Orbital-Selective Coexistence of Interlayer Spin-Singlet Formation and SDW Order with Anomalous Spin Reconfiguration in Bilayer Nickelate La₃Ni₂O₇ Revealed by 17O-NMR

Summary

Site-selective (17)O-NMR measurements of the inner apical O(1), outer apical O(2), and planar O(3,4) sites were used to investigate the spin density wave order in the bilayer nickelate La(3)Ni(2)O(7). Below (Trm SDW=150) K, all planar O(3,4) sites broaden significantly due to the appearance of internal magnetic fields, while the O(2) sites show almost no internal field, consistent with coherent spin density wave order with single-spin–no-spin (or large–small spin) stripes; the internal field at the O(1) sites bridging the NiO(2) planes nearly cancels, indicating an antiparallel spin configuration between adjacent planes. Below (Trm Asim 115) K, however, although the in-plane spin density wave order remains robust, the O(1) spectrum disappears, indicating that the antiparallel spin arrangement through the Ni–O(1)–Ni bonds is not particularly stable. In particular, the O(2) sites, which are strongly covalently bonded to the (d3z2-r2) orbitals, exhibit extremely small local spin susceptibility, suggesting that the Ni-(d3z2-r2) orbitals bridging the NiO(2) planes already form a well-developed interlayer spin singlet. These results reveal interlayer spin singlet formation and an anomalous spin reconstruction through the Ni–O(1)–Ni bonding orbitals, reflecting the orbital-selective nature of this bilayer nickelate.

Materials

• La₃Ni₂O₇+δ polycrystalline

Methods

• 17O-NMR
• 139La-NMR
• 139La-NQR
• DFT
• thermogravimetric analysis

Keywords

• spin density wave
• interlayer spin singlet formation
• orbital selective
• spin reconfiguration
• canted afm stacking
• dzyaloshinskii moriya interaction
• single spin spinless stripe
• large tiny spin stripe

Highlights

• Site-selective 17O-NMR successfully distinguishes the inner apical O(1), outer apical O(2), and planar O(3,4) oxygen sites in bilayer La₃Ni₂O₇.
• The extremely small local spin susceptibility at O(2), despite strong covalency with the d3z2-r2 orbital, reveals a well-developed interlayer spin-singlet formation.
• The interlayer spin alignment changes from antiparallel to canted AFM below about 115 K while the in-plane SDW order remains robust.
• The proposed orbital-selective coexistence of primarily dx2-y2-derived SDW order and d3z2-r2-derived interlayer spin-singlet formation offers key insight into which orbitals may drive high-T_c superconductivity under pressure.

Conclusions

• Below T_SDW = 150 K, 17O-NMR spectra show that all planar O(3,4) sites broaden due to finite internal magnetic fields, while outer apical O(2) sites show no or negligibly small internal field, consistent with commensurate SDW order with a single spin-spinless or large-tiny spin stripe.
• The inner apical O(1) site internal field is nearly canceled below T_SDW, indicating an antiparallel spin configuration between adjacent NiO₂ planes; below T_A ≈ 115 K, the O(1) spectrum broadens/disappears while the in-plane SDW order remains robust, indicating collapse of the antiparallel interlayer spin alignment and a transition to canted AFM stacking.
• The local spin susceptibility at the O(2) site is extremely small, indicating well-developed interlayer spin-singlet formation in Ni d3z2-r2 orbitals bridging the NiO₂ planes.
• The results reveal an orbital-selective nature in La₃Ni₂O₇: Ni dx2-y2 spins primarily drive the in-plane SDW order, while Ni d3z2-r2 orbitals govern interlayer spin-singlet formation, quenching most d3z2-r2 spins and reducing the effective interlayer coupling, which enables DMI-induced spin canting.

