Tunneling and enhanced magnetoresistance in Nd_2/3Sr_1/3MnO₃ thin films with microcracks

We found that microcracks in thin Nd_2/3Sr_1/3MnO₃ films create a series of intrinsic break junctions which are ideal for investigating tunneling phenomena in this system. A comparison of films with and without cracks, which have similar ferromagnetic Curie temperature (T_c) of 140-150 K, shows that the cracked film has a lower insulator to metal transition temperature T_M (97 K vs 140 K), three orders of magnitude higher resistivity at T_M, and two times larger magnetoresistance at 1 T near T_M. At T>T_M we observed that lnραT^-1/4 in the uncracked film while in the cracked film a lnραT^-1/2 dependence was found. This indicates that the conductivity in the first case is due to variable range hopping in three dimension, while in the second case it is dominated by thermally activated tunneling across the insulating barriers (the microcracks).