Direct observation of the flux line lattice in Al-doped YBa2Cu3O7-δ

There has been much interest in the effect of partial substitution of Cu atoms in YBa2Cu3O7-δ (YBCO) by dopant atoms such as Fe, Al and Co. It has been found that an increase in the doping level alters the structure of YBCO - the twin domain size drops to 30-100Å for x>0.025 and the domains form the so-called 'tweed' structure. HREM has confirmed that within the domains the structure remains orthorhombic but on a macroscopic scale is quasi-tetragonal. This, in turn, leads to a change in superconducting properties: a rapid decrease in the transition temperature Tc, critical current, etc.In the present contribution the first results are reported on the direct observation of the magnetic flux structure in YBa2(Cu1-xAlx)3O7-δ single crystals (x=0.04) which was also expected to be influenced by Al doping. The flux line (vortex) distribution was observed using the high resolution Bitter pattern technique which involves evaporation of fine ferromagnetic particles (both Fe and Co were used in our experiments) to ‘decorate’ vortices at the surface of a superconductor.