Dr Dharmalingam Prabhakaran

Abstract:

Commercially produced magnesium diboride powder was modified by attrition milling and ball milling in propan-2-ol for various durations. These powders were characterized by means of particle size distribution measurements using laser diffraction, impurity analysis using x-ray diffraction, energy dispersive spectroscopy and x-ray photoelectron spectroscopy, and scanning electron microscopy, and were then used to produce magnesium diboride samples through pressureless heat treatment at peak temperatures up to 1100 °C. X-ray diffraction, scanning electron microscopy, Vickers hardness measurements, and density measurements using the Archimedes method were used to characterize the properties of the samples, and a determination of their superconducting properties using the magnetization method was carried out. Magnesium diboride produced from powder milled under certain conditions had a higher J c, mag than samples produced from as-purchased powder, but the relationship between the milling duration or energy and the superconducting performance is a complex one, affected by both the particle size and the impurity content of the starting powder. © 2010 IOP Publishing Ltd.

Abstract:

The temperature dependence of magnetization (M) and resistivity (ρ) of La1-xSrxCoO3(x=0.10, 0.33) single crystals have been analyzed. For x=0.10, the temperature dependence of field-cooled magnetization (MFC) and zero-field-cooled magnetization (M ZFC) is similar to that expected for a canonical spin-glass system. The thermal response of MZFC for x=0.33 indicates a glassy ferromagnetic state. We observe that the ferromagnetic transition temperature TC decreases and ρ increases rapidly with increasing pressure (P) for the metallic sample (x=0.33), while the dependence of ρ on P for the insulating sample (x=0.10) is quite complicated; the pressure coefficient of resistivity (dρ/dT) is sensitive to temperature and applied pressure due to the strong interplay between the pressure-induced band broadening and spin-state transition phenomenon. dρ/dT is large and negative at low-pressure and low-temperature regime while small and positive at high pressures (P>5.4 GPa) and high temperatures (T>110 K). © 2010 IOP Publishing Ltd.

Abstract:

Magnetic and phonon excitations in the antiferromagnet CoO with an unquenched orbital angular momentum are studied by neutron scattering. Results of energy scans in several Brillouin zones in the (HHL) plane for energy transfers up to 16 THz are presented. The measurements were performed in the antiferromagnetic ordered state at 6 K (well below TN ∼290 K) as well as in the paramagnetic state at 450 K. Several magnetic excitation modes are identified from the dependence of their intensity on wavevector and temperature. Within a Hund's rule model, the excitations correspond to fluctuations of coupled orbital and spin degrees of freedom, whose bandwidth is controlled by interionic superexchange. The different <111> ordering domains give rise to several magnetic peaks at each wavevector transfer.

Abstract:

A superconducting ring could be used to simulate the supercurrent conduction, and its interaction with magnetic vortices in a superconducting solenoid, allowing one to investigate the nature of the vortex structure and its pinning in the presence of the persistent supercurrent. The dissipation of the persistent supercurrent has been studied in a ring-shaped high purity single crystal of Bi2 Sr2 CaCu2 Ox in order to obtain the information about the exponent μ, a parameter in the scaling relation between the effective energy barrier against vortex motion U effand the persistent current density J. The measurements of the persistent supercurrent decay show a transition from a strongly nonlogarithmic to a logarithmic decay regime with an increasing temperature. In response to a small increase in the concentration of oxygen vacancies μ decreases in the logarithmic decay regime but remains almost constant in the nonlogarithmic one. © 2010 American Institute of Physics.

Abstract:

We present the results of muon-spin relaxation (μ+ SR) measurements on the hexagonal manganite HoMnO3. Features in the temperature-dependent relaxation rate λ correlate with the magnetic transitions at 76, 38, and 34 K. The highest temperature transition, associated with the ordering of Mn3+ moments has the largest effect on λ. The application of a static electric field of E=5× 106 Vm-1 below T=50K causes a small reduction in λ which is suggestive of coupling between ferroelectric and magnetic domain walls in the ordered state of the material. © 2010 The American Physical Society.