Magnetic and structural instabilities in the stripe-phase region of La_1.875Ba_0.125-ySr_yCuO₄ (0 ≤ y ≤ 0.1)

Zero-field positive muon spin relaxation (ZF-μ⁺SR) experiments were performed to investigate the magnetic properties of the low-temperature structural modifications of the La_1.875Ba_0.125-ySr_yCuO₄ (y = 0.0, 0.025, 0.050, 0.075, 0.100) series, in which the total hole concentration is close to 1/8. Together with high-resolution time-of-flight neutron powder diffraction measurements, the results imply that the interplay among lattice distortions, doping and superconductivity is intimately related to the magnetic correlations of the Cu spins, which are interpreted in terms of a stripe-phase model. Materials with y ≤ 0.075 exhibit an incomplete structural transition at 7_d2, from a low-temperature orthorhombic (LTO, Bmab) to a low-temperature tetragonal (LTT, P4₂/ncm) phase. Diffraction patterns collected while approaching T_d2 reveal a rapid reduction of the orthorhombicity in the LTO phase with a residual fraction always surviving to low temperatures. Different behaviour is shown by the composition with y = 0.1. Rietveld analysis shows coexistence of the LTO phase with a less distorted low-temperature orthorhombic phase (LTO-2, Pccn). In the LTT phase, a fraction of muons senses regions with purely static spin correlations due to an incommensurate spin-density wave. The muon spin depolarization suggests that in the spatially separated remnant LTO, hole-rich domains, dynamical correlations of charge density give rise to superconductivity. As the spin fluctuations become static at y ≤ 0.075, the London penetration depth, λ calculated from the transverse-field μ⁺SR depolarization rate, σ (T → 0) ∝ λ^-2 ∝ n_s/m*, reveals a decrease in the superconducting carrier density. The magnetic freezing temperature, T_f is suppressed and the magnetic phase fraction shrinks as y increases, whereas the superconducting correlations persist in a larger sample volume. The disappearance of long-range magnetic order at y = 0.1 and the growth of a quasistatic component correlate well with the presence of the LTO-2 microstructure which behaves as a buffer phase out of which the LTT domains become dominant in the Ba-rich compositions.