Pranab Choudhury

Ferromagnetism in nanoscale BiFeO3

A remarkably high saturation magnetization of ∼0.4μB∕Fe along with room temperature ferromagnetic hysteresis loop has been observed in nanoscale (4–40nm) multiferroic BiFeO3 which in bulk form exhibits weak magnetization (∼0.02μB∕Fe) and an antiferromagnetic order. The magnetic hysteresis loops exhibit exchange bias and vertical asymmetry which could be because of spin pinning at the boundaries between ferromagnetic and antiferromagnetic domains. Interestingly, both the calorimetric and dielectric permittivity data in nanoscale BiFeO3 exhibit characteristic features at the magnetic transition point. These features establish the formation of a true ferromagnetic-ferroelectric system with a coupling between the respective order parameters in nanoscale BiFeO3.

Large magnetocaloric effect in Sm0.52Sr0.48MnO3 in low magnetic field

This letter reports on the magnetocaloric properties of Sm0.52Sr0.48MnO3 single crystal. A magnetic field of only 1T yields a change in the magnetic entropy by 5.9J∕(kgK) at TC (=124K), which is higher than those observed in several other perovskite manganites and rare earth alloys of comparable TC. This change originates from a sharp magnetization jump, associated with a first-order metamagnetic transition. Such a large change in entropy at a low magnetic field makes this material useful for magnetic refrigeration.

Multiferroic coupling in nanoscale BiFeO3

Using the results of x-ray and neutron diffraction experiments, we show that the ferroelectric polarization, in ∼22 nm particles of BiFeO3, exhibits a jump by ∼30% around the magnetic transition point TN (∼635 K) and a suppression by ∼7% under 5T magnetic field at room temperature (≪TN). These results confirm the presence of strong multiferroic coupling even in nanoscale BiFeO3 and thus could prove to be quite useful for applications based on nanosized devices of BiFeO3.

Particle size dependence of magnetization and noncentrosymmetry in nanoscale BiFeO3

The saturation magnetization (MS), antiferromagnetic transition point (TN), and the off-center displacements of Bi and Fe ions have been measured as a function of particle size in nanoscale BiFeO3. TN decreases down to ∼550 K for particles of size ∼5 nm from ∼653 K in bulk, while MS rises by more than an order of magnitude. Analysis of crystallographic structure from Rietveld refinement of x-ray diffraction patterns shows significant rise in off-center displacements of Bi (δBi) and Fe (δFe) ions within a unit cell with the decrease in particle size. The net unit-cell polarization PS too, is found to be larger in nanoscale regime.

Excess conductivity and thermally activated dissipation studies in Bi2Sr2Ca1Cu2Ox single crystals

Abstract The resistivity of Bi 2 Sr 2 Ca 1 Cu 2 O x single crystals has been measured in the ab -plane and along the c -axis. In all the directions the resistivity shows metallic behavior in the normal state. Room temperature resistivity along the c -direction is about 64 times higher than that in the ab -plane. Departure from the linear behavior occurs at 96 K and the zero resistance is obtained at 76 K. Excess conductivity analysis shows that the system is 2-dimensional in the temperature range 78.7–96 K and a 3-dimensional one below 78.7 K. Thus, a crossover from 2D to 3D is observed at 78.7 K in the Bi-2212 single crystal. This crossover was not detected in Bi 2212 or 2223 polycrystalline samples. The width of the transition (Δ T ) varies with the application of magnetic field ( H ) as H 0.85 in Bi 2212 single crystal. The pinning potential ( U 0 ) varies as H -α where α =0.3 for the Bi-2212 single crystal and 0.47 for polycrystalline sample.

Electrical resistivity, magnetoresistance, magnetisation, hall coefficient and excess conductivity in Pb-doped Bi-Sr-Ca-Cu oxides

The electrical resistance, Meissner signal, magnetoresistance and Hall coefficient of Bi1.75Pb0.25Sr2Ca2Cu3Ox and Bi1.5Pb0.5Sr2Ca2Cu3Ox (nominal compositions) have been measured. Resistance of the Bi1.75Pb0.25 sample becomes zero for T≤106 K while the Bi1.5Pb0.5 sample shows a 65% drop in the resistance around 106 K and TR = 0c ≈ 73 K. Powder X-ray diffraction analysis reveals that the major phase in the Bi1.75Pb0.25 sample is the high-Tc phase and in the Bi1.5 Pb0.5 sample the low-Tc phase. The Hall coefficients (RH) of both samples are positive with R−1H> linear in temperature. The temperature dependence of RH is stronger in the Bi1.75Pb0.25 sample than in the Bi1.5Pb0.5 sample. The carrier concentration determined R−1H for the Bi1.75Pb0.25 and the Bi1.5Pb0.5 samples at 300 K are 1.59 × 1021 cm−3 and 3.66 × 1021 cm−3, respectively. The excess conductivity of both samples is analyzed using the Aslamazov-Larkin expression. The critical exponent λ obtained for the Bi1.75Pb0.25 sample is 0.78 and that for the Bi1.5Pb0.5 sample, in the temperature region where the high-Tc phase contributes only, is 0.77.

Resistivity saturation in PrFeAsO1−xFy superconductor: evidence of strong electron–phonon coupling

We have measured the resistivity of PrFeAsO1−xFy samples over a wide range of temperature in order to elucidate the role of electron–phonon interaction on normal- and superconducting-state properties. The linear T dependence of ρ above 170 K followed by a saturation-like behavior at higher temperature is a clear signature of strong electron–phonon coupling. From the analysis of the T dependence of ρ, we have estimated several normal-state parameters that are useful for understanding the origin of superconductivity in this system. Our results suggest that Fe-based oxypnictides are phonon-mediated BCS superconductors like Chevrel phases and A15 compounds.

Magnetotransport properties of La2/3Sr1/3MnO3 thin film

We report the temperature and magnetic field dependence of the resistivity of La2/3Sr1/3MnO3 thin film over a wide temperature range and in high magnetic fields up to 20 T. Both above and below the ferromagnetic (FM) transition, the field dependence of the magnetoresistance (MR) can be described well using the extended Mott hopping conduction model. The large MR is proportional to the Brillouin function BJ in the FM phase and to BJ2 in the paramagnetic state which is consistent with this model. The value of spin moment J deduced from the temperature and magnetic field dependence of the MR is large (∼60) in the vicinity of the FM transition. This large value of J has been attributed to the presence of magnetic clusters due to the short-range FM ordering.

Excess conductivity in Tl1Ca3Ba1Cu3Oχ and Bi1Ca1Sr1Cu2Oχ systems due to thermodynamic fluctuations

The excess conductivity (Δσ) in Tl1Ca3Ba1Cu3Oχ and Bi1Ca3Sr1Cu2Oχ (nominal compositions) is determined using the linear extrapolation method and Anderson-Zou formula. Δσ for the TI-system shows a 2-dimensional character and is well described by the Aslamazov-Larkin (A-L) contribution for a layered superconductor. The interlayer coupling for the TI-system is found to be very small. The excess conductivity in a magnetic field perpendicular to the current direction Δσ⊥(H), for T≤Tc and H≥62; Hc2 shows a 3-dimensional character. Similar behavior is also observed in the Bi-system.

Quantum magnetoresistance of the PrFeAsO oxypnictide

We report the observation of an unusual