An effective medium model applied to the conductivity of La-Ca-Mn-O across the phase transition temperature

Colossal magnetoresistance is closely related to (but is not) the abrupt change of electrical resistivity in the vicinity of Curie temperature, which is caused by the temperature-dependent paramagnetic-ferromagnetic phase transition and concurrent change of electrical conducting mechanism. A model equation based on the effective medium theory is presented to explain the abrupt change of resistivity in the vicinity of Curie temperature of La-Ca-Mn-O (LCMO). It is assumed that LCMO is in the form of mixture of ferromagnetic and paramagnetic phase in the magnetic transition region. The main ingredients of the equation are a simple effective medium theory and a function for the temperature-dependent fraction of ferromagnetic phase. This model fits very well to the measured resistance-temperature curves of La0.7Ca0.3MnO3