Magnetotransport correlations in Pr- and La-based manganites

The nature of the insulating regime in certain Pr- and La-based manganites having ferromagnetic transition temperatures (TC) close to room temperature (RT) and at TC⪡RT was studied. It is seen that in a small temperature regime close to TC and/or TP (peak resistivity temperature) the conduction mechanism is of the 3D- variable range hopping (VRH) type while far away from TP, the insulating regime is governed by an Arrhenius and/or polaronic type of behaviour. In the presence of a magnetic field the resistivity data do not fit well to either the VRH or the polaron mechanisms, however, a better fit is obtained using the magnetic localization model. Inverse ac-susceptibility (χ′) vs temperature curves show a deviation from the Curie–Weiss behaviour at a temperature (θP∼paramagnetic Curie temperature) well above TC of the respective compounds, indicating the formation of superparamagnetic clusters close to RT. Our recent studies on Ho- and Y-doped manganites (La0.7−xRxCa0.3MnO3, R=Ho and Y and x=0.05, 0.075 and 0.1) unequivocally show coupling of Ho-magnetic moment with that of Mn in the lattice [Phys. Rev. B 63 (2001) 184 434].