SPIN-DISORDER SCATTERING AND LOCALIZATION IN MAGNETORESISTIVE (LA1-XYX)(2 3)CA1/3MNO3 PEROVSKITES/

We present experimental results of the temperature and magnetic-field dependences of the colossal magnetoresistance (CMR) and the magnetizat ion in a series of ceramic samples with composition (La1-xYx)(2/3)Ca1/ 3MnO3 (0<x<0.25). The samples have very different ferromagnetic orderi ng temperatures and magnetoresistance values (Delta R(T)/R=[R(T-C)-R(3 00 K)]/R(300 K) up to 5X10(5)%) but they display a common mechanism fo r field and temperature dependences of Delta R(T,H). We provide eviden ce for the existence of different microscopic mechanisms for the CMR i n the paramagnetic and the ferromagnetic phases. Below T-C the magneti zation controls the electrical resistivity through a law characteristi c of localization of magnetic polarons moving by a hopping mechanism. Well above T-C the key phenomenological parameter relating magnetic an d transport properties is the differential magnetic susceptibility (ch i d)(T) and not the magnetization. The substitution of La by the small er Y results in an enhanced magnetic disorder and a concomitant higher residual resistivity. We have found that the temperature dependence o f the magnetization can be understood as the arising of spin waves whi ch become softer when the conducting bandwidth is narrowed.