THE LARGE MAGNETORESISTANCE OF R2MO2O7 (R=GD, TB) PYROCHLORES AT LOW-TEMPERATURES

Magnetization and magnetoresistance (MR) measurements were performed o n pyrochlores Gd2Mo2O7, Gd-2(MO0.6V0.4)(2)O-7 and Tb2Mo2O7. It was sho wn that Gd-based compounds are ferromagnets below Te = 80 Ii with the same alignment of magnetic moments of 4f and 4d ions. Tb2MO2O7 is char acterized by a very weak spontaneous magnetization at 4.2 Ii being inc ompatible with ferromagnetic behavior. Gd2Mo2O7, exhibits a large nega tive magnetoresistance ratio, up to 20% and a field of 1 T,which incre ases strongly with decreasing temperature. The substitution of Mo ions by V leads to a change of the magnetoresistance ratio from negative t o positive. At 4.2 Ii Tb2Mo2O7 shows a crossover of magnetoresistance from positive (up to 10% in a field of 2 T) to negative at H = 4 T The se data indicate that there are two different mechanisms responsible f or the magnetotransport properties of molybdates with pyrochlore struc ture.