Role of spin polarized tunneling in magnetoresistance and low temperature minimum of polycrystalline La_1−x K _x MnO₃ (x=0·05, 0·1, 0·15) prepared by pyrophoric method

Abstract

The low temperature resistivity and magnetoresistance of bulk samples of La_1−x K _x MnO₃ has been investigated between 10 K and 300 K with and without the magnetic field (H=0·8 T). All the samples show metal-insulator transitions with Curie temperature (T _C) ranging between 260 K and 309 K. At temperature below 60 K, the K-doped manganites exhibit a shallow minimum, which disappears under an applied field of 0·8 T. This field dependent minimum in resistivity, observed in K-doped lanthanum manganites is explained in the light of intergrain tunneling of the charge carriers between anti-ferromagnetically coupled grains of the polycrystalline samples. The field variation of magnetoresistance below T _C follows a phenomenological model which considers spin polarized tunneling at the grain boundaries. The intergranular contribution to the magnetoresistance is separated out from that due to spin polarized tunneling part at the grain boundaries. The temperature dependence of intrinsic contribution to the magnetoresistance follows the prediction of the double exchange model for all values of field at T<T _C.