Local magnetic properties of PrBa2Cu3O6+x: NMR and NQR in crystals

NQR spectra of Cu-chain sites in PrBa2Cu3O6 crystals show below the Neel temperature of Pr the presence of a transferred hyperfine field of approximate to 0.12 T from the Pr sublattice. The fields at the Cu-plane sites observed below 4.2 K are an order of magnitude larger. In contrast no field could be detected at the Cu chains in PrBazCu(3)O(7). The spectrum at the Cu-plane sites for high oxygen concentration indicates the existence of a transferred field similar to the one in the reduced sample. The large transferred fields are in direct contradiction to the antiferromagnetic structure generally assumed for the Pr moments. They strongly support the model of a broken Pr point symmetry at least in PrBa2Cu3O6 The most simple distortion of the tetragonal PrO8 complex (D-4h), which may give rise to the fields, is not to orthorhombic (D-2h) as is usually assumed, but to C-4v. The results indicate that the Pr moments are perpendicular to the c axis and that the antiferromagnetic coupling along the c axis is distorted for the reduced sample.