Evidence of Spin Density Waves in La₃Ni₂O_7-δ

Summary

The recently discovered superconductivity with critical temperature 𝑇𝑐 up to 80 K in the double-layer Nickelate La₃⁢Ni₂⁢O₇−𝛿 under pressure has drawn great attention. Here, we report the positive muon spin relaxation (𝜇+⁢SR) study of polycrystalline La₃⁢Ni₂⁢O₆.92 under ambient pressure. Zero-field 𝜇+⁢SR experiments reveal the existence of magnetic order in La₃⁢Ni₂⁢O₆.92 with 𝑇𝑁=154 K. The weak transverse field 𝜇+⁢SR measurements reveal the bulk nature of magnetism. In addition, a small quantity of oxygen deficiencies can greatly broaden the internal magnetic field distribution sensed by muons.

Materials

• La₃Ni₂O₆.92

Methods

• Positive muon spin relaxation (μ+SR)
• Weak transverse field μ+SR

Keywords

• spin density waves
• magnetic order
• oxygen deficiencies

Highlights

• Bulk nature of magnetism confirmed by weak transverse field μ+SR.

Conclusions

• Magnetic order exists in La₃Ni₂O₆.92 with TN=154 K.
• Oxygen deficiencies greatly broaden the internal magnetic field distribution.

Main claims

• Bulk commensurate magnetic order (spin density wave) exists in La₃Ni₂O₆.92 at ambient pressure with TN=154 K.
  • Evidence: ZF-μSR spectra show oscillations below 148 K,wTF-μSR shows magnetic volume fraction reaches 90%,Fitting gives TN=154 K from Bfast1 temperature dependence (Eq.2)
• Oxygen deficiencies broaden the internal magnetic field distribution sensed by muons.
  • Evidence: Bfast2 component with large relaxation rate attributed to oxygen vacancies,Comparison of field distribution calculations with perfect crystal and vacancy model
• The magnetic structure is consistent with a spin-charge stripe model with moments along c-axis, and the ordered moment is about 0.22-0.42 μB.
  • Evidence: Dipole field calculation reproduces observed field strengths,Muon stopping site analysis

Workflow

• Sample synthesis — High-quality polycrystalline sample with controlled oxygen deficiency.
  • Materials: La₂O₃ (99.999%); NiO (99.99%)
  • Methods: Solid-state reaction; Sintering at 1100°C in air for 50h, repeated 3 times
  • Observations: Powder X-ray diffraction confirms orthorhombic structure (space group Amam); TGA gives oxygen content La₃Ni₂O₆.92
• Physical property characterization — Bulk property measurements do not show clear anomaly at the magnetic transition due to oxygen inhomogeneity.
  • Materials: Polycrystalline pellet
  • Methods: Resistivity (four-probe method); Magnetization (SQUID)
  • Observations: Resistivity shows negative temperature coefficient, no anomaly near 150K; Magnetic susceptibility shows no magnetic phase transition down to 2 K, upturn at low T
• Muon spin relaxation experiments — Evidence for bulk commensurate magnetic order with TN=154 K.
  • Materials: Polycrystalline La₃Ni₂O₆.92 powder
  • Methods: Zero-field (ZF) and weak transverse field (wTF) μ+SR
  • Observations: ZF-μSR: Below 170 K, exponential decay; below 148 K, oscillations appear; Fourier transform shows multiple internal fields (10 mT, 140 mT, broad 160 mT); wTF-μSR: Oscillating signal decreases with cooling, reaching 90% magnetic volume fraction below 93K
• Data analysis and interpretation — The magnetic order is commensurate SDW with possible stripe order; oxygen deficiencies significantly affect the internal field distribution.
  • Materials: μSR spectra
  • Methods: Least-squares fitting with Eq.(1) for ZF and Eq.(3) for wTF; MuFinder and CASTEP for muon stopping sites; DipoleCal for internal field calculation
  • Observations: Four magnetic components (three oscillating, one non-oscillating); Critical exponent β=0.26(2) suggests reduced dimensionality; Dipole field calculation consistent with spin-charge stripe model; Oxygen vacancies cause inhomogeneous broadening (Bfast2 component)