Long- and short-range magnetic order, charge transfer, and superconductivity in YBa2Cu3-xCoxO7 -: art. no. 094506

We have performed powder-neutron-diffraction measurements in YBa2Cu3-xCoxO7 for a large range of Co concentrations (x=0.04-0.72) and determined the magnetic phase diagram. As previously observed, superconductivity disappears for xgreater than or equal to0.42 and antiferromagnetic long-range order emerges above this value. The antiferromagnetic order shows noticeable differences compared to that in unsubstituted YBa2Cu3O6+delta and surprisingly survives to a much higher hole doping. In contrast to recent results in YBa2Cu3-xCoxO7 single crystals, there is no evidence of any long-range antiferromagnetic order in the superconducting phase. The Co valence evolves from 4+ to 3+ with increasing Co concentration. We studied the short-range chemical and magnetic correlations between the Co ions in superconducting (xless than or equal to0.36) and antiferromagnetic (xgreater than or equal to0.42) samples. The Co ions predominantly occupy the Cu1 sites and their moments remain mostly disordered, forming random antiferromagnetic next nearest neighbor pairs, which compete with the long-range order. This can explain the low value for the ordered Cu1 site moment.