Dr Dharmalingam Prabhakaran

Abstract:

We have investigated the spin dynamics of Tb 2Ti 2O 7 as a function of magnetic field applied along the [111] axis using ac susceptibilty and muon-spin relaxation measurements. We find a significant increase in the imaginary part of the magnetic susceptibility at low fields at our lowest investigated temperature and an increased muon-spin relaxation rate in the same field range persisting to higher temperature. Comparing the data from the two techniques we identify three field regions where the dynamic properties of Tb 2Ti 2O 7 appear to be evolving in different ways with crossovers between the regions at B 1=15 and B 2∼60 mT. © 2012 American Physical Society.

Abstract:

This report presents azimuthal dependent and polarization dependent x-ray resonant magnetic scattering at the Ir L(3) edge for the bilayered iridate compound Sr(3)Ir(2)O(7). The two magnetic wave vectors, k1 = (1/2, 1/2, 0) and k2 = (1/2, -1/2, 0), result in domains of two symmetry-related G-type antiferromagnetic structures, denoted A and B, respectively. These domains are approximately 0.02 mm(2) and are independent of the thermal history. An understanding of this key aspect of the magnetism is necessary for an overall picture of the magnetic behaviour in this compound. The azimuthal and polarization dependence of the magnetic reflections, relating to both magnetic wavevectors, show that the Ir magnetic moments in the bilayer compound are oriented along the c axis. This contrasts with single layer Sr(2)IrO(4) where the moments are confined to the ab plane.

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.