Ultrasonic evidence of an uncorrelated cluster formation temperature in manganites with first-order magnetic transition at the Curie temperature

Ultrasonic attenuation and phase velocity measurements have been carried out in the ferromagnetic perovskites La 2 / 3 Cc 1 / 3 MnO 3 and La 2 / 3 Sr 1 / 3 MnO 3 . Data show that the transition at the Curie temperature T C changes from first to second order as Sr replaces Ca in the perovskite. The compound with first-order transition shows also another transition at a temperature T*>T C . We interpret the temperature window T C <T<T* as a region of coexistence of a phase-separated regime of metallic and insulating regions, in the line of recent theoretical proposals.