Relation of conduction changes and magnetic properties in Nd1-xPbxMnO3 (0.10 ≤ x ≤ 0.25)

Measurements of bulk conductivity and dielectric relaxation processes by ac methods including complex-plane impedance analyses have been carried out on insulating Nd1-xPbxMnO3 (0.10 ≤ x ≤ 0.25), together with measurements of four-probe dc conductivity and magnetic susceptibility, so as to investigate the correlation between bulk conduction and short-range ferromagnetism due to double-exchange (DE) interactions working even above ferromagnetic ordering temperature (Tc). In bulk conduction, there are three temperature regimes where different processes dominate the electronic conduction: (i) the paramagnetic regime above the temperature at the onset of deviation of inverse magnetic susceptibility from the Curie-Weiss law, (ii) the intermediate regime and (iii) the temperature region below Tc. Though the polaronic conduction occurs in both high-temperature and intermediate regions, the hopping energy and the energy required to create a free hopping polaron differ considerably in these regimes. An anomaly emerges in both bulk conduction and dielectric relaxation processes around Tc at x = 0.20 and this anomaly becomes more clear at x = 0.25. By taking account of electron-phonon interactions, these results are discussed mainly in terms of the DE mechanism that brings about short-range ferromagnetism