Dr Dharmalingam Prabhakaran

Abstract:

Electronic and magnetic properties of the charge ordered phase of LuFe2 O4 are investigated by means of x-ray spectroscopic and theoretical electronic structure approaches. LuFe2 O4 is a compound showing fascinating magnetoelectric coupling via charge ordering. Here, we identify the spin ground state of LuFe2 O4 in the charge ordered phase to be a 2:1 ferrimagnetic configuration, ruling out a frustrated magnetic state. An enhanced orbital moment may enhance the magnetoelectric coupling. Furthermore, we determine the densities of states and the corresponding correlation potentials by means of x-ray photoelectron and emission spectroscopies, as well as electronic structure calculations. © 2009 The American Physical Society.

Abstract:

The magnetic structures which endow TbMnO3 with its multiferroic properties have been reassessed on the basis of a comprehensive soft x-ray resonant scattering (XRS) study. The selectivity of XRS facilitated separation of the various contributions (Mn L2 edge, Mn 3d moments; Tb M4 edge, Tb 4f moments), while its variation with azimuth provided information on the moment direction of distinct Fourier components. When the data are combined with a detailed group theory analysis, a new picture emerges of the ferroelectric transition at 28 K. Instead of being driven by the transition from a collinear to a noncollinear magnetic structure, as has previously been supposed, it is shown to occur between two noncollinear structures. © 2009 The American Physical Society.

Abstract:

Photoemission data taken with hard x-ray radiation on cleaved single crystals of the bilayered, colossal magnetoresistant manganite La 2-2xSr 1+2xMn 2O 7 (LSMO) with 0.30≤x<0.50 are presented. Making use of the increased bulk sensitivity upon hard x-ray excitation it is shown that the core-level footprint of the electronic structure of the LSMO cleavage surface is identical to that of the bulk. Furthermore, by comparing the core-level shift of the different elements as a function of doping level x, it is shown that microscopic phase separation is unlikely to occur for this particular manganite well above the Curie temperature. © 2009 The American Physical Society.

Abstract:

A detailed spectroscopic and structural characterization of ultrathin cobalt oxide films grown by O-assisted molecular-beam epitaxy on α- Al22 O3 (0001) single crystals is reported. The experimental results show that the cobalt oxide films become progressively more disordered with increasing thickness, starting from the early stages of deposition. Low-energy electron-diffraction patterns suggest that the unit cell remains similar to that of α -Al22 O3 (0001) up to a thickness of 17Å, while at larger thicknesses a pattern identified with that of Co3 O4(111) becomes visible. X-ray photoelectron spectroscopy reveals sudden changes in the shape of the Co2p lines from 3.4 to 17Å cobalt oxide thickness, indicating the transition from an interfacial cobalt oxide layer toward [111]-oriented Co3 O4. In particular, the absence of characteristic satellite peaks in the Co 2p lines indicates the formation of a trivalent, octahedrally coordinated, interfacial cobalt oxide layer during the early stages of growth, identified as the Co 3 O4 corundum phase. © 2009 The American Physical Society.

Abstract:

Spin-ice materials are magnetic substances in which the spin directions map onto hydrogen positions in water ice. Their low-temperature magnetic state has been predicted to be a phase that obeys a Gauss' law and supports magnetic monopole excitations: in short, a Coulomb phase. We used polarized neutron scattering to show that the spin-ice material Ho2Ti2O7 exhibits an almost perfect Coulomb phase. Our result proves the existence of such phases in magnetic materials and strongly supports the magnetic monopole theory of spin ice.