The
precursor state of the CMR metallic state is poorly understood. In La1-xBxMnO3,(
B=Ca, Sr), in the concentration range 0.125≤xCa≤0.22
and 0.1≤xSr≤0.17 where the compounds are ferromagnetic, a very
unusual temperature behaviour is observed. The systems evolve from a
quasi-metallic state below TC towards an insulating state at lower
temperatures. The double-exchange alone is insufficient to describe the physics
of these compounds and other interactions have to be considered, the origin of
which is still unclear. We will review and discuss neutron scattering studies
performed on three compounds La0.83Ca0.17MnO3
La7/8Sr 1/8MnO3 and La0.8Ca 0.2MnO3.
In the three systems and over the whole temperature range the spin wave
spectrum appears splitted into several branches more or less dispersed. In the
quasi-metallic state, these levels can be characterised as spin waves confined
within nano-size ferromagnetic domains. Within some model, a quantitative
analysis of these excitations is proposed. It determines their size, which
evolves from 4 lattice spacing (xCa=0.17 and xSr=0.125)
to 2 ones (xCa=0.2), and
their magnetic couplings. These couplings are found to be anisotropic, with a
strong two-dimensional (2D) character in the xCa=0.17 and xSr=0.125
cases. These observations are interpreted by a charge segregation picture
consisting of hole-poor domains, spatially ordered, the mobile holes forming
their boundaries. In the case of xSr=0.125, these observations lead
to a picture of 2D stripe structure.