Vortex phase diagram in impurity doped YBa2Cu3Oy single crystals

Impurity substituted single crystals of untwinned YBa2(Cu1−xMx)3O6.95 (M = Zn, Ni) have been prepared and the Zn and Ni substitution effect on the vortex phase diagram has been studied as a function of impurity concentration x. In YBa2(Cu1−xMx)3O6.95 with small amount of impurity [x(Zn) = 0.0003 (Tc = 91 K), x(Ni) = 0.003 (Tc = 90.5 K)], although the first-order vortex lattice melting transition Hm(T) disappears, the order-disorder transition line H*(T) between the Bragg glass and the vortex glass phases has been observed. The obtained results suggest that Zn and Ni impurities strongly affect the vortex system in comparison with the oxygen vacancy in pure YBa2Cu3Oy.