In situ and ex situ wires were prepared by using the powder-in-tube method with Cu or Ag tubes as cladding material. Alternating-current susceptibility measurements revealed a small anomalous decrease, with its onset around 50K. This effect persisted when the wires were ground into powders. Electron microscopy and X-ray studies were performed on Cu-clad samples. Spectroscopic measurements in a scanning electron microscope showed that regions contained either Cu or Mg and B. X-ray diffraction gave the major crystalline phases as Cu, MgCu2 and MgB2. The diffraction evidence for Cu substituting at Mg sites was inconclusive.

50K Anomalies in Superconducting MgB2 Wires in Copper and Silver Tubes. M.Majoros, B.A.Glowacki, M.E.Vickers: Superconductor Science and Technology, 2002, 15, 269-75