Griffiths phase, metal-insulator transition, and magnetoresistance of doped manganites

A phenomenological model is developed for systematic study of the universal features in metal-insulator transition and magnetoresistivity of mixed-phase manganites. The approach is based on utilization of some hypothesis appropriate to the Preisach picture of the magnetization process for half-metallic ferromagnets and an assumption that in doped manganites a Griffiths-type phase exists just above the magnetic-ordering temperature. Within the model, the system is considered as a random three-dimensional resistor network where a self-consistent formation of paths with metal and polaron types of conductivity is not only due to magnetic field variation but also due to temperature changes, as well. Both mechanisms of intrinsic percolation transition are considered on one basis. The theory is able to replicate the basic regularities found experimentally for doped manganites resistivity dependence on temperature and magnetic field without the need for empirical input from the magnetoresistive data. Within the approach a natural basis has arisen for a qualitative classification of magnetoresistive materials into those, such as La0.7Sr0.3MnO3, showing modest magnetoresistivity, and those, such as La0.7Ca0.3MnO3, showing large magnetoresistivity.