Modeling the conductive channel as a series of ferromagnetic (FM) domain with resistivity of ${\rho }_{\mathrm{FM}}$ and paramagnetic (PM) region with resistivity of ${\rho }_{\mathrm{PM}},$ we propose a phenomenological equation for resistivity as $\rho =\left[1-f\right(T,H\left)\mathrm{}\right]{\rho }_{\mathrm{PM}}+f(T,H)\mathrm{}{\rho }_{\mathrm{FM}},$ where $f(T,H)\mathrm{}$ is the volume fraction of FM domains. This allows us to account quantitatively for reports of CMR in the optimally doped manganese perovskites. We present temperature-dependent magnetoresistance data measured in $({\mathrm{La}}_{1-x}{\mathrm{Y}}_x{)}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_3$ $\mathrm{}(x=0.195\mathrm{})\mathrm{}$ and demonstrate that the model yields excellent agreement with experiments over the whole range covering the high-temperature insulating and the low-temperature metallic regimes. The physical basis for this model is discussed on the basis of dynamic phase segregation and the CMR is argued to be originated mainly from the application of magnetic fields that accelerates the growth of the more conductive FM domains.

• Received 6 July 2000

DOI:https://doi.org/10.1103/PhysRevB.63.172415