G. Kalai Selvan

Influence of Si substitution on the structure, magnetism, exchange bias and negative magnetoresistance in Ni48Mn39Sn13 Heusler alloys

The effect of partial Si substitution for Sn on the structure, magnetism, magnetic entropy change, exchange bias and negative magnetoresistance in Ni48Mn39Sn13?xSix(1???x???4) Heusler alloy systems is investigated. A small amount of Si in the Sn position influences the characteristic transition temperatures and the magnetocaloric effect. A maximum positive entropy change of 5.13?J?kg?1?K?1 is observed for a field change of 9?T in the x?=?1 alloy. An increase in Si content increases the exchange bias field. The exchange bias field and the coercive field strongly depend on the Si content. For the x?=?4 alloy, an exchange bias field of 354?Oe is observed. In addition, a negative magnetoresistance is observed in this alloy system, which decreases with Si content. The increase in resistivity at the martensitic transition could be attributed to the formation of superzone boundary gaps that changes the density of states near the Fermi surface.

Functionalization of Multi-walled Carbon Nanotubes with 6-Aminobenzothiazole and their Temperature-dependent Magnetic Studies

Magnetically responsive functionalized multi-walled carbon nanotubes (Bet-f-MWCNTs) were prepared using 6-Aminobenzothiazole (ABT) through acid functionalization. Bet-f-MWCNTs were characterized by Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, Ultra violet-visible-near infrared spectroscopy (UV-VIS-NIR) and powder X-Ray Diffraction (XRD). The transmission electron microscopy (TEM) images were visualized as further evidence for the successful modifications on the sidewalls of the MWCNTs. The temperature dependent magnetization and hysteresis studies of multiwalled carbon nanotubes (MWCNTs), carboxylic acid functionalized MWCNTs (Acid-f-MWCNTs) and Bet-f-MWCNTs were investigated for the first time. The change in the magnetic behavior of Bet-f- MWCNTs at lower temperature was attributed to the functionalization of MWCNTs by the organic moiety benzothiazole (BT). Interestingly, Bet-f-MWCNTs showed higher saturation magnetization value of 0.94 emu/g compared to MWCNTs and Acid-f-MWCNTs at low temperature. Moreover, Bet-f-MWCNTs exhibited higher coercive value of 109 Oe compared with the value of 90 Oe for MWCNTs indicating the functionalized MWCNTs as a promising candidate in electrical motors and magnetic recording media.

Crystal Growth, Structure, Resistivity, Magnetic, and Photoelectric Properties of One‐Dimensional Selenometallate Ba2BiFeSe5

Low-dimensional materials have attracted extensive research interest in recent years owing to their interesting structural chemistry and physical properties, which will greatly deepen our knowledge of these materials and could lead to additional breakthroughs in the future. Herein we have synthesized and characterized Ba2 BiFeSe5 , which adopts a quasi-one-dimensional structure and possesses some fascinating physical properties. The sharp divergences between the field-cooled (FC) and the zero-field-cooled (ZFC) data and the rather small magnetic moment per Fe3+ (0.07 μB ) strongly suggest that the title compound is weakly ferromagnetic with a high magnetic transition temperature above room temperature, which is controlled by competing super-exchange interactions within and between [FeBiSe5 ]∞ anionic ladders. Moreover, with its narrow bandgap of 0.95 eV, Ba2 BiFeSe5 shows photoelectric properties with a photocurrent density of approximately 30 mA cm2 at 5 V. Our study demonstrates that Ba2 FeBiSe5 might be a new type of multifunctional material that deserves further investigation.

Investigations on the electronic transport and piezoresistivity properties of Ni2−XMn1+XGa (X = 0 and 0.15) Heusler alloys under hydrostatic pressure

The resisitivity of Ni2−XMn1+XGa (X = 0 and 0.15) magnetic shape memory alloys has been investigated as a function of temperature (4–300 K) and hydrostatic pressure up to 30 kilobars. The resistivity is suppressed (X = 0) and enhanced (X = 0.15) with increasing pressure. A change in piezoresistivity with respect to pressure and temperature is observed. The negative and positive piezoresistivity increases with pressure for both the alloys. The residual resistivity and electron-electron scattering factor as a function of pressure reveal that for Ni2MnGa the electron-electron scattering is predominant, while the X = 0.15 specimen is dominated by the electron-magnon scattering. The value of electron-electron scattering factor is positive for both the samples, and it is decreasing (negative trend) for Ni2MnGa and increasing (positive trend) for X = 0.15 with pressure. The martensite transition temperature is found to be increased with the application of external pressure for both samples.

Effect of pressure on the magnetic and superconducting transitions of GdFe1−xCoxAsO (x = 0, 0.1, 1) compounds

We have investigated the effect of applied pressure (P) on the magnetic and superconducting transitions of GdFeCoxAsO (x = 0, 0.1, 1) compounds by measuring the temperature dependence of resistivity. At ambient pressure, GdFeAsO undergoes a spin-density wave transition at associated with the 3d electrons of Fe. Under external pressure, TN decreases from 128.2 K at 0 GPa to 114.5 K at 2.5 GPa at the rate = 6 K/GPa. Unlike GdFeAsO, the isostructural GdCoAsO exhibits a ferrimagnetic ordering of the Co moments at 76 K which decreases from 76 K to 71 K as pressure increases from 0 to 2.5 GPa. For the GdFeCoAsO sample, the superconducting onset temperature Tcon decreases monotonically from 19 to 17.1 K, and the temperature Tczero at which the resistivity disappears decreases from 16.7 to 10.5 K as pressure increases from 0 to 2.9 GPa. For all the three samples, both the value of resistivity and the strength of electron–electron correlation decrease monotonically with increasing pressure.

