Crossover effect of inter-granular transport and quantum correction in Co-doping La2/3Ca1/3MnO3 manganites

Abstract

The dependence of electrical transport on temperature and magnetic field is studied systemically in La2/3Ca1/3Mn1−xCoxO3 (x=0–0.15) systems between 2.5 and 300K in magnetic field ranging from 0 to 6T. The substitution of Co at Mn site results in weakening the ferromagnetism, a deterioration of the metallic conductivity. The PM–FM transition is broadened with increasing Co doping content x, and the magnetic spontaneous state has changed into a cluster glass state for the sample of x=0.15. The resistivity minima are clearly observed at low temperature (T<50K) for all the samples. An unusual modulation effect under magnetic field for low temperature resistivity minimum of the Co-doping samples is observed. The abnormal behavior of Co-doped samples is that the resistivity minimum temperature (Tmin) shifts to higher temperatures with the increase in magnetic field. It is distinctly different from undoped and low doped samples where Tmin shifts to lower temperature under higher magnetic fields. In other words, the resistivity upturn of Co-doped samples is enhanced by magnetic field instead of the general field suppression effect. The observed resistivity minimum at low temperatures is partly due to the coexistence of electron–electron interaction enhanced by an intrinsic magnetic inhomogeneity and spin polarization tunneling through grain boundaries. The electron–electron scattering due to the Coulomb interaction is the intrinsic mechanism for the low temperature resistivity upturn. Electrical resistivity behavior of the compounds as function of temperature at paramagnetic state could be explained on the basis of variable-range hopping model.