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$ $$|${\bf q$_0$}$|$, 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.