Synthesis and crystal growth

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.

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.