Magnetic properties of PrBa₂Cu₂MO_8−y(M = Ta, Nb)

Abstract

We have investigated the magnetic properties of PrBa₂Cu₂MO_8−y(PrBCMO) (M = Ta, Nb), in which MO₆ octahedra replace the square planar Cu in the parent compound, PrBa₂Cu₃O_7−y(PrBCO). Signature of an antiferromagnetic (AFM) transition at 9 K for Pr_1.1Ba_1.9Cu_2.1Ta_0.9O_8−y (Pr_1.1BCTO) and at 10 K for PrBa₂Cu₂NbO_8−y (PrBCNO) is found in the magnetic susceptibility versus temperature curve. The value for the latter is somewhat lower than that observed recently. The transition temperatures are approximately half the value of 17 K observed for PrBCO. This transition in the absence Cu(1)=O(1) chains in PrBCMO means that it is due to the Pr sublattice ordering as confirmed recently. Our results also imply that the copper chains are not required for the ordering of the Pr sublattice. However, a reduction in T_N indicates that the Cu(1)=O(1) chains, acting as separator layers between the CuO₂ planes, may play a role. The effect of separator layers, Cu(1)=O(1)/MO₆, on the Pr-containing block, CuO₂=Pr=CuO₂, is to change the double-layer CuO₂ spacing, which in turn may control T_N through hybridization effects. The Néel point seems to be related to this CuO₂ interlayer spacing in Pr-based cuprates. The effective paramagnetic moment of the Pr ion is found to be 3.06 μ_B and 2.85 μ_B, respectively, for Pr_1.1BCTO and PrBCNO, a value somewhat higher than that found for PrBCO.