K. Vinod

Pressure-induced metallization of BaMn2As2

The temperature and pressure dependent electrical resistivity rho(T,P) studies have been performed on BaMn2As2 single crystal in the 4.2 to 300 K range upto of 8.2 GPa to investigate the evolution of its ground state properties. The rho(T) shows a negative co-efficient of resistivity under pressure upto 3.2 GPa. The occurrence of an insulator to metal transition (MIT) in an external P ~4.5 GPa is indicated by a change in the temperature co-efficient in the rho(T) data at ~36 K . However complete metallization in entire temperature range is seen at a P~5.8 GPa. High pressure XRD studies carried out at room temperature also shows an anomaly in the pressure versus volume curve around P ~ 5 GPa, without a change in crystal structure, indicative of an electronic transition. Further, a clear precipitous drop in rho(T) at ~17 K is seen for P ~5.8 GPa which suggests the possibility of the system going over to a superconducting ground state.

Structural investigations in BaFe2 − xRuxAs2 as a function of Ru and temperature

We present the results of synchrotron X-ray diffraction (XRD) measurements on powdered single-crystal samples of BaFe(2-x)Ru(x)As2, as a function of Ru content, and as a function of temperature, across the spin-density wave transition in BaFe(1.9)Ru(0.1)As2. The Rietveld refinements reveal that with Ru substitution, while the a-axis increases, the c-axis decreases. In addition, the variation of positional coordinates of As (z(As)), the Fe-As bond length and the As-Fe-As bond angles have also been determined. In the sample with x = 0.1, temperature-dependent XRD measurements indicate that the orthorhombicity shows the characteristic increase with a decrease in temperature, below the magnetic transition. It is seen that the c-axis, the As-Fe-As bond angles, Fe-As bond length and positional coordinates of the As show definite anomalies close to the structural transition. The observed anomalies in structural parameters are analysed in conjunction with restricted geometric optimization of the structure using ab initio electronic structure calculations.

Critical current density and magnetic phase diagrams of BaFe1.29Ru0.71As2 single crystals

The critical current density has been measured on single crystals of Ru-substituted BaFe2As2 superconductor at several temperatures and in fields up to 16?T. The magnetization versus field isotherms reveal the occurrence of a clear second magnetization peak (SMP), also known as the fish-tail effect, for both H parallel and perpendicular to the c-axis of the crystal. The in-field resistance and magnetization data are used to draw a vortex phase diagram. The nature of the vortices has been determined from the scaling behaviour of the pinning force density extracted from the JC?H isotherms.

Magnetic behavior of the metal organic framework [(CH3)2NH2]Co(HCOO)3

In this study we examine the phase transitions in single crystals of [(CH3)2NH2]Co(HCOO)3, using magnetization and specific heat measurements as a function of temperature and magnetic field. Magnetisation measurements indicate a transition at 15 K that is associated with an antiferromagnetic ordering. Isothermal magnetization versus magnetic field curves demonstrate the presence of a single-ion magnet phase, coexisting with antiferromagnetism. A peak in specific heat is seen at 15 K, corresponding to a magnetic transition and the enthalpy of the transition evaluated from the area under the specific heat peak which decreases with the application of a magnetic field of up to 8 T. This is suggestive of long range antiferromagnetic magnetic order, giving way to single-ion magnetic behavior under an external magnetic field. At high temperatures, the specific heat measurements show a peak at ∼155 K that is insensitive to the applied magnetic field. Raman scattering studies confirm the presence of a structural transition. The magnetisation in this temperature range, while exhibiting a paramagnetic behavior, shows a distinct jump and the paramagnetic susceptibility changes across the structural transition.

