Oxygen dependence of the magnetic anomalies in ${\mathrm{Pr}}_{1.5}$${\mathrm{Ce}}_{0.5}$${\mathrm{Sr}}_2$${\mathrm{NbCu}}_2$${\mathrm{O}}_{10}$

We investigated ${\mathrm{Pr}}_{1.5}$ ${\mathrm{Ce}}_{0.5}$ ${\mathrm{Sr}}_2$ ${\mathrm{NbCu}}_2$ ${\mathrm{O}}_{10}$ and the superconducting ${\mathrm{Eu}}_{1.5}$ ${\mathrm{Ce}}_{0.5}$ ${\mathrm{Sr}}_2$ ${\mathrm{NbCu}}_2$ ${\mathrm{O}}_{10}$ materials by several complementary experimental techniques. Oxygen-rich ${\mathrm{Pr}}_{1.5}$ ${\mathrm{Ce}}_{0.5}$ ${\mathrm{Sr}}_2$ ${\mathrm{NbCu}}_2$ ${\mathrm{O}}_{10}$ is not superconducting and the magnetic-susceptibility data reveal two magnetic anomalies at 11 and 54 K. Both anomalies always appear together and are sensitive to oxygen concentration, whereas for oxygen-poor material both anomalies are absent. The presence of 0.5% Fe dramatically affects only the transition at 54 K and shifts it to 94 K, and Mössbauer studies indicate that this transition is related to antiferromagnetic ordering of the Cu sublattice. The peak in the susceptibility at 11 K is probably due to spin reorientation of Cu moments. While the anomalies are clearly identified by magnetic measurements, no specific-heat anomaly was observed at either temperature. A strong two-dimensional quantum spin fluctuation is assumed as the cause of the unusual behavior at ${\mathit{T}}_{\mathit{N}}$ of Cu. A high linear term of 265 mJ/mol Pr ${\mathit{K}}^2$ is obtained in the specific heat of the oxygen-rich ${\mathrm{Pr}}_{1.5}$ ${\mathrm{Ce}}_{0.5}$ ${\mathrm{Sr}}_2$ ${\mathrm{NbCu}}_2$ ${\mathrm{O}}_{10}$ material.