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

In low doping regime, manganites present a very particular spin dynamics wi th two spin wave branches and peculiar static features associated to ferrom agnetic clusters. We point out the existence of an average canted antiferro magnetic state in a large domain of concentration and an insulator ferromag netic state preceeding the metal-insulator transition (x similar or equal t o 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 sys tem. 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 thres hold of the ferromagnetic clusters has been reached, announcing the metal-i nsulator transition. (C) 2000 Elsevier Science B.V. All rights reserved.