Ultrafast optical detection of magnetic inhomogeneity in ferromagnetic La

Optically excited spin wave resonances in ferromagnetic La0.67Ca0.33MnO3 (LCMO) thin films are probed by subpicosecond Kerr-rotation experiments at 10 K. We identify an extra fundamental mode along with the confined spin wave modes, whose frequency and intensity are independent of film thickness, but depend strongly on the applied magnetic field. We attribute the origin of this mode to magnetic inhomogeneity at the FM ground state. Our results show that the picosecond time-resolved magneto-optical method is a powerful tool for detecting dynamic magnetic inhomogeneity in colossal magneto-resistance manganites