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Brecht, E.; Schmahl, Wolfgang W.; Fuess, H.; Casalta, H.; Schleger, P.; Lebech, B.; Andersen, N. H. und Wolf, Th. (1995): Significance of Al doping for antiferromagnetic AFII ordering in YBa2Cu3-xAlxO6+ materials: A single-crystal neutron-diffraction study. In: Physical Review B - Condensed Matter and Materials Physics, Vol. 52, Nr. 13: S. 9601-9610
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Aluminum-doped, oxygen-deficient YBa2Cu3-xAlxO6+ single crystals with different Al contents x (0x0.19) and O contents (0.180.36) were studied by magnetic neutron diffraction. All of the Al-doped single crystals show two magnetic transitions, the first between the paramagnetic state and the AFI phase, and a second transition at low temperatures between the AFI and the AFII phase. The Néel temperature TN of the antiferromagnetic AFI phase is found to be insensitive to the Al content x as well as the O content 6+ in the x- region investigated so far. In a limited temperature interval the order parameter shows the components of both the AFI and AFII phases indicating competing interactions. For some crystals a complete reordering to the AFII phase at 4.2 K can be observed. Although the antiferromagnetic ordering pattern is different for the AFI and AFII phase, the ordered moments on the Cu sites are within the experimental error (SCu(2)0.56B, SCu(1)0B) identical in the two phases. Comparison of Al-doped crystals with pure crystals of the same oxygen content shows that the presence of excess oxygen only (>0) is not sufficient to obtain the AFII ordering at low temperatures. Instead, the AFII ordering is triggered by the creation of the local magnetic moments in Cu-O-Al chain fragments.