Magnetic and structural instabilities in the stripe-phase region of La1.875Ba0.125-ySryCuO4 (0 <= y <= 0.1)

Zero-field positive muon spin relaxation (ZF-mu(+)SR) experiments were perf ormed to investigate the magnetic properties of the low-temperature structu ral modifications of the La1.875Ba0.125SryCuO4 (Y = 0.0, 0.025, 0.050, 0.07 5, 0.100) series, in which the total hole concentration is close to 1/8. To gether with high-resolution time-of-flight neutron powder diffraction measu rements, the results imply that the interplay among lattice distortions, do ping and superconductivity is intimately related to the magnetic correlatio ns of the Cu spins, which are interpreted in terms of a stripe-phase model. Materials with y less than or equal to 0.075 exhibit an incomplete structu ral transition at T-d2, from a low-temperature orthorhombic (LTO, Bmab) to a low-temperature tetragonal (LTT, P4(2)/ncm) phase. Diffraction patterns c ollected while approaching T-d2 reveal a rapid reduction of the orthorhombi city in the LTO phase with a residual fraction always surviving to low temp eratures. Different behaviour is shown by the composition with y = 0.1. Rie tveld analysis shows coexistence of the LTO phase with a less distorted low -temperature orthorhombic phase (LTO-2, Peen). In me LTT phase, a fraction of muons senses regions with purely static spin correlations due to an inco mmensurate spin-density wave. The muon spin depolarization suggests that in the spatially separated remnant LTO, hole-rich domains, dynamical correlat ions of charge density give rise to superconductivity. As the spin fluctuat ions become static at y less than or equal to 0.075, the London penetration depth, h calculated from the transverse-field mu(+)SR depolarization rate, sigma(T --> 0) proportional to lambda(-2) proportional to n(s)/m*, reveals a decrease in the superconducting carrier density. The magnetic freezing t emperature, T-f is suppressed and the magnetic phase fraction shrinks as y increases, whereas the superconducting correlations persist in a larger sam ple volume. The disappearance of long-range magnetic order at y = 0.1 and t he growth of a quasistatic component correlate well with the presence of th e LTO-2 microstructure which behaves as a buffer phase out of which the LTT domains become dominant in the Ba-rich compositions.