Dr Dharmalingam Prabhakaran

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.

Abstract:

Resonant X-ray scattering measurements have been performed at the (011) Bragg forbidden reflection of YVO3 at the vanadium K-edge. Data were taken above and below the orbital, magnetic and structural transition occurring at 77 K. Energy spectra calculated by the FDMNES code are shown to be in excellent agreement with our experimental data, conclusively showing the signal to originate from anisotropic tensor of susceptibility scattering; ie. solely due to distortions of the lattice and hence the crystal field. We thus resolve the ambiguous origin of the resonant energy spectra in the literature. © 2010 IOP Publishing Ltd.