Effects of Frequency on AC Conductivity and Magnetoresistance in Doped La0.65Ca0.35MnO3 Manganites

The effects of frequency on the alternating-current (AC) conductivity of polycrystalline La0.65Ca0.35MnO3 compounds prepared by solid-state reaction with various heat treatments have been studied. The AC conductivity and magnetoresistance (MR) are affected by the heat treatment temperature. The MR is enhanced at and below the transition temperature (TC). The AC transport behavior of this system shows pronounced negative MR even at small applied magnetic fields for temperatures well below TC. It is observed that such effects originate from spin-dependent scattering at grain boundaries as a consequence of spin disorder at the interfaces of the boundaries and spin scattering within the grains. The real part of the AC conductivity σ′ exhibits a pronounced frequency dependence, measured in the frequency range f < 100 kHz. The minimum occurs at a frequency that is temperature dependent. The MR below TC and the frequency dependence of the AC electrical conductivity σ′(f) are interpreted as being consequences of cluster and grain boundary effects.