Elastic neutron scattering experiments performed in semi-conducting La1−xCaxMnO3 single crystals (x=0.05, 0.08), reveal new features in the problem of electronic phase separation and metal insulator transition. Below TN , the observation of a broad magnetic modulation in the q-dependent scattering intensity, centered at nearly identical qm whatever the q direction, can be explained by a liquid-like spatial distribution of similar magnetic droplets. A semi-quantitative description of their magnetic state, diameter, and average distance , can be done using a two-phase model. Such a picture can explain the anomalous characteristics of the spin wave branches and may result from unmixing forces between charge carriers predicted from the s-d model.

We describe the evolution of the static and dynamic spin correlations of La1−xCaxMnO3, 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 x<0.1. The anisotropic q-dependence of the intensity of the low-energy spin wave suggests a bidimensionnal character for the static inhomogeneities. At x=0.125, which corresponds to the occurence of a ferromagnetic and insulating state, the two spin wave branches reduce to a single one, but anisotropic. At this concentration, an anomaly appears at q0=(1.25,1.25,0), that could be related to an underlying periodicity, as arising from (1.5,1.5,0) superstructures. At x=0.2, the spin-wave branch is isotropic. In addition to the anomaly observed at q0, extra magnetic excitations are observed at larger q, forming an optical branch. The two dispersion curves suggest an anti-crossing behavior at some q0' value, which could be explained by a folding due to an underlying perodicity involving four cubic lattice spacings.