for the x=1.25 samples. A negative field hysteresis magnetoresistance in close correlation with the coercive field has been observed and can be explained by the grain boundary tunneling effect. The first-principles band structure calculations were carried out for Sr1.5La0.5CoO4 and the results indicate that the system is metallic with a high spin polarization which is responsible for the obser...

Extended dislocation-based pinning mechanism in superconducting YBa2Cu3O7 films

To describe the critical current density (Jc) as the function of applied magnetic field (Ba) in high quality YBa2Cu3O7 (YBCO) superconducting films, the vortex pinning mechanism along the structural domain boundaries of the films is developed. The boundaries, assumed to have low misorientation angles, are quantitatively considered to consist of individual edge dislocations acting as pinning wells, rather than a continuous boundary. This extended model accurately describes the experimental Jc(Ba) over the wide field and temperature ranges. Marginal deviations of the model from the experimental Jc(Ba) curves are observed at high fields and temperatures where thermally activated depinning is significant. This pinning model is verified to provide precise structural properties of the films which can be obtained by other considerations.

Nanocoating of particles for optimal doping and universal enhancement of current-carrying ability in “organic” MgB2−xCx superconductors

The universal approach of liquid mixing has been found to produce “organic” MgB2−xCx superconductors with structural and supercurrent-carrying properties unattainable by other techniques. We show by transmission electron microscopy combined with energy dispersive spectroscopy that these unique properties are enabled through coating of boron particles with a nanolayer of carbon. The subsequent reaction between carbon-encapsulated boron and magnesium results in enhanced carbon content x in the crystal lattice, as determined by x-ray diffraction pattern analysis. Among various carbon-containing compounds investigated, polycarbosilane has been found to be the optimal dopant for MgB2 superconductor, producing the largest critical current density due to the formation of wide range of nanodefects with pinning properties within the MgB2 crystal lattice, as well as due to minimal formation of current-blocking layers around the grains.

Magnetic phase diagrams based on static and dynamic magnetic behaviour in Ru-based superconducting ferromagnets

In this work, we present magnetic phase diagrams of a RuSr(2)Eu(1.5)Ce(0.5)Cu(2)O(10-δ) (Ru-1222) superconducting ferromagnet derived from its static and dynamic magnetic responses, measured by temperature and field dependences of dc magnetization and nonlinear ac susceptibility in both low and high magnetic fields. Comparison of magnetic phase diagrams of phase pure and impure samples singles out the intrinsic and extrinsic magnetic features, naturally proposing a unified model of Ru-1222 magnetic behaviour. The results considered within the proposed interpretation indicate full agreement between static and dynamic properties which, if measured in combination, effectively complement each other, uncovering existing ambiguities.

Comparative study of magnetic behavior of RuSr2RE1.5Ce0.5Cu2O10-δ where RE=Eu and Sm

Systematic investigation of structural, magnetic and transport properties of RuSr2RE2-xCexCu2O10 with RE being Sm and Eu has been performed. We present the experimental evidence that the synthesis of RuSr2Eu1.5Ce0.5Cu2O10 under non-optimised heat treatment conditions may results in the formation of secondary RuSr2EuCu2O8 phase. In contrast, similar preparation conditions for RuSr2Sm1.5Ce0.5Cu2O10 samples lead to the pure phase. Consequently, RuSr2Eu1.5Ce0.5Cu2O10 samples exhibit a small peak around 125 K in the temperature dependence of dc magnetization, pronounced irreversibility between zero-field cooled and field cooled curves at temperatures higher than the peak temperature, and the bell shaped behavior of the coercive field as the function of temperature above Curies temperature. These anomalous features, which may arise due to the superposition effect of the main and secondary phases, which are both magnetic, are completely absent for RuSr2Sm1.5Ce0.5Cu2O10 sample, which does not consist of the secondary phase.