Dr Dharmalingam Prabhakaran

Abstract:

Ultra-small superparamagnetic iron oxide nanoparticles (SPIOs) were synthesized by co-precipitation of iron chloride salts with ammonia and then encapsulated with thin (~2nm) layers of silica. The particles have been characterized for size, diffraction pattern, surface charge, and magnetic properties. This rapid and economical synthesis has a number of industrial applications; however, the silica-coated particles have been optimized for use in medical applications as MR contrast agents, biosensors, DNA capturing, bioseparation and enzyme immobilization.

Abstract:

We have investigated charge dynamics in the static stripe ordered phase of La2-x Srx Ni O4+δ at lattice temperatures below the charge ordering transition, via optical conductivity measurements at low energies (1-10 meV). The thermally activated dynamic response of the charge stripes is found to be characteristic of a collective mode such as a pinned charge density wave. At incommensurate doping levels, the pinning energy is reduced, owing to the presence of real-space defects in the stripe order, and a pronounced increase in the oscillator strength is seen. The results provide compelling evidence for the existence of low-energy collective modes of the charge stripes. © 2008 The American Physical Society.

Abstract:

An ac photopyroelectric calorimeter has been used to simultaneously measure the specific heat (cp), thermal conductivity (K), and thermal diffusivity (D) around the antiferromagnetic to paramagnetic phase transition in CoO and NiO single crystals. Up to now, no agreement on the critical behavior of both oxides has been obtained. The results for seven samples grown in different laboratories have been compared. We have found that, irrespective of the origin of the samples, the critical exponent and amplitude ratio of cp, D, and K in NiO agree with the predictions of the three-dimensional Heisenberg model for isotropic antiferromagnets, which contradicts previous results. On the other hand, the sharpness of the peak of specific heat and of the dip of thermal diffusivity, together with the value of the critical exponent α in CoO, highly increase as crystal quality improves. For the best sample, a value α=0.81 has been obtained, which is not only higher than previously reported results, but far away from any universality class. © 2008 The American Physical Society.

Abstract:

By means of neutron scattering we have determined new branches of magnetic excitations in orbitally active CoO (TN=290 K) up to 15 THz and for temperatures from 6 to 450 K. Data were taken in the (1 1 1) direction in six single-crystal zones. From the dependence on temperature and Q we have identified several branches of magnetic excitation. We describe a model for the coupled orbital and spin states of Co2+ subject to a crystal field and tetragonal distortion. © 2007.

Abstract:

Polarized- and unpolarized-neutron scattering was used to study the temperature evolution of the magnetic excitations of charge ordered La5/3Sr1/3NiO4. We studied two features in detail: (i) the resonance-like scattering at 27 meV in the quasi-two-dimensional magnetic excitations from the ordered Ni2+ (S = 1) spins, and (ii) the diffuse scattering associated with quasi-one-dimensional antiferromagnetic correlations along the stripes. Although both these features persist up to at least similar to 125 K, their temperature dependences are found to be quite different. We argue from the results that the resonance- like feature observed in La2-xSrxNiO4 (x similar to 1/3) is not caused by a coupling between the spin dynamics of the two magnetic sub-systems.