Brajesh Tiwari

HoCrO3 and YCrO3: a comparative study

A comparative study on structural, optical and magnetic properties of orthorhombic perovskite-like isostructural compounds HoCrO3 and YCrO3 was carried out, wherein Ho(3(+)) is a magnetic ion and Y(3+) is non-magnetic. We found almost identical structural parameters for both compounds, however a significant difference was observed in low wavenumber Raman active phonon modes. The effect of local field on optical transitions of Cr ion is studied by diffused reflectance spectroscopy. In order to understand the effect due to the Ho(3(+)) local magnetic field on the magnetic ordering of Cr(3(+)) in ACrO3 (A=Ho, Y), dc magnetization measurements were carried out in a temperature range of 5-300 K and several isotherms were analyzed. Magnetization measurements reveal that Cr(3+) magnetic ordering temperature is insensitive to local fields (due to Ho(3+)) and very sensitive to thermal cycling in the presence of an external applied magnetic field. This behavior is related to magnetic disorder near the phase transition due to weak ferromagnetism and/or structural distortions which we observed as thermal hysteresis. The importance of this work lies with the fact that both Ho(3+) (10.6 μB) and Y(3+) (0 μB) have identical ionic radii and the electron correlation effects in both the systems have been studied using different experimental techniques.

Appearance of bulk superconductivity under hydrostatic pressure in Sr0.5RE0.5FBiS2 (RE = Ce, Nd, Pr, and Sm) compounds

We report the appearance of superconductivity under hydrostatic pressure (0–2.5 GPa) in Sr0.5RE0.5FBiS2 with RE = Ce, Nd, Pr, and Sm. The studied compounds, being synthesized by solid state reaction route, are crystallized in tetragonal P4/nmm space group. At ambient pressure, though the RE = Ce exhibits the onset of superconductivity below 2.7 K, the Nd, Pr, and Sm samples are not superconducting down to 2 K. With application of hydrostatic pressure (up to 2.5 GPa), superconducting transition temperature (Tc) is increased to around 10 K for all the studied samples. The magneto-resistivity measurements are carried out on all the samples under 2.5 GPa pressure and their upper critical fields (Hc2) are determined. The superconductivity of these compounds appears to be quite robust against magnetic field. Summarily, the Sr0.5RE0.5FBiS2 compounds with RE = Ce, Nd, Pr, and Sm are successfully synthesized and superconductivity is induced in them under hydrostatic pressure.

Impact of Hydrostatic Pressure on Superconductivity of Sr0.5La0.5FBiS2

We report the impact of hydrostatic pressure (0–1.97 GPa) on superconductivity of recently discovered 2 K superconductor Sr0.5La0.5FBiS2. Resistivity under pressure measurements are performed by using HPC-33 Piston type pressure cell with Quantum design DC resistivity Option. The superconducting transition temperature (Tc) is increased by 5 fold to around 10 K with just above 1 GPa pressure, which remains nearly unaltered for studied higher pressures of up to 1.97 GPa. The fivefold increase in Tc accompanied with decrease in normal state resistivity of Sr0.5La0.5FBiS2 with just above 1 GPa pressure calls for the attention of solid state physics community.

Mechanical milling assisted synthesis of Ba?Mn co-substituted BiFeO3 ceramics and their properties

Samples of composition Bi1−xBaxFe1−xMnxO3 (x = 0, 0.1, 0.2) were synthesized by initial mixing of precursors by high-energy ball milling and subsequent sintering of the milled powders. The co-substitution of Ba–Mn controls the formation of impurity phases, as evident from x-ray diffraction analysis. Evidence of Fe in mixed oxidation states of +3 and +2 and Mn in +3 state is found from x-ray photoelectron spectroscopy. Electron microscopy exhibits a decrease in grain size due to inhibition in grain growth by Ba–Mn co-substitution. The magnetization value at 20 kOe increases as the percentage of substitution increases. The x = 0.2 sample exhibits a comparable and stable resistivity curve in the experimental temperature range and has a higher value of remanent polarization (Pr) when compared with the x = 0 sample.

