Approach to the metal-insulator transition in La 1 − x Ca x MnO 3 ( 0 x 0 . 2 ) : Magnetic inhomogeneity and spin-wave anomaly

We describe the evolution of the static and dynamic spin correlations of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ for $x=0.1,$ 0.125, and 0.2, where the system evolves from the canted magnetic state towards the insulating ferromagnetic state, approaching the metallic transition $(x=0.22).$ In the $x=0.1$ sample, the observation of two spin-wave branches typical of two distinct types of magnetic coupling and of a modulation in the elastic diffuse-scattering characteristic of ferromagnetic inhomogeneities confirms the static and dynamic inhomogeneous features previously observed at $xl0.1.$ The anisotropic q dependence of the intensity of the low-energy spin wave suggests a bidimensional character for the static inhomogeneities. At $x=0.125,$ which corresponds to the occurrence of a ferromagnetic and insulating state, one spin wave branch, anisotropic, is detected. At this concentration, an anomaly appears at ${\mathbf{q}}_{0}=(1.25,1.25,0).$ At $x=0.2,$ the spin-wave branch appears as isotropic. In addition to the anomaly observed at ${q}_{0},$ extra magnetic excitations are observed at larger q, forming an optical branch.