Magnetic and transport properties of the 1201-phase Tl0.94Sr2Cu0.4Co0.6O5−δ

We present the magnetic and transport properties of a mixed oxide of Cu and Co with the prototypic structure of the Tl containing copper oxide superconductors. This insulating phase shows a transition to a spin-glass state with an equilibrium transition temperature $T_c=30.9\mathrm{}\left(1\right)\mathrm{}$ K. Interestingly, the electrical transport, which seems to be dominated by disorder effects, demonstrates giant negative magnetoresistance (GMR) at low temperatures, with the magnitude of the GMR increasing as the temperature is decreased. The possible cause for the spin-glass behavior as well as the GMR seems to be site disorder of Cu/Co and the simultaneous presence of ferro- and antiferromagnetic interactions. The GMR seems to be different from what is observed in the ferromagnetic manganites in that the transport properties do not closely trace the magnetic behavior.