La139 NMR is suited for investigations into magnetic properties of La2CuO4-based cuprates in the vicinity of their magnetic instabilities, owing to the modest hyperfine interactions between La139 nuclear spins and Cu electron spins. We report comprehensive La139 NMR measurements on a single-crystal sample of high-Tc superconductor La1.885Sr0.115CuO4 in a broad temperature range across the charge and spin order transitions (Tcharge≃80 K, Tspinneutron≃Tc=30 K). From the high-precision measurements of the linewidth for the nuclear spin Iz=+1/2 to -1/2 central transition, we show that paramagnetic line broadening sets in precisely at Tcharge due to enhanced spin correlations within the CuO2 planes. Additional paramagnetic line broadening ensues below ∼35 K, signaling that Cu spins in some segments of CuO2 planes are on the verge of three-dimensional magnetic order. A static hyperfine magnetic field arising from ordered Cu moments along the ab plane, however, begins to develop only below TspinμSR=15-20 K, where earlier muon spin rotation measurements detected Larmor precession for a small volume fraction (∼20%) of the sample. Based on the measurement of La139 nuclear-spin-lattice relaxation rate 1/T1, we also show that charge order triggers enhancement of low-frequency Cu spin fluctuations inhomogeneously; a growing fraction of La139 sites is affected by enhanced low-frequency spin fluctuations toward the eventual magnetic order, whereas a diminishing fraction continues to exhibit a behavior analogous to the optimally superconducting phase even below Tcharge. These La139 NMR results corroborate our recent Cu63 NMR observation that a very broad, anomalous winglike signal gradually emerges below Tcharge, whereas the normally behaving, narrower main peak is gradually wiped out [T. Imai, Phys. Rev. B 96, 224508 (2017)10.1103/PhysRevB.96.224508]. Furthermore, we show that the enhancement of low-energy spin excitations in the low-temperature regime below Tspinneutron(≃Tc) depends strongly on the magnitude and orientation of the applied magnetic field.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics