Magnetic ordering in PrBa₂Cu_3-yAl_yO_6+x

The magnetic ordering in single crystals of PrBa2CU3O6+x has been investigated by elastic neutron scattering over the full range of temperatures for reduced and oxygenated crystals. The crystals were grown in alumina crucibles and therefore contained dissolved aluminum on the Cu(1) site. Both aluminum and oxygen contents were analyzed in detail in order to establish their effects on the magnetic ordering, Our crystals exhibited Pr ordering and the two types of antiferromagnetic Cu ordering frequently reported in related compounds, but our results differ in several respects from previous studies. We observed three-dimensional (3D) collinear ordering of the Pr moments in the oxygenated crystal, with a finite correlation length of (34+/-3) Angstrom parallel to the c axis, but in contrast to earlier neutron scattering results, which tentatively placed the Pr moment as being parallel to the c axis, we find the moment to be aligned well away from the c axis, in agreement with recent Yb-170(3+) Mossbauer spectroscopy results. Ridges of scattering indicative of 2D magnetic ordering were seen in both oxygenated and reduced crystals, though we believe different magnetic moments are responsible in each case. The Pr Neel temperatures were suppressed compared with those reported for nominally pure samples, We have observed a very small 3D-ordered moment on the Cu(1) site in the second type of Cu antiferromagnetic ordering (in the reduced crystal), but an ordered Cu(1) moment may not necessarily be a characteristic of this structure. Our results are discussed with reference to previous works, and in the light of the Al substitution.