The conductivity and magnetoresistance of La0.5Pb0.5Mn1−xCrxO3 (0.0⩽x⩽0.45) measured at 0.0 and 1.5 T magnetic field have been reported. All the oxide samples except x=0.45, showed metal insulator transition (MIT) between 158–276 K, depending on x. In contrast to the behavior of a similar sample La0.7Ca0.3Mn1−xCrxO3 showing no (MIT) for x⩾0.3, the Pb doped samples showed MIT even with x=0.35. The MIT peak temperature (Tp) shifts towards lower temperature with increasing x while magnetic field shifts Tp to the high temperature regime. The metallic (ferromagnetic) part of the temperature dependent resistivity (ρ) curve (below Tp) is well fitted with ρ(T)=ρ0+ρ2.5T2.5 indicating the importance of electron–magnon interaction (second term). We have successfully fitted the high temperature (T>θD/2, θD is Debye temperature) conductivity data, both in presence and in absence of magnetic field, with small polaron hopping conduction mechanism. Adiabatic small polaron hopping conduction mechanism is followed by the s...

Nature of small-polaron hopping conduction and the effect of Cr doping on the transport properties of rare-earth manganite La0.5Pb0.5Mn1−xCrxO3

A systematic investigation of lanthanum-based manganite, La0.67Ca0.33MnO3, has been undertaken with a view to understand the influence of varying crystallite size, in the nanoscale, on various physical properties. The materials were prepared by the sol–gel route by sintering at four different temperatures starting from 800 to 1100 °C, with an interval of 100 °C. After the usual characterization of these materials structurally by XRD, their metal-insulator transition (TP) as well as magnetic transition (TC) temperatures were determined. Surprisingly these materials are found to exhibit two different types of behaviors, viz, while TC is found to decrease from 253 to 219 K, TP is increasing from 145 to 195 K with increasing sintering temperature. A systematic study of electrical conductivity of all four materials was undertaken not only as a function of temperature (80–300 K), but also as a function of magnetic field up to 7 T mainly to understand the detailed conduction mechanism in these materials. On analyzing the data by using several theoretical models, it has been concluded that the metallic (ferromagnetic) part of the resistivity (ρ) (below TP) fits well with the equation ρ ( T ) = ρ 0 + ρ 2.5 T 2.5 , indicating the importance of grain/domain boundary effects (ρ0) and electron–magnon scattering processes (∼T2.5). On the other hand, in the high temperature (T>TP) paramagnetic insulating regime, the adiabatic small polaron and VRH models fit well in different temperature regions, thereby indicating that polaron hopping might be responsible for the conduction mechanism.

Effect of sintering temperature on electrical transport properties of La0.67Ca0.33MnO3

The structural and electrical transport properties of LaMn1−xFexO3 (0.1 ≤ x ≤ 0.6) bulk samples have been investigated. The powder x-ray diffraction patterns at room temperature show that all samples are formed in single phase. The temperature dependent resistivity data have been fitted with the Mott's variable-range hopping (VRH) model for an entire studied range of the temperature (77–300 K) to calculate the hopping distance (Rh) and the density of states at Fermi level (N(EF)). It is found that all parameters vary systematically with the increase in Fe concentration. Moreover, the resistivity data were also fitted in the small polaron hopping (SPH) model. The non-adiabatic SPH conduction mechanism is followed by all samples. This type conduction mechanism is far accompanied by subtle electronically induced structural changes involving in Fe–O–Fe and Fe–O–Mn bond angles and bond lengths. Thus we suggest that the transport properties can be explained according to the additional localization of charge car...

Woo-Hwan Jung

Papers

Evaluation of Mott's Parameters for Hopping Conduction in La0.67Ca0.33MnO3 Above Tc