Nature of magnetism in bilayer nickelate La₃Ni₂O₇ single crystals

Summary

This study employs neutron scattering techniques to elucidate the spin order and dynamics in bilayer nickelate La₃Ni₂O₇ single crystals. In the ambient-pressure parent phase, clear spin excitations are observed at the reciprocal-space position Q = (0, 0.5, 2.5), exhibiting a spin gap of approximately 5 meV and pronounced in-plane anisotropic dispersion—band-edge softening along the transverse direction reveals competing exchange interactions. The excitations display an out-of-plane modulation with bilayer periodicity, directly confirming antiferromagnetic interlayer coupling. Based on linear spin-wave theory, the experimental dispersion can be accurately described by a bilayer Heisenberg Hamiltonian incorporating strong interlayer exchange and competing in-plane couplings, with a stripe-type magnetic order. After normalizing the spectral intensity to absolute units, it is found that although the spin-wave bandwidth is only 25% of that in cuprates, the local dynamical magnetic susceptibility is significantly enhanced at comparable energies, and the total fluctuating magnetic moment is comparable to that of cuprates. These results reveal that intermediate-energy spin excitations originating from strong electronic correlations are an intrinsic feature of this system, establishing a magnetic framework fundamentally distinct from that of cuprates and providing direct evidence for understanding the pairing mechanism of superconductivity in this system.

Materials

• La₃Ni₂O₇

Methods

• Neutron scattering
• Inelastic neutron scattering
• Elastic neutron diffraction
• Linear spin wave theory

Keywords

• spin excitations
• spin gap
• stripe type magnetic order
• antiferromagnetic interlayer coupling
• magnetic exchange interactions
• spin fluctuations
• magnetic anisotropy

Highlights

• First direct evidence of antiferromagnetic interlayer coupling from pronounced out-of-plane modulations in spin excitations.
• Quantitative determination of exchange interactions (Jc, J₁a, J₁b, J₂) using linear spin wave theory.
• Comparison with cuprates reveals a distinct magnetic framework: smaller bandwidth but comparable fluctuating moment suggests dense mid-energy spin fluctuations relevant for pairing.

Conclusions

• Single-crystalline La₃Ni₂O₇ exhibits stripe-type magnetic order with a spin gap of ≈5 meV and anisotropic in-plane dispersions indicative of competing exchange interactions.
• Out-of-plane modulations with bilayer periodicity provide direct evidence for antiferromagnetic interlayer coupling.
• Spin-wave bandwidth is only ≈25% of that in cuprates, but the total fluctuating magnetic moment is comparable, with enhanced mid-energy local dynamic susceptibility.
• The system is well described by a bilayer Heisenberg model with strong interlayer exchange and competing in-plane couplings.

Main claims

• La₃Ni₂O₇ exhibits a stripe-type magnetic ground state with a single magnetic phase (Q = (0,0.5,0.5)) and no phase separation.
  • Evidence: Elastic neutron diffraction shows only one magnetic peak at (0,0.5,0.5); no evidence of (0,0.5,0)
• The spin wave dispersion is described by a bilayer Heisenberg model with strong interlayer exchange (SJc ≈40 meV) and competing in-plane couplings, supporting a single-stripe order.
  • Evidence: Inelastic neutron scattering data fit well to single-stripe model; double-stripe model fails to reproduce anisotropic dispersion and intensity distribution
• Despite spin-wave bandwidth only 25% of cuprates, local dynamic susceptibility is significantly enhanced at mid energies, and total fluctuating moment is comparable to cuprates.
  • Evidence: Absolute normalization gives χ''(ω) integrated moment of 1.75 μ_B²; comparison with cuprate data shows enhanced mid-energy susceptibility

Workflow

• crystal_growth — Millimeter- to centimeter-sized La₃Ni₂O₇ single crystals.
  • Materials: La₂O₃; NiO
  • Methods: solid-state reaction; optical floating-zone growth under O₂ pressure
  • Observations: large high-quality single crystals
• elastic_neutron_diffraction — Antiferromagnetic order with stripe-type structure.
  • Materials: La₃Ni₂O₇ single crystal
  • Methods: triple-axis spectrometer TOPAN at JRR-3
  • Observations: magnetic Bragg peak at Q = (0, 0.5, 2.5); onset at 151K
• inelastic_neutron_scattering — Spin wave dispersion mapped across Brillouin zone.
  • Materials: 25 co-aligned single crystals, total mass 1.26 g
  • Methods: time-of-flight spectrometers ARCS and MAPS; absolute normalization to phonons
  • Observations: spin excitations with gap ≈5 meV; anisotropic dispersion; bilayer periodicity in L
• spin_wave_modeling — Single-stripe order with competing exchange interactions.
  • Materials: experimental dispersion
  • Methods: linear spin wave theory using SpinW; fitting exchange parameters
  • Observations: SJc = 39.86, SJ₁a = 2.36, SJ₁b = 3.71, SJ₂ = 4.63 meV
• absolute_units_normalization — Mid-energy spin excitations are a defining feature of La₃Ni₂O₇.
  • Materials: acoustic phonon normalization
  • Methods: integration of magnetic signal
  • Observations: total fluctuating moment 1.75 μ_B², comparable to cuprates