SPIN DEPENDENT HOPPING AND CMR-SCALING IN Nd_{0.5}Sr_{0.5}Mn0_{3} THIN FILMS

__Y. Bruynseraede__, P. Wagner, I. Gordon, L. Trappeniers, J. Vanacken, and V.V. Moshchalkov, Laboratorium voor Vaste-Stoffysica en Magnetisme, KU Leuven, Celestijnenlaan 200 D, 3001 Leuven/Belgium.

The magnetic perovskite Nd_{0.5}Sr_{0.5}MnO_{3} is characterized by a crossover from a semiconducting to a quasi-metallic state at the para- (PM) to ferromagnetic (FM) transition around Tc=225K. The colossal negative magnetoresistivity (CMR) scales differently in the two magnetic regimes, i.e. with a parabolic curvature above, and a hyperbolic-like curvature below Tc. From Hall effect data we found that the carrier mobility is extremely low while the carrier density is not affected by external fields. Both observations justify the use of an extended Mott model with spin-dependent hopping. Calculations of the average spin misorientation in terms of the external magnetic field and the interval Weiss field predict a simple CMR scaling with the linear (FM) or quadratic (PM) Brillouin function. Both predictions have been confirmed by pulsed-field measurements up to 50 T. –Supported by the Belgian IUAP-and Flemish GOA- and FWO programmes.