The charge-ordered perovskite Pr0.65Ca0.28Sr0.07MnO3 was investigated by means of magnetic susceptibility, specific heat, dielectric and optical spectroscopy and electron-spin resonance techniques. Under moderate magnetic fields, the charge order melts yielding colossal magnetoresistance effects with changes of the resistivity over eleven orders of magnitude. The optical conductivity is studied from audio frequencies far into the visible spectral regime. Below the phonon modes hopping conductivity is detected. Beyond the phonon modes the optical conductivity is explained by polaronic excitations out of a bound state. ESR techniques yield detailed informations on the (H,T ) phase diagram and reveal a broadening of the linewidth which can be modeled in terms of activated polaron hopping. PACS. 63.20.-e Phonons in crystal lattices -71.30.+h Metal-insulator transitions and other electronic transitions -72.60.+g Mixed conductivity and conductivity transitions -78.30.-j Infrared and Raman spectra

High-field antiferromagnetic-resonance ͑AFMR͒ spectra were obtained in the frequency range 40 GHz ϽϽ700 GHz and for magnetic fields up to 8 T in twin-free single crystals of La 0.95 Sr 0.05 MnO 3 . At low temperatures two antiferromagnetic modes were detected, which reveal different excitation conditions and magnetic field dependencies. No splitting of these modes was observed for any orientation of the static magnetic field excluding the phase-separation scenario for this composition. Instead, the full data set including the anisotropic magnetization can be well described using a two-sublattice model of a canted antiferromagnetic structure.

High-field electron-spin-resonance ͑ESR͒ experiments have been carried out in single crystals of La 1Ϫx Sr x MnO 3 in the concentration range 0рxр0.175. Different quasioptical arrangements have been utilized for frequencies 40рр700 GHz and for magnetic fields Bр12 T. A splitting of the antiferromagnetic resonance ͑AFMR͒ mode is observed in the magnetic field for the parent compound LaMnO 3 , in agreement with the antiferromagnetic structure of this material. Abrupt changes in the AFMR frequencies have been observed around xӍ0.025, and were attributed to a transition between a pure antiferromagnetic and a canted state. For increasing Sr doping the observed AFMR modes are split even in a zero field, which can be naturally explained using a concept of a canted magnetic structure for xϽ0.1. In La 0.825 Sr 0.175 MnO 3 the ESR spectra are consistent with the ferromagnetic and metallic state. The lines of ferromagnetic resonance and ferromagnetic antiresonance can be clearly observed. For intermediate concentrations 0.1рxр0.15 complicated ESR spectra were observed, which can be well explained by a single ferromagnetic resonance mode and taking electrodynamic effects into account.

Magnetic field and temperature dependence of the Terahertz conductivity and permittivity of the colossal magnetoresistance manganite Pr0.65Ca0.28Sr0.07MnO3 (PCSMO) is investigated approaching the metal-to-insulator transition (MIT) from the insulating side. In the charge-ordered state of PCSMO both conductivity and dielectric permittivity increase as function of magnetic field and temperature. Universal scaling relationships ∆ε ∝ ∆σ are observed in a broad range of temperatures and magnetic fields. Similar scaling is also seen in La1−xSrxMnO3 for different doping levels. The observed proportionality points towards the importance of pure ac-conductivity and phononic energy scale at MIT in manganites.

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