Untersuchung der magnetischen Ordnung in Yttrium-Barium-Kupfer-Oxid und verwandten Verbindungen mittels Kernresonanz
Schmenn, S. (Corresponding author)
Publikationen vor 2000; PRE-2000; Retrocat
Jülich Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag 1996
140 p.
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Berichte des Forschungszentrums Jülich 3320
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Please use the identifier: http://hdl.handle.net/2128/21787 in citations.


The antiferromagnetic structure of REBa$_{2}$Cu$_{3}$O$_{6+x}$ powder samples and REBa$_{2}$Cu$_{3-y}$Al$_{y}$O$_{6+x}$ single crystals (RE = Y, Tm, Gd, Yb, Nd, La) is studied by $^{63,65}$Cu NQR and NMR, as well as NMR ofthe aluminium impurity and ofthe rare earth atom lanthanum. The magnetic hyperfine field and the electric field gradient (EFG) are investigated. The antiferromagnetic ground structure of pure YBa$_{2}$Cu$_{3}$O$_{6+x}$ is the AF 1 phase for all oxygen contents. This phase is characterized by a stacking sequence +o - +o - of the Cu(2) magnetic moments along the c-axis of the crystal. The syrnbol o denotes non magnetic Cu(1) ions. The two- and threefold coordinated Cu(1) ions - denoted by Cu(1)$_{2}$ and Cu(1)$_{3}$ respectively - carry no static magnetic moment. This is for the first time proved by NMR for the Cu(1)$_{3}$. The EFG-tensor, with an asymmetry parameter of 0.3, and the shift tensor of Cu(1)$_{3}$ reveal the orthorhombic symmetry of this site. In the aluminium doped samples a few percent of the Cu ions at the Cu(1)-places are substituted by Al ions. This results at low temperatures in a change to the AF 2 phase with an ordering ofthe magnetic moments in a stacking sequence + o + - o -. In YBa$_{2}$Cu$_{3-y}$Al$_{y}$O$_{6+x}$ a plausible explanation for the AF2-structure is the existence of magnetic moments in the Cu(1) layers. These must be localized at the Cu and not at the AI sites. In the samples with an extremely reduced oxygen content all the Al ions are shifted away from their symmetric lattice sites along the <110>-direction, in order to get a more tetrahedral oxygen coordination. This can result in a distortion of the symmetric sites of the nearest Cu(1) ions and therefore in aninduced magnetic moment at these sites. The NMR signal ofthese moment bearing Cu ions is not found yet. With increasing oxygen content the oxygen surrounding of the Al ions changes to a very symmetric octahedral. The Al ions are not shifted and the AF 1 phase stays stable down to the lowest temperatures. It is shown why the NQR ofthe threefold coordinated Cu(1) cannot be observed in the AF 2 phase ofthe sample. The Cu(2) moments are oriented perpendicular to the c-axis of the crystal, independent of the Al doping and the antiferromagnetic phase. In the pure YBa$_{2}$Cu$_{3}$O$_{6.0}$ the moments are aligned along <100> and in YBa$_{2}$Cu$_{2.94}$AL$_{0.06}$O$_{6.18}$ along <110>. This change can be explained by the local symmetry of the Al ions in crystals where the oxygen content has been extremely reduced. The temperature dependence of the transition from the AF 1 to the AF 2 phase can be detected by the Zeeman splitting ofthe Cu(1)$_{2}$ NQR lines. The measurements show a hyperfine field distribution which originates from local fluctuations in the Al and oxygen content within the samples. All field values between zero and the maximum value are observed. The magnetic moments of the Cu(2) ions in neighboring planes change their relative orientation continuously during a second order phase transition, from 0° in the AF 1-structure to 90° in the AF 2-structure. A very good agreement is observed between the canting angles derived from the NQR measurements and those derived from neutron diffraction experiments.