Gallium nitride doped with oxygen (unintentionally), silicon and magnesium was grown by metalorganic chemical vapor deposition on the conductive single crystals of GaN grown at high hydrostatic pressure. The layers were examined using X-ray diffraction, photoluminescence and far-infrared reflectivity. It was found that the incorporation of silicon depends on the side used for deposition. For the two Si-doped layers grown in the same run, the one grown on the (00.1) side (gallium-terminated) had always smaller free electron concentration with respect to the (00.1) side (nitrogen-terminated). This conclusion could be drawn from the lattice expansion by free electrons, the photoluminescence peak shift by Burstein-Moss effect and the position of plasma edge in farinfrared reflectivity.

S e w measurements of magnetic properties of pure FeCr,S, and CoCr2S, have been carried out. The g-factors of cobalt, iron, and chromium in these compounds were evaluated. Exchange interactions were deduced from computation of the experimental paramagnetic and magnetization curves. De nouvelles mesures des propriiftifs magnktiques ont iftif effectukes sur les coniposirs FeCr,S, et CoCr,S, obtenus A 1'6tat pur. Les facteurs g du cobalt, fer et chrome dans ces composes ont At8 irvaluifs. Le calcul des courbes de paramagnetisme et d'aimantation permet de difduire les interactions d'bchange. lntroductionIron and cobalt thiochromite (FeCr,S, and CoCr,S,) exhibit two crystallographic structures : a ferrimagnetic spinel phase and an antiferromapnetic nickel arsenide defect structure phase.These two thiochromites were first prepared in powder form from high purity elements (Fe, Co, Cr at 99.999%, S at 99.99999%). X-ray examination showed that no binary compounds were present.Octahedral single crystals up to 5 mni were grown by vapour transport using chlorine (FeCr,S,) or bromine (CoCr,S,) [I, 21. Low temperature X-ray photographs (down to 77 OK) show that no .Jahn-Teller distortion appears [ l , 21. The absence of any Jahn-Teller distorsion is consistent with the neutron diffraction results [3, 41, no distorsion appears down to a temperature of 4.2 OK. These compounds present semiconductor-like conductivity [5, 6, 21. Magnetic MeasurementsAll measurements were carried out on powdered monocrystalline material. PeC'r,Y,The paramagnetic susceptibility was measured between 200 and 1100 OK.The Curie temperature was found to be T , = 195 "K and Op = -290 OK. The reciprocal molar susceptibility vs. T (for 1' > 300 OK) can be fitted by the equation (Fig. 1 ) Taking into account the temperature dependence of the molecular field coefficients : n = no (1 + y T) (y = -lo-, in FeCr,S,, calculated from the NBel relations [IS]) the relation becomes 1 690 -~ + 41.8 -___ 1 _ -XJI 6.92T -208 '

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