Transport and magnetism correlations in thin-film ferromagnetic oxides

To determine the T_c dependence of the colossal magnetoresistance (CMR) exhibited by the ferromagnetic La_0.7A_0.3MnO_3+δ (A=Ba, Ca, Sr) system, we examine the magnetic-field and temperature-dependent resistivity and magnetization of a series of thin films that were grown via pulsed-laser deposition. The films had magnetic ordering temperatures (T _c) ranging from 150 to 350 K. All samples display a large negative MR that is largest near T_c, and samples with a low T_c display significantly larger MR values than do samples with large T_c's. The quantity p(T_c)/ρ(4 K), the amount by which the resistivity is reduced by full ferromagnetic order, varies as exp(E_a/T_c) with an activation energy E_a=0.1 eV. These results indicate that the magnitude of the CMR effect in a given specimen is controlled not by ρ(T_c), but by T_c via the ratio ρ(T_c)/ρ(4 K). Phenomenological scaling relationships are also reported that link ρ(H,T) to both H and M(H, T).