Random existence of charge ordered stripes and its influence on the magnetotransport properties of La0.6Sr0.4MnO3 perovskite substituted with diamagnetic ions at Mn sublattice.

Phase-singular solid solutions of La0.6Sr0.4Mn1-yMeyO3 (0≤y≤0.3) [Me=Li1+, Mg2+, Al3+, Ti4+, Nb5+, Mo6+ or W6+] [LSMey] perovskite of rhombohedral symmetry (space group: R<OVERLINE>3</OVERLINE>c) have been prepared wherein the valence of the diamagnetic substituent at Mn site ranged from 1 to 6. With increasing y-content in LSMey, the metal-insulator (TM-I) transition in resistivity-temperature ρ(T) curves shifted to low temperatures. The magnetization studies M(H) as well as the M(T) indicated two groups for LSMey. (1) Group A with Me=Mg, Al, Ti, or Nb which are paramagnetic insulators (PIs) at room temperature with low values of M (<0.5μB/Mn); the magnetic transition [ferromagnetic insulator (FMI)-PI] temperature (TC) shifts to low temperatures and nearly coincides with that of TM-I and the maximum magnetoresistance (MR) of ∼50% prevails near TC (≈TM-I). (2) Group-B samples with Me=Li, Mo, or W which are FMIs with Ms=3.3–3.58μB/Mn and marginal reduction in TC∼350 K as compared to the undoped LSMO (TC∼378 K). The latter samples show large temperature differences ΔT=Tc-TM-I, reaching up to ∼288 K. The maximum MR (∼60%) prevails at low temperatures corresponding to the M-I transition TM-I rather than around TC. High resolution lattice images as well as microscopy analysis revealed the prevalence of inhomogeneous phase mixtures of randomly distributed charge ordered-insulating (COI) bistripes (∼3–5 nm width) within FMI charge-disordered regions, yet maintaining crystallographically single phase with no secondary precipitate formation. The averaged ionic radius <rB>, valency, or charge/radius ratio <CRR> cannot be correlated with that of large ΔT; hence cannot be used to parametrize the discrepancy between TC and TM-I. The M-I transition is controlled by the charge conduction within the electronically heterogeneous mixtures (COI bistripes+FMI charge disordered); large MR at TM-I suggests that the spin-ordered FM-insulating regions assist the charge transport, whereas the TC is associated with the bulk spin ordered regions corresponding to the FMI phase of higher volume fraction of which anchors the TC to higher temperatures. The present analysis showed that the double-exchange model alone cannot account for the wide bifurcation of the magnetic and electric transitions, contributions from the charge as well as lattice degrees of freedom to be separated from spin/orbital ordering. The heterogeneous phase mixtures (COI+FMI) cannot be treated as of granular composite behavior [ABSTRACT FROM AUTHOR]