The precursor phase of the CMR metallic state probed by spin and lattice dynamics
Résumé
The origin of the colossal magnetoresistance (CMR) observed in La1-xCaxMnO3, at x approximate to 0.3, is still largely debated. Actually, the precursor phase of the ferromagnetic ( F) metallic state, defined as the concentration ranges 0.125 <= x(Ca) < 0.22 and 0.1 <= x(Sr) < 0.17 in La1-xCaxMnO3 and La1-xSrxMnO3 systems respectively, is poorly understood. At these concentrations where the compounds are F, a very specific temperature behaviour is observed, the systems evolving from a quasi-metallic state below T-C towards an insulating state at lower temperatures. The double-exchange coupling alone is insufficient to describe the physics of these compounds and other interactions have to be considered, the origin of which is still unclear. In this paper, we mainly review and discuss neutron scattering studies performed on three compounds, La0.83Ca0.17MnO3, La0.875Sr0.125MnO3 and La0.8Ca0.2MnO3. The magnetic excitations, as well as the dispersion of acoustic and lower optical phonons have been determined using inelastic neutron scattering. In the three systems, and over the whole temperature range, the spin-wave excitation spectrum is characterized by a splitting into several levels or several branches which are more or less dispersed. In the quasi-metallic state, particularly studied in the two Ca-doped compounds, these levels can be characterized as spin-waves confined within nanosize F spin domains. Within some model, a quantitative analysis of these excitations is proposed which determines their sizes, shapes and magnetic couplings. These couplings are found to be anisotropic, a feature characteristic of some orbital-ordering. These observations are interpreted by a charge segregation. Moreover, in the low-temperature state of La0.83Ca0.17MnO3, the existence of an underlying periodicity for the magnetic characteristics is revealed, suggesting a charge-ordered state similar to the case of La0.875Sr0.125MnO3. Finally, a coupling of magnons with acoustic and lower optical phonon branches is strongly suggested, particularly in La0.875Sr0.125MnO3, where a coincidence between the magnon and acoustic phonon energies is observed in a large q-range along the [ 1 0 0] direction.
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