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

Single crystals of La1-xSrxMnO3 (0 less than or equal to x less than or equ al to 0.35) and Pr0.67Ca0.33MnO3 were studied by various techniques includi ng SQUID magnetometry, resistivity and Mn-55 nuclear magnetic resonance (NM R). In an untwinned LaMnO3 single crystal, the magnetization versus field a t 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 insu lating and metallic phases. The drastic effect of oxygen deficiency on the Curie temperature is discussed. Slow thermal relaxation at the metal-insula tor transition of Pr0.67Ca0.33MnO3 is studied by magnetization and resistiv ity measurements as a function of time. The experiments support the coexist ence of metallic and insulating phases and a percolative behaviour is evide nced in the time variation of resistivity. A tentative interpretation by a phenomenological two-level model is given. (C) 1999 Elsevier Science S.A. A ll rights reserved.