Dr Dharmalingam Prabhakaran

Abstract:

Nonresonant x-ray magnetic scattering has been used to study the magnetic structure of multiferroic TbMnO3 in its ferroelectric phase. Circularly polarized x rays were combined with full polarization analysis of the scattered beam to reveal important new information on the magnetic structure of this canonical multiferroic. An applied electric field is shown to create essentially a single magnetic domain state in which the cycloidal order on the Mn sublattice rotates either clockwise or anticlockwise depending on the sign of the field. It is demonstrated how this technique provides sensitivity to the absolute sense of rotation of the Mn moments and to components of the ordering on the Tb sublattice and phase shifts that earlier neutron diffraction experiments could not resolve.

Abstract:

The low-temperature magnetic behavior of LaCoO3, containing oxygen vacancies, is reported. Magnetic-susceptibility measurements made in the temperature range of 0.5-35 K on a single crystal and a polycrystalline sample provide strong evidence for the existence of magnetic excitons as fundamental entities within the bulk of the material system. Specifically, two distinct types of excitons form, isolated and interacting excitons, both of which are associated with oxygen vacancies. Isolated magnetic excitons act as high-spin paramagnetic particles while the interacting excitons appear to be coupled antiferromagnetically. It is proposed that the interaction arises from the overlap of magnetic excitons as a consequence of the statistical clustering of oxygen vacancies. The striking similarity of these results with those of the lightly doped La0.97 Sr0.03 CoO3 suggests that the observed excitons are a precursor to magneto-electronic phase separation and supports the idea that phase separation is initiated by disorder in the material system. © 2009 The American Physical Society.

Abstract:

The sodium reordering in Nax CoO2 in the vicinity of room temperature is rationalized at high x in terms of phase transitions between square and striped phases. A striking hexagon-of-hexagons diffraction pattern observed for x=0.78 can be reproduced using coexisting square and striped phases that are related by simple shear deformations. All compositions exhibit a partial melting transition to a disordered stripe phase just below room temperature, which alters the topology of the electrical conduction pathways. © 2009 The American Physical Society.

Abstract:

We have investigated orbital ordering in the bilayer manganite, Pr (Sr0.1 Ca0.9) 2 Mn2 O7, using synchrotron x-ray diffraction techniques, in particular resonant soft x-ray diffraction. Diffraction signals are observed at (0, 1 2, 0) above 300 K and (1 2, 0,0) below 300 K, corresponding to orbital stripes along the a and b axes, respectively. The previously observed transition at 300 K is evidence of a rotation of the orientation of the 3d orbital stripes. Large resonances of the orbital signals are observed at the MnL edges, with a complex dependence on incident photon energy indicative of a weakly Jahn-Teller distorted system. A structural transition observed at 92 K is found not to involve a further change in the orbital order. © 2009 The American Physical Society.

Abstract:

Strongly correlated electron systems give an opportunity to manipulate charge, orbital, magnetic and structural phases of matter. Here we show that the insulating phase where charges are localized can be delocalized through photo-excitation which, in turn changes the structure locally, inducing an orthorhombic to rhombohedral phase transition. The I-M transition was witnessed for La(1-x)Sr(x)MnO(3) compounds in Raman spectra and photo-induced conduction simultaneously. A simple continuous argon ion laser source was used for optical excitation. The photon energy was 2.53 eV and the power can be chosen anywhere between 5 and 45 mW. Our studies clearly bring out the role of local disorder in the form of Jahn-Teller distortion in the localization of electrons.