Dr Dharmalingam Prabhakaran

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.

Abstract:

We describe neutron-scattering experiments performed to investigate the magnetic order and dynamics of half-doped La1.5Sr0.5CoO4. This layered perovskite exhibits a near-ideal checkerboard pattern of Co2+/Co3+ charge order at temperatures below ~800 K. Magnetic correlations are observed at temperatures below ~60 K but the magnetic order only becomes established at 31 K, a temperature at which a kink is observed in the susceptibility. On warming above 31 K we observed a change in the magnetic correlations which we attribute either to a spin canting or to a change in the proportion of inequivalent magnetic domains. The magnetic excitation spectrum is dominated by an intense band extending above a gap of approximately 3 meV up to a maximum energy of 16 meV. A weaker band exists in the energy range of 20–30 meV. We show that the excitation spectrum is in excellent quantitative agreement with the predictions of a spin-wave theory generalized to include the full magnetic degrees of freedom of high-spin Co2+ ions in an axially distorted crystal field, coupled by Heisenberg exchange interactions. The magnetic order is found to be stabilized by dominant antiferromagnetic Co2+–Co2+ interactions acting in a straight line through Co3+. No evidence is found for magnetic scattering from the Co3+ ions, supporting the view that Co3+ is in the S=0 state in this material.

Abstract:

The transport of electrically charged quasiparticles (based on electrons or ions) plays a pivotal role in modern technology as well as in determining the essential functions of biological organisms. In contrast, the transport of magnetic charges has barely been explored experimentally, mainly because magnetic charges, in contrast to electric ones, are generally considered at best to be convenient macroscopic parameters, rather than well-defined quasiparticles. However, it was recently proposed that magnetic charges can exist in certain materials in the form of emergent excitations that manifest like point charges, or magnetic monopoles. Here we address the question of whether such magnetic charges and their associated currents-'magnetricity'-can be measured directly in experiment, without recourse to any material-specific theory. By mapping the problem onto Onsager's theory of electrolytes, we show that this is indeed possible, and devise an appropriate method for the measurement of magnetic charges and their dynamics. Using muon spin rotation as a suitable local probe, we apply the method to a real material, the 'spin ice' Dy(2)Ti(2)O(7) (refs 5-8). Our experimental measurements prove that magnetic charges exist in this material, interact via a Coulomb potential, and have measurable currents. We further characterize deviations from Ohm's law, and determine the elementary unit of magnetic charge to be 5 mu(B) A(-1), which is equal to that recently predicted using the microscopic theory of spin ice. Our measurement of magnetic charge and magnetic current establishes an instance of a perfect symmetry between electricity and magnetism.

Abstract:

We present the magnetic phase diagram of multiferroic TbMnO3 for H parallel to b determined using X-ray resonant scattering at the Tb L-3-edge. Investigations of two different magnetic domains, C-type and F-type, demonstrate very similar behaviour in high applied magnetic fields, with a low temperature incommensurate to commensurate transition at H similar or equal to 4.5T, concurrent with the polarisation flop observed in bulk magnetoelectric measurements. (C) 2009 Elsevier B.V. All rights reserved.