  We investigated the magnetoresistance (MR) effect of the pyrochlore oxide
Nd2Ir2O7, which shows a metal-insulator transition at T_MI =33 K. A small
positive MR effect was observed in the metallic state above T_MI, while a large
negative MR effect was observed in the insulating state below T_MI . MR effects
exceeding 3000% were found at 1 K at a field of 9 T. As a result, we confirmed
the crossover from the insulating state to a state with a small or partial band
gap in a field up to 56 T. Furthermore, from the MR effect in Eu2Ir2O7 (T_MI =
120 K) and Gd$_2$Ir$_2$O$_7$ (T_MI = 127 K), we revealed that the large
negative MR effect of the pyrochlore iridate Ln2Ir2O7 depends on the magnetism
of the lanthanide Ln^{3+} ion. The d-f interaction plays a significant role in
the large negative MR effect in the insulating state.
