Santhosh Raj

Bio: Santhosh Raj is an academic researcher. The author has contributed to research in topics: Magnetoresistance & Transition temperature. The author has an hindex of 2, co-authored 3 publications receiving 12 citations.

Aging effect in magnetotransport property of oxygen adsorbed BaFe2As2

Abstract: Presence of oxygen (O2) has been found by Energy Dispersive X-ray Analysis (EDAX) on the surfaces of flux grown BaFe2As2 single crystals which were kept in air ambience for several months Transport studies show that the O2 adsorbed crystals are more resistive and do not display any sharp slope change near 140 K which is the well known Spin Density Wave (SDW) transition temperature (TSDW) accompanying structural transition for as grown BaFe2As2 An anomalous slope change in resistivity is observed around 18 K at 0 and 5T Magnetoresistance (MR) is noticed to increase as a function of applied field (H) quite differently than that for as grown crystals below TSDW which may be attributed to aging effect

Aging effect in Magnetotransport Property of Oxygen adsorbed BaFe2As2

Abstract: Presence of Oxygen (O2) has been found by Energy Dispersive X-ray Analysis (EDAX) on the surfaces of flux grown BaFe2As2 single crystals which were kept in air ambience for several months. Transport studies show that the O2 adsorbed crystals are more resistive and do not display any sharp slope change near 140 K which is the well known Spin Density Wave (SDW) transition temperature (TSDW) accompanying structural transition for as grown BaFe2As2. An anomalous slope change in resistivity is observed around 18 K at 0 and 5T. Magnetoresistance (MR) is noticed to increase as a function of applied field (H) quite differently than that for as grown crystals below TSDW which may be attributed to aging effec

The effects of post-growth annealing on the structural and magnetic properties of BaFe2As2.

Abstract: We investigate the effects of post-growth annealing on the structural and magnetic properties of BaFe2As2. Magnetic susceptibility measurements, which exhibit a signal corresponding to the magnetic phase transition, and high-resolution x-ray diffraction measurements, which directly probe the structural order parameter, show that annealing causes the ordering temperatures of both the phase transitions to increase, sharpen and converge. In the as grown sample, our measurements show two distinct transitions corresponding to structural and magnetic ordering, which are separated in temperature by approximately 1 K. After 46 days (d) of annealing at 700 °C, the two become concurrent in temperature. These measurements demonstrate that the structural phase transition is second-order like when the magnetic and structural phase transitions are separated in temperature, and first-order like when the two phase transition temperatures coincide. This observation indicates that annealing causes the system to cross a hitherto undiscovered tricritical point. In addition, x-ray diffraction measurements show that the c-axis lattice parameter increases with annealing up to 30 d, but remains constant for longer annealing times. Comparisons of BaFe2As2 to SrFe2As2 are made when possible.

Aging effect in Magnetotransport Property of Oxygen adsorbed BaFe2As2

Abstract: Presence of Oxygen (O2) has been found by Energy Dispersive X-ray Analysis (EDAX) on the surfaces of flux grown BaFe2As2 single crystals which were kept in air ambience for several months. Transport studies show that the O2 adsorbed crystals are more resistive and do not display any sharp slope change near 140 K which is the well known Spin Density Wave (SDW) transition temperature (TSDW) accompanying structural transition for as grown BaFe2As2. An anomalous slope change in resistivity is observed around 18 K at 0 and 5T. Magnetoresistance (MR) is noticed to increase as a function of applied field (H) quite differently than that for as grown crystals below TSDW which may be attributed to aging effec

Investigation of Oxygen-Adsorbed Iron Pnictide Crystals

Abstract: In this present study, we synthesized a new compound of YbFe2As2 crystals by using a melt growth technique. The YbFe2As2 crystals had been characterized by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX). The presence of oxygen was found by EDAX on the surfaces of grown YbFe2As2 crystals which had been kept in air ambience for few months. The measurement of magnetization (M) versus temperature (T) using a superconducting quantum interference device (SQUID) at constant magnetic field (H = 100 Oe) for oxygen-adsorbed YbFe2As2 (YbFe2As2:O2) had revealed an occurrence of sharp slope change around 140 K. An additional slope change had been observed around 40 K. We had carried out magnetization and transport measurements for oxygen-adsorbed YbFe2As2 (YbFe2As2:O2) and oxygen-adsorbed BaFe2As2 (BaFe2As2:O2) for comparative study also. M versus T data at H = 10,000 Oe had exhibited a paramagnetic behavior for both YbFe2As2:O2 and BaFe2As2:O2. The result of M versus H measurements at 2 K had shown that the saturation had not been achieved for YbFe2As2:O2 at H = 80,000 Oe. There was a slope change observed in transport measurement for YbFe2As2:O2 at 15 K which was not noticed for BaFe2As2:O2.

Structural and chemical properties of superconducting Co-doped BaFe2As2 thin films grown on CaF2

Abstract: Thin films of Co-doped BaFe$_2$As$_2$ of similar thickness (~40 nm) were grown with different growth rates (0.4 A s$^{-1}$ and 0.9 A s$^{-1}$) by pulsed laser deposition on CaF$_2$(001) substrates. Analytical transmission electron microscopy (TEM) was applied to analyze the microstructure and secondary phases. The formation of BaF$_2$ and a high concentration of planar defects (mainly stacking faults) are observed for the sample grown at a low rate. A higher growth rate results in high-quality epitaxial films with only few antiphase boundaries. A higher $T_\text{c}$ was measured for the sample grown at a low growth rate, which is attributed to the difference in strain state induced by the high concentration of defects. Large crystalline Fe precipitates are observed in both samples. Chemical analysis shows a pronounced O and slight F content at the planar defects which highlights the role of O in defect formation. Electron-beam-induced irradiation damage during TEM measurements is observed and discussed.

X-ray photoelectron spectroscopy study on YbFe2As2 crystals prepared by different growth temperatures

Abstract: Polycrystalline samples of YbFe2As2 were synthesized by double step solid-state reaction method. Two different samples were grown at the temperatures of 900 and 1200 °C. The synthesized YbFe2As2 polycrystalline samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX) and X-ray photoelectron spectroscopy (XPS). SEM images revealed that the YbFe2As2 compound was formed as polycrystalline with typical sizes of granules ranging from 5 to 10 μm. The Rietveld refinement of the XRD data of YbFe2As2 crystals was performed to understand the structural phase. The oxygen adsorption in to the surface of YbFe2As2 crystals was studied by means of XPS analysis. A possible explanation was given for the synthesis temperature dependent oxygen adsorption in to the YbFe2As2 based on the results of XPS, EDAX and resistance versus temperature measurements.