Robust superconductivity with large upper critical field in Nb2PdS5

We report synthesis, structural details, and complete superconducting characterization of very recently discovered [Q. Zhang, Sci. Rep. 3, 1446 (2013)] Nb2PdS5 new superconductor. The synthesized compound is crystallized in monoclinic structure with C2/m (#12) space group. Bulk superconductivity is seen in both ac/dc magnetic susceptibility and electrical resistivity measurements with superconducting transition temperature (Tc) at 6 K. The upper critical field (Hc2) being estimated from high field magneto-transport [ρ(T)H] measurements is above 240 kOe. The estimated Hc2(0) is clearly above the Pauli paramagnetic limit of ∼1.84Tc. Heat capacity (Cp) measurements show clear transition with well defined peak at Tc, but with lower jump than as expected for a Bardeen–Cooper–Schrieffer type superconductor. The Sommerfeld constant (γ) and Debye temperature (ΘD) as determined from low temperature fitting of CP(T) data are 32 mJ/mole-K2 and 263 K, respectively. Hall coefficients and resistivity in conjugation wit...

Integration of perovskite PZT thin films on diamond substrate without buffer layer

Integration of lead zirconate titanate (PZT) thin film on diamond substrate offers a great deal of potential for the application of multifunctional devices under extreme conditions. However, fabrication of perovskite PZT thin films on diamond substrate without a buffer layer has not been realized to date. We report for the first time on the successful deposition of PZT thin film directly on a diamond substrate without any buffer layer using the pulsed-laser deposition technique. The perovskite phase was realized only under specific growth conditions. X-ray diffraction and Raman studies confirmed the perovskite phase. The ferroelectric behaviour of the deposited PZT thin film was confirmed using piezo response microscope phase image and ferroelectric hysteresis loop.

Specific Heat of Robust Nb2PdS5 Superconductor

We report specific heat under different magnetic fields for recently discovered quasi-one-dimensional Nb2PdS5 superconductor. The studied compound is superconducting below 6 K. Nb2PdS5 is quite robust against magnetic field with dHc/dT of −42 kOe/K. The estimated upper critical field [ Hc2(0)] is 190 kOe, clearly surpassing the Pauli-paramagnetic limit of 1.84 Tc. Low-temperature heat capacity in superconducting state of Nb2PdS5 under different magnetic fields showed s-wave superconductivity with two different gaps. Two quasilinear slopes in Somerfield coefficient (γ) as a function of applied magnetic field and two-band behavior of the electronic heat capacity demonstrate that Nb2PdS5 is a multiband superconductor in weak coupling limit with ΔC/ γTc = 0.9.

Dielectric and optical phonon anomalies near antiferromagnetic ordering in LaCrO3: A possible near room temperature magnetodielectric system

We report on anomalies in the dielectric and optical phonon spectra in bulk orthorhombic distorted perovskite LaCrO3 near an antiferromagnetic transition at 290 K. These anomalies were tracked using temperature dependent (200 K–320 K) dielectric and Raman spectroscopy. We observed a blue-shift for the B3g(3) optical phonon mode coincident with the onset of antiferromagnetic ordering of LaCrO3 in conjunction with a broad dielectric anomaly near the Neel temperature. The dielectric and optical phonon anomalies are discussed in the context of the observed antiferromagnetic transition in temperature dependent magnetic studies.

Anomalous room temperature magnetoresistance in brownmillerite Ca2Fe2O5

We demonstrate the hitherto unreported field induced magnetoresistance in wet chemical synthesized bulk polycrystalline brownmillerite Ca2Fe2O5. A magnetoresistance of 12% is observed at room temperature, well below its antiferromagnetic transition temperature of 775 K. The variation in resistivity with increasing and decreasing magnetic field shows a significant memory effect. When synthesized in a nanostructured form it exhibits weak ferromagnetism due to the appearance of an Fe oxide phase, besides exhibiting a shift to a higher antiferromagnetic Neel temperature of 820 K. Spectroscopic investigations (X-ray photoelectron and Raman spectroscopy) as well as magnetic measurements have been employed to investigate the above behavior of brownmillerite Ca2Fe2O5.

Magnetostructural and magnetocaloric properties of bulk LaCrO3 system

We studied magnetic properties of bulk LaCrO3; a GdFeO3-type distorted perovskite, with a predominant antiferromagnetic phase transition at ?290 K. The bulk LaCrO3 exhibits intrinsic weak ferromagnetism at room temperature, which may arise due to the tilting of CrO6 octahedra, resulting in a non-zero net magnetic moment, as confirmed from the magnetization measurements. A broad magnetically-induced entropy change (??S) is observed with the maxima at 290 K, close to room temperature in LaCrO3 system.