Spin-disorder scattering and localization in magnetoresistive (${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Y}}_{\mathit{x}}$${)}_{2/3}$${\mathrm{Ca}}_{1/3}$${\mathrm{MnO}}_3$ perovskites

We present experimental results of the temperature and magnetic-field dependences of the colossal magnetoresistance (CMR) and the magnetization in a series of ceramic samples with composition (${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Y}}_{\mathit{x}}$ ${)}_{2/3}$ ${\mathrm{Ca}}_{1/3}$ ${\mathrm{MnO}}_3$ (0<x<0.25). The samples have very different ferromagnetic ordering temperatures and magnetoresistance values (ΔR(T)/R=[R(${\mathit{T}}_{\mathit{C}}$)-R(300 K)]/R(300 K) up to 5×${10}^5$%) but they display a common mechanism for field and temperature dependences of ΔR(T,H). We provide evidence for the existence of different microscopic mechanisms for the CMR in the paramagnetic and the ferromagnetic phases. Below ${\mathit{T}}_{\mathit{C}}$ the magnetization controls the electrical resistivity through a law characteristic of localization of magnetic polarons moving by a hopping mechanism. Well above ${\mathit{T}}_{\mathit{C}}$ the key phenomenological parameter relating magnetic and transport properties is the differential magnetic susceptibility ${\mathrm{\chi }}_{\mathit{d}}$(T) and not the magnetization. The substitution of La by the smaller Y results in an enhanced magnetic disorder and a concomitant higher residual resistivity. We have found that the temperature dependence of the magnetization can be understood as the arising of spin waves which become softer when the conducting bandwidth is narrowed. © 1996 The American Physical Society.