Universität Stuttgart, Germany and General Physics Institute (Moscow)
A systematic study of the low-energy electrodynamic response of a series of La0.25Ca0.75MnO3 samples of different morphology (pressed pellets) is discussed covering the terahertz-subterahertz range from 4 to 700 cm-1 (quantum energies from 0.4 meV to 90 meV) with particular attention focused at frequencies below 40 cm-1. In the antiferromagnetically ordered phase below 140 K, a resonance-like absorption band appears in the spectra around 20 - 40 cm-1 which is assigned to acoustical phonons that acquire optical activity due to a 4a superstructure appearing in the crystallographic a direction. Nano-size effects in charge and magnetic subsystems are investigated by reducing the sample's grains diameters from several micrometers down to 40 nm. With decreasing the grains, the absorption band around 20 - 40 cm-1 and hence the 4a-superlattice are gradually suppressed while the charge and the AFM orders survive, indicating weak coupling of the corresponding order parameters to the crystal lattice. At the lowest frequencies 4 - 20 cm-1 a strong relaxation in the conductivity and permittivity spectra is observed in all samples. Possible mechanisms responsible for the relaxation are discussed.