Magnetic properties and magnetoresistance of the Ru-substituted Tl2Mn2-xRuxO7 pyrochlore

We report on the magnetotransport properties of the Tl2Mn(2-x)Ru(x)O(7) pyr ochlore as a function of the substitution degree 0 x less than or equal to 2 . The evolution of the paramagnetic moment as a function of x in this ser ies of samples points to an itinerant character of the Ru 4d electrons for x less than or equal to 1. For larger values of x some localization of Ru 4 d electrons sets in and its contribution to the paramagnetic moment is dete cted. The appearance of a magnetic moment in Ru atoms promotes a remarkable magnetic irreversibility, which is attributed to the reinforcement of near est-neighbor antiferromagnetic interactions within the Mn/Ru-O sublattice a nd the concomitant magnetic frustration. In agreement with this hypothesis, the ferromagnetic transition temperature T-c and the saturation magnetizat ion M-s both decrease with x. Noticeable magnetoresistance is only found fo r x < 0.3, being maximum around x = 0.05.