50 K anomalies in superconducting MgB2 wires in copper and silver tubes

In situ and ex situ MgB2 wires were prepared by the powder-in-tube method. Copper and silver tubes were used as a cladding material. AC susceptibility measurements revealed a small anomalous decrease with onset around 50 K. This effect persisted also when the wires were ground into powders. Electron microscopy and x-ray studies were performed on copper clad samples. Spectroscopic measurements in a SEM showed that regions contained either Cu or Mg and B. X-ray diffraction gave the major crystalline phases as Cu, MgCu2 and MgB2. Diffraction evidence for Cu substituting in the Mg position was inconclusive.