Intercluster conduction in lightly doped La 1 − x Ca x MnO 3 manganites in the paramagnetic temperature range

The magnetotransport and magnetic properties of La 1 − x Ca x MnO 3 polycrystalline samples ( x = 0–0.3) annealed under vacuum and in the oxygen environment are investigated in the temperature range from 77 to 400 K. The magnetic studies of lightly doped manganites reveal persistence of short-range magnetic order up to a temperature T * ≈ 300 K, which is about 2–3 times higher than their Curie temperature T C . The temperature dependence of the electrical resistivity measured from T * down to nearly T ≈ T C is fitted by the relation logρ ∼ T −1/2 , which is characteristic of granular metals with electrons tunneling among nanoclusters of magnetic metals embedded in a dielectric host. The magnetoresistance of polycrystalline samples annealed in the oxygen environment has been observed to increase. The electrical, magnetic, and magnetotransport properties of the manganites can be accounted for by the formation of magnetic nanoclusters below T *, tunneling (or hopping) of carriers among the nanoclusters, variation in the magnetic cluster size, and tunneling barrier thickness with variations in temperature and magnetic field strength, as well as by the effect of annealing in different media on the cluster properties.