Ferromagnetic (FM) manganites are of great interest because of the varied phenomena associated with the strong interplay between charge, spin and structure in these materials. Doped manganites have a metallic ground state associated with FM order, and this was originally understood by the so-called double-exchange (DE) mechanism proposed by Zener in the 1950s. This mechanism has been an essential ingredient for understanding the coupled ferromagnetic-paramagnetic and metallic-insulator transitions. It also provided insight into the colossal magnetoresistance (CMR) effect which the manganites exhibit – the very large changes in electrical resistance produced when magnetic fields are applied. However, the nature of the FM metallic ground state, especially the spin dynamics, is still not understood. Using the time-of-flight neutron scattering technique, we have been able to map out the complete pattern of spin behaviour in the prototype CMR manganite compound La1-xCaxMnO3. Abnormal spin behaviour is revealed by observed deviations from the expectations of the simple DE model. None of the prevailing theoretical approaches can account for our experimental observations.
F Ye (Oak Ridge National Laboratory, USA), Pengcheng Dai, JA Fernandez-Baca (University of Tennessee and Oak Ridge National Laboratory, USA), DT Adroja (ISIS), TG Perring (ISIS), Y Tomioka, Y Tokura (Correlated Electron Research Centre, Japan)
Contact: Dr F Ye, email@example.com
Research date: December 2008
F Ye et al., Phys. Rev. B 75 (2007) 14440