Experimental studies of colossal magnetoresistance manganites: effects of oxygen non-stoichiometry, 55Mn nuclear magnetic resonance, slow relaxation near the metal-insulator phase transition

Single crystals of La1−xSrxMnO3 (0≤x≤0.35) and Pr0.67Ca0.33MnO3 were studied by various techniques including SQUID magnetometry, resistivity and 55Mn nuclear magnetic resonance (NMR). In an untwinned LaMnO3 single crystal, the magnetization versus field at low T exhibits a strongly anisotropic behaviour and a weak ferromagnetic moment depending on oxygen stoichiometry is observed along c axis. The NMR spectra of La0.85Sr0.15MnO3 reveal a coexistence between ferromagnetic insulating and metallic phases. The drastic effect of oxygen deficiency on the Curie temperature is discussed. Slow thermal relaxation at the metal-insulator transition of Pr0.67Ca0.33MnO3 is studied by magnetization and resistivity measurements as a function of time. The experiments support the coexistence of metallic and insulating phases and a percolative behaviour is evidenced in the time variation of resistivity. A tentative interpretation by a phenomenological two-level model is given.