Random existence of charge ordered stripes and its influenceon the magnetotransport properties of La 0.6 Sr 0.4 Mn O 3 perovskitesubstituted with diamagnetic ions at Mn sublattice

Phase-singular solid solutions of La 0.6 Sr 0.4 Mn 1 − y Me y O 3 ( 0 ⩽ y ⩽ 0.3 ) [ Me = Li 1 + , Mg 2 + , Al 3 + , Ti 4 + , Nb 5 + , Mo 6 + or W 6 + ] [ LSMe y ] perovskite of rhombohedral symmetry (space group: R 3 ¯ c ) have been prepared wherein the valence of the diamagnetic substituent at Mn site ranged from 1 to 6. With increasing y -content in LSMe y , the metal-insulator ( T M - 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 LSMe y . (1) Group A with Me = Mg , Al, Ti, or Nb which are paramagnetic insulators (PIs) at room temperature with low values of M ( < 0.5 μ B ∕ Mn ) ; the magnetic transition [ferromagnetic insulator (FMI)-PI] temperature ( T C ) shifts to low temperatures and nearly coincides with that of T M - I and the maximum magnetoresistance (MR) of ∼ 50 % prevails near T C ( ≈ T M - I ) . (2) Group-B samples with Me = Li , Mo, or W which are FMIs with M s = 3.3 - 3.58 μ B ∕ Mn and marginal reduction in T C ∼ 350 K as compared to the undoped LSMO ( T C ∼ 378 K ) . The latter samples show large temperature differences Δ T = T c - T M - I , reaching up to ∼ 288 K . The maximum MR ( ∼ 60 % ) prevails at low temperatures corresponding to the M-I transition T M - I rather than around T C . 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 ⟨ r B ⟩ , valency, or charge/radius ratio ⟨CRR⟩ cannot be correlated with that of large Δ T ; hence cannot be used to parametrize the discrepancy between T C and T M - I . The M-I transition is controlled by the charge conduction within the electronically heterogeneous mixtures (COI bistripes + FMI charge disordered); large MR at T M - I suggests that the spin-ordered FM-insulating regions assist the charge transport, whereas the T C is associated with the bulk spin ordered regions corresponding to the FMI phase of higher volume fraction of which anchors the T C 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