Spin-dependent hopping in the paramagnetic state of the bilayer manganite (La _0.4 Pr _0.6 ) _1.2 Sr _1.8 Mn ₂ O ₇

Abstract

We report on the scaling behaviour of the colossal negative magnetoresistance effect (CMR) in a single crystal of the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7. The high Pr content is responsible for the absence of long-range ferromagnetism and for a CMR effect in the order of 106. The system is down to the Neél temperature (TN = 50 K) in a superparamagnetic state with spin clusters extending over 6–9 unit cells. The coefficient C, which describes the magnetization-induced resistance drop in the paramagnetic state, has absolute values up to C = 15 at 100 K. The temperature dependence C ∝ T−1 is explained by spin-dependent hopping between magnetic clusters. Spin-dependent scattering and Kondo-lattice models do not account for the magnitude of C and its variation with temperature.