Influence of the aluminium doping on the antiferromagnetic structure of YBa2Cu3O6+x

The magnetic structure of YBCO with low oxygen content is severely influenced by Aluminium doping. The Al-NMR spectra confirm the position of the non-magnetic Al impurities in the Cu(1) layer with a shift from the regular Cu(1) site in <110> direction. The Cu(1) ions in the Al-O-Cu(1) clusters may be responsible for the transition between the antiferromagnetic AFI and AFII phases in the doped crystals. From the Cu-NMR in pure and Al doped single crystals different spin directions in the AFI and AFII phases have been determined, <100> for the pure YBCO and <110>for the Al doped in the AFII state.