Dr Dharmalingam Prabhakaran

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.

Abstract:

We investigate the spin state of LaCo O3 using state-of-the-art photoemission spectroscopy and ab initio band structure calculations. The GGA+U calculations provide a good description of the ground state for the experimentally estimated value of electron correlation strength U. In addition to the correlation effect, spin-orbit interaction is observed to play a significant role in the case of intermediate spin and high spin configurations. The comparison of the calculated Co 3d and O 2p partial density of states with the experimental valence band spectra indicates that at room temperature, Co has dominant intermediate spin state configuration and that the contribution from high spin configuration may not be significant at this temperature. The line shape of the La 5p and O 2s core level spectra could be reproduced well within these ab initio calculations. © 2008 The American Physical Society.

Abstract:

We report the complex refractive index of La5/3Sr 1/3NiO4 over the terahertz frequency range, obtained using time-domain spectroscopy. Negligible change in the complex refractive index with magnetic flux densities up to 6 T was seen, while changes were observed as the lattice temperature was increased from 1.5K to the charge-ordering temperature at 220 K. The terahertz frequency response therefore originates from the dielectric function rather than the magnetic permeability.

Abstract:

We report on an extensive investigation of the multiferroic compound TbMn O3 using x-ray scattering techniques. Nonresonant x-ray magnetic scattering (NRXMS) was used to characterize the domain population of the single crystal used in our experiments. This revealed that the dominant domain is overwhelmingly A type. The temperature dependence of the intensity and wave vector associated with the incommensurate magnetic order was found to be in good agreement with neutron scattering data. X-ray resonant scattering experiments were performed in the vicinity of the Mn K and Tb L3 edges in the high-temperature collinear phase, the intermediate temperature cycloidal and ferroelectric phase, and the low-temperature phase. In the collinear phase, where according to neutron diffraction only the Mn sublattice is ordered, resonant E1-E1 satellites were found at the Mn K edge associated with A -type but also F -type peaks. Detailed measurements of the azimuthal dependence of the F -type satellites (and their absence in the NRXMS experiments) leads us to conclude that they are most likely nonmagnetic in origin. We suggest instead that they may be associated with an induced charge multipole. At the Tb L3 edge, resonant A - and F -type satellites were observed in the collinear phase again associated with E1-E1 events. We attribute these to a polarization of the Tb 5d states by the ordering of the Mn sublattice. On cooling into the cycloidal and ferroelectric phase, a new set of resonant satellites appear corresponding to C -type order. These appear at the Tb L3 edge only. In addition to a dominant E1-E1 component in the σ- π′ channel, a weaker component is found in the preedge with σ- σ′ polarization and displaced by -7 eV with respect to the E1-E1 component. Comprehensive calculations of the x-ray scattering cross section were performed using the FDMNES code. These calculations show that the unrotated σ- σ′ component of the Tb L3 C -type peaks appearing in the ferroelectric phase contains a contribution from a multipole that is odd with respect to both space and time, known in various contexts as the anapole. Our experiments thus provide tentative evidence for the existence of a type of anapolar order parameter in the rare-earth manganite class of mulitferroic compounds. © 2007 The American Physical Society.