Magnetoresistance in Tl2Mn2O7 pyrochlore: magnetic and charge density

Magnetic and transport properties of T1(2)Mn(2-x)Ru(x)O(7) pyrochlore are s tudied as a function of the Ru substitution rate x. The use of Ru4+ substit ution in the Mn4+ network provides a convenient way to modify the charge de nsity in the T1-O band without disrupting it with foreign cations, a fact w hich allows to study the effects of the charge carriers density on the tran sport properties of the system. Magnetic mesurements indicate that in the l ow substitution rate (x less than or equal to 1) Ru 4d electrons have an it inerant character but for higher a they become localized leading to a stron g reinforcement of antiferromagnetic interactions. Resistivity also changes drastically with 1 and noticeable magnetoresistance is only found for x < 0.3. Indications in favor of activated behavior and charge carrier localiza tion, suggesting the existence of spin polarons, are found above T-c. (C) 2 000 Elsevier Science B.V. All rights reserved.