Dr Dharmalingam Prabhakaran

Abstract:

The growth of single crystals of solid solutions of LiNbO 3 and LiTaO 3 has been studied by using a number of different techniques: Czochralski pulling, Top-Seeded Solution Growth, flux growth from LiBO 2 and LiVO 3, and by optical floating zone. Crystals with homogenous Li, Ta and Nb compositions were obtained by several techniques. Analysis of local and average compositions of Ta, Nb and Li by Raman spectroscopy, birefringence imaging and X-ray diffraction, in addition to EDAX, is described. The effects of replacing Nb by Ta and of Li non-stoichiometry on the lattice parameters and the compositional disorder over the whole composition range 0 ≤ x ≤ 1 are discussed. © 2012 Elsevier B.V. All rights reserved.

Abstract:

We have investigated the critical behavior of ferromagnetic La 0.75Sr 0.25CoO 3 and La 0.79Sr 0.21CoO 3 single crystals from the bulk magnetization measurements around their Curie temperature (T C). The detailed analysis of the dc magnetization data using different techniques such as the Kouvel-Fisher, the Arrott-Noaks, and critical isotherm plots yield the critical exponents of β=0.362±0.002, γ=1.304±0.006, and δ=4.75±0.01 with T C=213.93±0.02 K for La 0.75Sr 0.25CoO 3 and β=0.491±0.004, γ=1.217±0.003, and δ=3.51±0.01 with T C=187.67±0.01 K for La 0.79Sr 0.21CoO 3, characterizing these second-order phase transitions. For both the crystals, the scaling of the magnetization data above and below T C obtained using the respective critical exponents and the consistency in the values of the critical exponents determined by different methods confirm that the calculated exponents are unambiguous and intrinsic. The obtained values of exponents suggest that for La 0.75Sr 0.25CoO 3 the transition falls into the three-dimensional Heisenberg universality class of the near-neighbor interaction as proposed for double-exchange systems, whereas in the case of La 0.79Sr 0.21CoO 3 the transition is characterized by mean-field-like values of the critical exponents. We have also estimated the reduced critical amplitudes and observed that for La 0.75Sr 0.25CoO 3 they fall well within the range of the Heisenberg model prediction for spin S1/2, whereas for La 0.79Sr 0.21CoO 3 they are found to be shifted toward the mean-field values. The deviation of the critical exponents from 3D Heisenberg values toward mean-field ones is attributed to the presence of magnetoelectronic phase inhomogeneity in the x=0.21 single crystal. The detailed analysis of the specific-heat data in the vicinity of T C for the x=0.33, 0.25, and 0.21 samples also supports the phase separation scenario at around x=0.21. © 2012 American Physical Society.

Abstract:

The magneto-electric multiferroic TbMn 2O 5 has a complex magnetic structure in three different magnetically ordered phases. We have determined the nature of the induced magnetic order on the oxygen sites in the commensurate magnetic phase through full linear X-ray polarisation analysis at the oxygen K edge. This has been achieved rotating the linear polarisation of the incident beam at the source, and using multilayers to analyse the polarisation state of the scattered X-ray beam. We have confirmed that the anisotropy of the magnetic scattering at the oxygen edge is consistent with the anisotropy of the manganese magnetic structure. © 2012 EDP Sciences, Springer-Verlag.

Abstract:

Through powder x-ray diffraction we have investigated the structural behavior of SmVO 3, in which orbital and magnetic degrees of freedom are believed to be closely coupled to the crystal lattice. We have found, contrary to previous reports, that SmVO 3 exists in a single, monoclinic phase below 200 K. The associated crystallographic distortion is then stabilized through the magnetostriction that occurs below 134 K. The crystal structure has been refined using synchrotron x-ray powder diffraction data measured throughout the structural phase diagram, showing a substantial Jahn-Teller distortion of the VO 6 octahedra in the monoclinic phase, compatible with the expected G-type orbital order. Changes in the vanadium ion crystal field due to the structural and magnetic transitions have then been probed by resonant x-ray diffraction. © 2012 American Physical Society.