Strong change in spin dynamics close to percolation in La1-xCaxMnO3

In low doping regime, manganites present a very particular spin dynamics with two spin wave branches and peculiar static features associated to ferromagnetic clusters. We point out the existence of an average canted antiferromagnetic state in a large domain of concentration and an insulator ferromagnetic state preceeding the metal-insulator transition (x similar or equal to 0.2). The high-energy spin wave exhibits a huge evolution between x = 0.1 and 0.15 announcing the disappearance of the antiferromagnetism in the system. The low-energy branch is still observed in these compounds and it can be related to static ferromagnetic clusters. The huge evolution of the high-energy spin wave can be interpreted by the fact that the percolation threshold of the ferromagnetic clusters has been reached, announcing the metal-insulator transition. (C) 2000 Elsevier Science B.V. All rights reserved.