GIANT NEGATIVE MAGNETORESISTANCE IN THE SPIN-GLASS INSULATOR TL(0.94)SR2CU0.4CO0.6O5-DELTA

An investigation of the ''1 2 0 1'' phase Tl0.94Sr2Cu0.4Co0.6O5-delta with the prototypic structure of the Tl based high T-c superconductors has revealed a transition to a spin glass state in the vicinity of 30 K. Interestingly, this phase which is insulating at all temperatures below 350 K shows giant negative magnetoresistance (GMR) at low temper atures. This GMR behaviour does not trace the magnetization unlike in the perovskite, pyrochlore and layered manganites. We present a first report of these properties and point out analogies between the title p hase and the manganites and certain other systems that show negative m agnetoresistance effects. The same site disorder that leads to the spi n glass behaviour is suggested to play an important role in the transp ort properties. (C) 1997 Elsevier Science Ltd.