Suppression of charge-ordering and appearance of magnetoresistance in a spin-cluster glass manganite La 0.3Ca 0.7Mn 0.8Cr 0.2O 3

Abstract

The magnetic properties of electron-doped manganite La 0.3Ca 0.7MnO 3 and La 0.3Ca 0.7Mn 0.8Cr 0.2O 3 polycrystalline samples prepared by sol–gel technique have been investigated between 5 and 300 K in magnetic fields ranging from 0 to 5 T. The transition at 260 K, attributed to charge ordering in La 0.3Ca 0.7MnO 3, is completely suppressed in the Cr-substituted sample while the onset of a magnetic remanence followed by the appearance of a magnetic irreversibility at lower temperatures is observed in both samples. These features indicate that ferromagnetic clusters coexist with either an antiferromagnetic phase for La 0.3Ca 0.7MnO 3 or a spin-cluster glass phase for La 0.3Ca 0.7Mn 0.8Cr 0.2O 3 at the lowest temperatures. The exponential temperature dependence of the resistivity for the Cr-substituted sample is consistent with the small polaron hopping model for 120 K< T<300 K, while the data are better described by Mott's hopping mechanism for T<120 K. Whereas the parent compound La 0.3Ca 0.7MnO 3 is known to show no magnetoresistance, a large negative magnetoresistance is observed in the La 0.3Ca 0.7Mn 0.8Cr 0.2O 3 sample below 120 K. The appearance of the CMR is attributed to spin dependent hopping between spin clusters and/or between ferromagnetic domains.