Main claims

• Site-selective 17O-NMR distinguishes inner apical O(1), outer apical O(2), and planar O(3,4) sites in La₃Ni₂O₇.
  • Evidence: At₁₆₀ K, three distinct peaks are observed and labeled peak 2, peak 1, and peak 3 from low to high frequency.,Peak 2 is assigned to O(2) based on the smallest NQR frequency and relaxation behavior; peak 1 and peak 3 match the 1:4 site ratio of O(1):O(3,4).
• The low-temperature SDW order is a commensurate single spin-spinless or large-tiny spin stripe, excluding double spin stripe and double spin-charge stripe models.
  • Evidence: At₁₂₀ K, all planar O(3,4) sites broaden due to finite internal magnetic field while O(2) sites retain narrow components with no or negligibly small internal field.,Double spin stripe would require all O(2) sites to be magnetic and half of O(3,4) sites nonmagnetic, contradicting the observed O(2) and O(3,4) responses.
• The interlayer spin configuration is antiparallel just below T_SDW but changes below T_A ≈ 115 K, with canted AFM stacking as the most probable state.
  • Evidence: The O(1) internal field is nearly canceled below T_SDW, indicating antiparallel spins between adjacent NiO₂ planes.,Below T_A, the O(1) spectrum disappears or broadens while in-plane SDW order remains robust, requiring a nonzero O(1) internal field.,139La-NMR La(1) broadening is only marginal, consistent with canted AFM but inconsistent with ferromagnetic or in-plane slipped stacking.
• The O(2) site has extremely small local spin susceptibility, indicating well-developed interlayer spin-singlet formation in the Ni-d3z2-r2 orbitals bridging the NiO₂ planes.
  • Evidence: Knight shift at O(2) is 0.04%, one order of magnitude smaller than the planar O(3,4) value of 0.33%.,O(2) relaxation times are much longer than O(3,4), corroborating small local spin susceptibility.,Strong covalency between O(2) and Ni-d3z2-r2 would transfer hyperfine field unless the d3z2-r2 spin is quenched.
• The results reveal an orbital-selective nature in which Ni-dx2-y2 carries the SDW spin while Ni-d3z2-r2 forms interlayer spin-singlets.
  • Evidence: O(3,4) sites show large Knight shift and internal fields, while the O(2) site shows negligible spin susceptibility.,The observed Ni moment is much smaller than the fully polarized moment expected from 1.5 electrons/Ni, implying singlet formation in the d3z2-r2 bonding channel.

Workflow

• sample_preparation — High-quality polycrystalline La₃Ni₂O₇+δ with 17O substitution was prepared for oxygen-site-resolved NMR.
  • Materials: Polycrystalline La₃Ni₂O₇+δ; 17O-enriched oxygen
  • Methods: Solid-state reaction; Thermogravimetric analysis for oxygen content; Annealing at 1000 °C for 4 h for 17O isotope exchange
  • Observations: Narrow 139La(2)-NQR spectrum after 17O substitution confirms high sample quality
• measurement — Oxygen-site-resolved NMR spectra and complementary La NMR were obtained across the SDW and lower spin-reconfiguration transitions.
  • Materials: 17O-enriched polycrystalline La₃Ni₂O₇+δ; Coarse powder sample
  • Methods: Site-selective 17O-NMR spin-echo spectroscopy; 139La-NMR spectroscopy; Frequency-swept RF pulses with selectively tuned pulse conditions
  • Observations: Three 17O-NMR peaks are observed at 160K; Planar O(3,4) peak broadens below T_SDW = 150K; O(2) peak remains narrow at 120K; O(1) peak disappears or broadens below T_A ≈ 115K; 139La-NMR shows only marginal La(1) broadening below T_SDW
• analysis — Spectral assignment and model comparisons show a commensurate single spin-spinless or large-tiny spin stripe SDW with antiparallel interlayer stacking that becomes canted below T_A.
  • Materials: 17O-NMR spectra; 139La-NMR spectra; WIEN₂k DFT electric-field-gradient calculations
  • Methods: Peak assignment using NQR frequencies, relaxation times, and intensity ratios; Spectral simulation of quadrupolar powder patterns; Knight shift and local spin susceptibility analysis; Comparison with stripe SDW and interlayer stacking models; Dipolar hyperfine field estimation for La(1)
  • Observations: Peak 2 is assigned to outer apical O(2), peak 1 to inner apical O(1), and peak 3 to planar O(3,4); O(2) Knight shift is about 0.04%, O(1) about 0.20%, and O(3,4) about 0.33%; O(3,4) sites experience finite internal magnetic field in the SDW state; O(2) sites show no or negligibly small internal field; O(1) internal field is nearly canceled below T_SDW but becomes finite below T_A; Canted AFM stacking explains the O(1) and La(1) observations
• interpretation — The results indicate orbital-selective coexistence of primarily dx2-y2-derived SDW order and d3z2-r2-derived interlayer spin-singlet formation, with DMI-induced canted AFM below T_A.
  • Materials: Knight shift data; Relaxation-time data; Density-wave stripe models; Interlayer stacking models
  • Methods: Orbital-selective spin analysis; Dzyaloshinskii-Moriya interaction scenario; Comparison of O(2) and O(3,4) local spin susceptibilities
  • Observations: O(2) local spin susceptibility is extremely small despite strong covalency with the d3z2-r2 orbital; O(3,4) sites carry substantial spin polarization; Large predicted interlayer coupling appears effectively reduced, enabling DMI-induced canting