TEMPERATURE PROFILE OF RESISTIVITY OF MANGANITES

Abstract

In the resistivity–temperature curves of La 0.7 Ca 0.3 MnO 3 and La 0.75 Ca 0.25 MnO 3, five distinct regions have been identified. For example, in La 0.7 Ca 0.3 MnO 3, the regions are: (1) from the lowest temperature of measurement to 200 K where resistivity is directly proportional to temperature with a small slope, (2) the reciprocal of resistivity follows Bloch's T3/2 law, (3) resistivity is directly proportional to temperature with a large slope, (4) the cusp region, where there are FM clusters of reduced dimensions and varying degrees of correlated polarons or bipolarons and small polarons, (5) the semiconducting region having small polarons. The regions 1, 2 and 3 together appear to replicate the spontaneous magnetization curves of the materials. The effect of magnetic field on the resistivity–temperature curves are as follows: (a) resistivity is reduced. In particular, the resistivity peak with magnetic field occurs at a temperature μH/K downward from the T c of the curve without magnetic field; (b) resistivity peak is displaced by μ · μH/K towards the higher temperature side compared to the resistivity peak without magnetic field; (c) cusp region is broadened; (d) magnetoresistance is appreciable only within 10% of Tp on both sides. All these observations suggest that temperature profile of resistivity of manganites is controlled by magnetic effects.