Significance of Al doping for antiferromagnetic AFII ordering in YBa2Cu3-xAlxO6+ delta materials: A single-crystal neutron-diffraction study.

Abstract

Aluminum-doped, oxygen-deficient YBa{sub 2}Cu{sub 3{minus}{ital x}}Al{sub {ital x}}O{sub 6+{delta}} single crystals with different Al contents {ital x} (0{le}{ital x}{le}0.19) and O contents (0.18{le}{delta}{le}0.36) 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 Neel temperature {ital T}{sub {ital N}} of the antiferromagnetic AFI phase is found to be insensitive to the Al content {ital x} as well as the O content 6+{delta} in the {ital x}-{delta} 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 ({l_angle}{ital S}{r_angle}{sub Cu(2)}{approx}0.56{mu}{sub {ital B}}, {l_angle}{ital S}{r_angle}{sub Cu(1)}{approx}0{mu}{sub {ital B}}) 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 ({delta}{gt}0) ismore » 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.« less