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(0.7)A(0.3)MnO(3+delta) (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 rho(T-c)/rho(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 rho(T-c), but by T-c via the ratio rho(T-c)/rho(4 K). Phenomenological scaling relationships are also reported that link rho(H, T) to both H and M(H, T). (C) 1996 American Institute of Physics.