We report on optical linear birefringence measurements in the nearly ideal S = 1 Heisenberg antiferromagnetic chain compounds: Ni(C2HsN2)2 NO2 (C104) and Ni (CzHsNz)2 N02PF6. The temperature dependence of the magnetic contribution, proportional to that of the magnetic energy, indicates J / k = -53 K and supports the Haldane conjecture.Article published online by EDP Sciences and available at http://dx.
Optical Bragg-regime diffraction of lighi from the domain structure of a twinned orthorhombic crystal is reported for the first time. Diffracted beams were observed in Dy AsO,, DyVO,. TbV04, and some mixed crystals below their orthorhombic phase transitions. The diffracted beams could be seen over a range of scattering angles, with peak intensity near 5' corresponding to an average domain spacing of about 3pm. The temperature-dependent intensity was measured to obtain values for the 0rder.parameter critical exponent
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