Superconducting RuSr2GdCu2O8 studied by SQUID magnetometry

For polycrystalline RuSr 2 GdCu 2 O 8 (Ru-1212), distinct peaks have been reported in dc magnetization in the superconducting state of the sample. Sr 2 GdRuO 6 (Sr-2116), the precursor for the preparation of Ru-1212, shows similar peaks in the same temperature regime. Based on measurements performed on both bulk and powdered samples of Ru-1212 and Sr-2116, we exclude the possibility that the observed behavior of the magnetization of Ru-1212 is due to Sr-2116 impurities. The effect is related to the superconductivity of Ru-1212, but it is not an intrinsic property of this compound. We provide evidence that the observation of magnetization peaks in the superconducting state of Ru-1212 is due to flux motion generated by the movement of the sample in an inhomogeneous field, during the measurement in the superconducting quantum interference device (SQUID) magnetometer. We propose several tests that help to decide whether the features observed in a SQUID magnetization measurement of Ru-1212 represent a property of the compound or not.