OXYGEN DEPENDENCE OF THE MAGNETIC-ANOMALIES IN PR1.5CE0.5SR2NBCU2O10

We investigated Pr1.5Ce0.5Sr2NbCu2O10 and the superconducting Eu1.5Ce0.5Sr2NbCu2O10 materials by several complementary experimental techniques. Oxygen-rich Pr1.5Ce0.5Sr2NbCu2O10 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 Mossbauer 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 T(N) of Cu. A high linear term of 265 mJ/mol Pr K2 is obtained in the specific heat of the oxygen-rich Pr1.5Ce0.5Sr2NbCu2O10 material.