Doping experiments on low-dimensional oxides and a search for unusual magnetic properties of MgAlB14

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2002-01-01
Authors
Hill, Julienne
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Physics and Astronomy
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Doping experiments on La2CuO4, Sr2CuO 3 and SrCu2(BO3)2 were performed with the intent of synthesizing new metallic low-dimensional cuprate oxide compounds. Magnetic susceptibility chi(T) measurements on a polycrystalline La 2CuO4 sample chemically oxidized at room temperature in aqueous NaClO showed superconductivity with a superconducting transition temperature Tc of 42.6 K and a Meissner fraction of 26%. We were unable to electrochemically oxidize La2CuO4 in a nonaqueous solution of tetramethylammonium hydroxide (TMAOH) and methanol. Sr2CuO 3 was found to decompose upon exposure to air and water. Electron paramagnetic resonance, isothermal magnetization M(H) and chi(T) measurements on the primary decomposition product, Sr2Cu(OH)6, were consistent with a nearly isolated, spin S = 1/2, local moment model for the Cu +2 spins. From a fit of chi(T) by the Curie-Weiss law and of the M(H) isotherms by a modified Brillouin function, the weakly antiferromagnetic exchange interaction between adjacent Cu+2 spins in Sr 2Cu(OH)6 was found to be J/kB = 1.06(4) K. Doping studies on SrCu2(BO3)2 were inconclusive. chi(T) measurements on an undoped polycrystalline sample of SrCu2(BO 3)2, a sample treated with distilled water, and a sample treated with aqueous NaClO showed no qualitative differences between the samples.;In addition, chi(T) and M(H, T) studies of the ultra-hard material MgAlB14 were carried out in search of superconductivity or ferromagnetism in this compound. chi(T) measurements on a powder sample revealed temperature-independent diamagnetism from 1.8 K up to room temperature with a Curie-Weiss impurity concentration equivalent to ~1 mol% of spin-1/2 ions. In contrast, M(H, T) data on hot pressed samples showed evidence of ferromagnetic transitions above ~330 K. Scanning electron microscopy and Auger microprobe analysis of the hot pressed samples indicated that the observed ferromagnetism was likely due to Fe impurities.

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Physics and astronomy, Condensed matter physics
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