Unusual magneto-elastic forces stabilize the insulating ferromagnetic state of manganites: the La(0.8)Ca(0.2)$MnO(3) case

The ferromagnetic and insulating state observed in La${}_{1-x}$Ca${}_x$MnO${}_3$, 0.125$<$x$<$0.2, is characterized by structural and magnetic anomalies below T${}_C$, similar to those observed in the x${}_{Sr}$$\approx$1/8. A neutron scattering study of the superlattice {\bf Q${}_0$}= (0,0,1+/4)${}_{cub}$ peak, and of the magnetic excitations are reported in the x${}_{Ca}$=0.2 sample. The occurrence of this superstructure is associated with the observation of a gap in the spin dynamics, at a {\bf q${}_0$} wave-vector ({\bf q${}_0$}={\bf Q${}_0$}-$\tau$) with the same modulus $\mathrm{\mid }${\bf q${}_0$}$\mathrm{\mid }$ in all directions, which divides the dispersion into two regimes. For $\mathrm{\mid }${\bf q}$\mathrm{\mid }$$<$$\mathrm{\mid }${\bf q${}_0$}$\mathrm{\mid }$ the dispersion is splitted into two or three curves. For $\mathrm{\mid }${\bf q}$\mathrm{\mid }$$>$$\mathrm{\mid }${\bf q${}_0$}$\mathrm{\mid }$, magnetic excitations lock on acoustic and optic phonon energies, revealing a new kind of magneto-vibrational coupling. We suggest an analysis in terms of two distinct magnetic couplings, associated with two ferromagnetic media involved into a collective state.