γ-MnS films with 3D microarchitectures: comprehensive study of the synthesis, microstructural, optical and magnetic properties

In the present work, for the first time, we report γ-MnS films with a 3D microsphere morphology prepared through a chemical bath deposition technique at low temperature under aqueous conditions. The X-ray diffraction study confirmed the formation of wurtzite phase γ-MnS. Growth temperature induces a change in the morphology of γ-MnS films from microspheres to microflakes, which may be due to the difference in the nucleation process. The obtained γ-MnS microspheres have a surface area of ∼25 m2 g−1. The photoluminescence spectra show a strong yellow emission band centered at about 550 nm. The obtained microsphere ensemble plays a crucial role in influencing the magnetic behavior of γ-MnS films. The highest coercive field (3320 Oe) and saturation magnetization (17 emu g−1) were obtained for γ-MnS microspheres prepared at 60 °C.

Effect of pressure on normal and superconducting state properties of iron based superconductor PrFeAsO0.6F y (y = 0.12, 0.14)

The effect of high pressure (up to 8 GPa) on normal and superconducting state properties of PrFeAsO0.6F0.12, an 1111-type iron based superconductor close to optimal doped region, has been investigated by measuring the temperature dependence of resistivity. Initially, the superconducting transition temperature (Tc) is observed to increase slowly by about 1 K as pressure (P) increases from 0 to 1.3 GPa. With further increase in pressure above 1.3 GPa, Tc decreases at the rate of ~1.5 K/GPa. The normal-state resistivity decreases monotonically up to 8 GPa. We have also measured the pressure dependence of magnetization (M) on the same piece of PrFeAsO0.6F0.12 sample up to 1.1 GPa and observed Tc as well as the size of the Meissner signal to increase with pressure in this low-pressure region. In contrast, for an over-doped PrFeAsO0.6F0.14 sample, magnetization measurements up to 1.06 GPa show that both Tc and the Meissner signal decrease with pressure. The present study clearly reveals two distinct regions in the dome-shaped (Tc-P) phase diagram of PrFeAsO0.6F0.12.

Possibility for conventional superconductivity in Sr0.1Bi2Se3 from high-pressure transport studies

Sr x Bi 2 Se 3 is recently reported to be a superconductor derived from topological insulator Bi 2 Se 3 . It shows a maximum resistive T c of 3.25 K at ambient pressure. We report magnetic (up to 1 GPa) and transport properties (up to 8 Gpa) under pressure for single crystalline Sr 0.1 Bi 2 Se 3 superconductor. Magnetic measurements show that T c decreases from ∼2.6 K (0 GPa) to ∼1.9 K (0.81 GPa). Similar behavior is observed in transport properties as well without much change in the metallic characteristics in normal-state resistivity. No reentrant superconducting phase (Zhou Y. et al. , Phys. Rev. B , 93 (2016) 144514) is observed at high pressure. Normal-state resistivity near T c is explained by Fermi liquid model. Band structure calculations indicate decreasing density of states at Fermi level with pressure. In consonance with transition temperature suppression in conventional BCS low- T c superconductors, the pressure effect in Sr x Bi 2 Se 3 is well accounted by pressure-induced band broadening.

Superconductivity in La1−x Sm x O0.5F0.5BiS2 (x = 0.2, 0.8) under hydrostatic pressure

We have investigated the pressure effect on the newly discovered samarium-doped La1−x Sm x O0.5F0.5BiS2 superconductors. More than a threefold increase in T c (10.3 K) is observed with external pressure (at ~1.74 GPa at a rate of 4.08 K GPa−1) for x = 0.2 composition. There is a concomitant large improvement in the quality of the superconducting transition. Beyond this pressure T c decreases monotonously at the rate of −2.09 K GPa−1. In the x = 0.8 sample, we do not observe any enhancement in T c with the application of pressure (up to 1.76 GPa). The semiconducting behavior observed in the normal-state resistivity of both samples is significantly subdued with the application of pressure which, if interpreted by invoking the thermal activation process, implies that the activation energy gap of the carriers is significantly reduced with pressure. We believe these observations should generate further interest in La1−x Sm x O0.5F0.5BiS2 superconductors.

Enhanced upper critical field, critical current density, and thermal activation energy in new ytterbium doped CeFeAsO0.9F0.1 superconductor

In this report, we have investigated the essential physical properties of Ce0.7Yb0.3FeAsO0.9F0.1 superconductor such as field dependent critical current density (Jc), thermal activation energy (U0), and upper critical field (Hc2). From the isothermal magnetization curves and size of the superconducting grains, the critical current density Jc of 2.3 × 106 A/cm2 at 2 K, 0.5 T was estimated using the Beans model for this Yb doped superconductor. A gradual decrease of Jc and absence of peak effect were found on increasing magnetic field up to 5 T. Thermal activation energy (U0/kB = ∼2500 K) calculated from Arrhenius plots at low magnetic field (0.5 T) indicates a strong flux pinning potential might be co-existing with applied magnetic field. Our results suggest that this new Yb doped superconductor is a possible practical high temperature superconductor under certain magnetic field and temperature.