Theoretical study of magnetotransport properties of colossal magnetoresistive manganites (Re1−xAxMnO3): A variational treatment

The magnetotransport properties like electrical resistivity and Thermoelectric power of rare earth manganites doped with alkaline earths namely Re1−xAxMnO3 (where Re=La, Pr, Nd etc., and A=Ca, Sr, Ba etc.) which exhibit colossal magnetoresistance (CMR), metal-insulator transition and many other poorly understood phenomena, have been studied in this work by means of a Variational method. We have used two band (ℓ-b) model for manganites in the strong electron-lattice Jahn-Teller (JT) coupling regime, an approach similar to the two-fluid models along with (ℓ-b) hybridization. We have already used this Variational method to study the zero field electrical resistivity ρ(T) of doped CMR manganites. In the present study we find that the resistivity decreases with increasing magnetic field and the metal-insulator transition temperature (Tρ) shifts towards higher temperature region. This may happen because of the delocalized charge carriers, induced by applied magnetic field, decreasing the resistivity which in turn leads to the local ordering of the electron spins. Due to this ordering, the ferromagnetic metallic state might have suppressed the paramagnetic insulating regime resulting in the observed increase in Tρ under applied magnetic field. We have also shown the temperature dependent magneto-resistance (MR) ratio at different magnetic fields. We obtained large MR value at low temperatures. An interesting feature of the Magnetic field dependent thermoelectric power Q(T) is that it rises gradually with decreasing temperatures and a broad peak appears at low temperatures and as the temperature is lowered further, the value of Q(T) drops sharply (i.e. Q→0 as T→0 ), which is suggested to originate from the weak carrier localization effect. In the external magnetic field, the phonon-drag effect is expected to become weaker so that value of Q(T) reduces.