Large magnetocaloric effect in hexagonal Yb1−xHoxMnO3

Magnetocaloric properties of polycrystalline hexagonal Yb1−xHoxMnO3 (x = 0.1, 0.2, and 0.3) compounds are studied through magnetization measurements. Temperature dependence of Zero Field Cooled magnetic moment measurements shows Neel temperature (TN1) of ∼83 K, corresponding to the Mn3+ antiferromagnetic ordering. At low temperatures (TN2 ∼ 5 K), all compounds show ferromagnetic ordering due to alignment of the Yb moments and the field induced magnetic transition is observed in the isothermal magnetization measurements. The maximum entropy change |ΔSMmax| and the relative cooling power (RCP) of Yb1−xHoxMnO3 are 3.75 ± 0.78 J/(mol K) and 90.0 ± 27 J/mol for x = 0.3 at ΔH = 100 kOe. Values of both |ΔSMmax| and RCP found to increase with increasing Ho content.

Studies on the magnetoelastic and magnetocaloric properties of Yb1−xMgxMnO3 using neutron diffraction and magnetization measurements

We report the magnetic ordering and magnetoelastic coupling of polycrystalline hexagonal Yb1−xMgxMnO3 (x = 0.00 and 0.05) compounds by using neutron diffraction measurements. The magnetocaloric properties of these Yb1−xMgxMnO3 compounds are also studied using magnetization measurements. The temperature dependence of the lattice parameters (a and c/a ratio) and unit cell volume V show anomalous behavior near TN1 ∼ 85 K (the Mn ordering temperature) due to the magnetoelastic effect. Also all the Mn–O bond distances display considerable variation at TN1. Isothermal magnetization curves measured near the Yb long range ordering temperatures indicate a field induced magnetic transition with applied field. The isothermal magnetic entropy change (−ΔSM) is calculated from the magnetization curves measured for different temperatures. Values of maximum entropy change (−ΔSmaxM), the adiabatic temperature change (ΔTad) and the relative cooling power (RCP) for these compounds are found to be 3.02 ± 0.37 J mol−1 K−1, 8.6 ± 0.95 K and 41 ± 9 J mol−1 for x = 0.00, and 2.63 ± 0.36 J mol−1 K−1, 9.06 ± 0.96 K and 40.0 ± 10 J mol−1 for x = 0.05, respectively, for ΔH = 100 kOe. Rescaling of the −ΔSM vs. T curves for various fields fit into a single curve, implying the second-order phase transition.

Structural, magnetic and magnetocaloric properties of hexagonal multiferroic Yb1−xScxMnO3 (x = 0.1 and 0.2)

We have studied the effect of Sc doping on the structural, magnetic and magnetocaloric properties of multiferroic Yb1−xScxMnO3 (x = 0.1 and 0.2). X-ray powder diffraction shows that both samples crystallize in the hexagonal phase with P63cm space group. The structural analysis shows a decrease in the lattice parameter a, a decrease in the cell volume of the hexagonal unit cell and a decrease in the average bond length between Mn–O, with Sc substitution. Magnetic measurements show that the Neel temperature (TN) increases from 90 K for x = 0.1 to 94 K for x = 0.2 samples. Isothermal magnetic curves show that the field variation in magnetization generates a metamagnetic transition. The maximum entropy change −ΔSmaxM and the relative cooling power (RCP) of Yb1−xScxMnO3 are found to be 2.46 ± 0.40 J mol−1 K−1 and 38.5 ± 9 J mol−1 for x = 0.1 and 1.87 ± 0.31 J mol−1 K−1 and, 30.1 ± 8 J mol−1 for x = 0.2 with ΔH = 10 T. The rescaled magnetic entropy change curves for different applied fields collapse onto a single curve for materials with second-order phase transitions.

Upper critical field anisotropy in BaFe 2 − x Co x As 2 single crystals synthesized without flux

The upper critical field was measured up to 12 T for three BaFe

Investigation of the effect of Ag addition on the critical current density of the high-temperature superconductor Nd1.85Ce0.15CuO4

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Structural studies of Nd1.85Ce0.15CuO4 + Ag superconducting system

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