Spin correlations in La₃Ni₂O₇ thin films

Summary

This work employs resonant inelastic X-ray scattering (RIXS) to systematically investigate the electronic and spin excitations in La₃Ni₂O₇ (LNO) thin films under epitaxial strain spanning from approximately –2% to +1.9%. In compressively strained LNO/SrLaAlO₄ films that exhibit ambient-pressure superconductivity with an onset critical temperature above 40 K, dd excitations and spin dynamics resembling those of bulk LNO are observed, yet the spin excitation bandwidth increases by about 10 meV, indicating an enhanced interlayer antiferromagnetic exchange coupling Jz; conversely, tensile-strained LNO/SrTiO₃ films display a pronounced suppression of both spin excitations and Ni 3dz²-related dd excitations. This evolution reflects how strain tunes the Ni 3dz²–O 2pz hybridization and the interlayer distance, thereby modulating the interlayer magnetic coupling strength. The results demonstrate that epitaxial strain effectively controls the interlayer antiferromagnetic superexchange in bilayer nickelates, and the strengthened Jz is closely correlated with the emergence of ambient-pressure superconductivity, lending support to the theoretical picture in which interlayer magnetic exchange facilitates interlayer pairing.

Materials

• La₃Ni₂O₇

Methods

• resonant inelastic X-ray scattering (RIXS)
• X-ray absorption spectroscopy (XAS)
• DFT
• spin-wave calculations (SpinW)

Keywords

• spin excitations
• interlayer antiferromagnetic superexchange
• superconductivity
• epitaxial strain
• magnetic correlations
• bilayer nickelates
• ni 3d o 2p hybridization

Highlights

• The LNO/SLAO thin film exhibits dispersive spin excitations similar to bulk La₃Ni₂O₇ but with a larger spin-excitation bandwidth, indicating enhanced interlayer antiferromagnetic exchange Jz under compressive strain.
• Tensile-strained LNO/STO shows a marked suppression of both spin excitations and Ni 3dz²-derived dd excitations, reflecting a collapse of magnetic correlations.
• The emergence of ambient-pressure superconductivity with onset >40 K in compressively strained La₃Ni₂O₇ films coincides with the strengthening of interlayer antiferromagnetic superexchange.
• The results demonstrate how epitaxial strain modulates interlayer magnetic coupling and provide direct spectroscopic evidence linking structural reconstruction, spin dynamics, and superconductivity.

Conclusions

• High-resolution Ni- RIXS reveals robust, dispersive spin excitations in the LNO/SLAO thin film and establishes the systematic evolution of magnetic correlations under epitaxial strain.
• The interlayer exchange is not dictated by a single geometric parameter, but is instead governed by the synergistic structural modulation of the Ni-OAP-Ni bond angle and the c-axis lattice constant, which collectively tune the effective Ni 3dz²–O 2pz hybridization.
• The enhancement of interlayer antiferromagnetic superexchange under compressive strain, and the contrasting profound collapse of overall magnetic spectral weight under tensile strain, closely track the emergence and disappearance of the superconducting phase.
• These direct spectroscopic constraints are highly consistent with pairing scenarios wherein interlayer magnetic exchange interactions, acting within a delicately balanced structural framework, serve as a key ingredient for superconductivity in bilayer nickelates.

Main claims

• Compressively strained LNO/SLAO thin films exhibit dispersive spin excitations similar to bulk LNO but with a larger bandwidth, indicating enhanced interlayer antiferromagnetic exchange Jz.
  • Evidence: RIXS spectra along (H,0) and (0,0,L) show dispersive magnon with band top energy ≈10 meV higher than bulk,Heisenberg model fitting gives Jz = 58 meV for LNO/SLAO vs 50 meV for bulk
• Tensile strain in LNO/STO suppresses both spin excitations and Ni 3dz2-derived dd excitations, reflecting weakened interlayer magnetic coupling and hybridization.
  • Evidence: RIXS spectra of LNO/STO show no distinct magnetic peak, DHO fit gives upper bound Jz ≤ 35 meV,dd excitation at 1.6 eV suppressed in tensile-strained films
• Epitaxial strain modulates interlayer magnetic coupling via synergistic effects on Ni-OAP-Ni bond angle and c-axis lattice constant, which collectively tune Ni 3dz2-O 2pz hybridization.
  • Evidence: Structural characterization: bond angle from 164° to 172° across strain range,dd excitation intensity tracks hybridization changes,Jz values correlate with strain and bond angle
• The correlation between enhanced Jz and superconductivity supports theoretical scenarios where interlayer antiferromagnetic superexchange promotes interlayer pairing in bilayer nickelates.
  • Evidence: Superconductivity (T_c,onset > 40 K) observed only in compressively strained LNO/SLAO,LNO/STO with suppressed Jz shows no superconductivity,Theoretical models: interlayer pairing mediated byJz

Workflow

• sample_preparation — Epitaxially strained La₃Ni₂O₇ thin films were grown on various substrates to systematically vary in-plane strain.
  • Materials: La₃Ni₂O₇ thin films; SrLaAlO₄ (SLAO) substrate; LaAlO₃ (LAO) substrate; (LaAlO₃)₀.3(Sr₂TaAlO₆)₀.7 (LSAT) substrate; SrTiO₃ (STO) substrate
  • Methods: reactive molecular-beam epitaxy (MBE); post-growth ozone annealing (for LNO/SLAO)
  • Observations: high-quality films confirmed by XRD and RSM
• measurement — High-resolution RIXS revealed strain-dependent electronic and spin excitations in LNO thin films.
  • Materials: resonant inelastic X-ray scattering (RIXS) at ID₃₂, ESRF; Ni L₃-edge X-rays
  • Methods: X-ray absorption spectroscopy (XAS) total fluorescence yield; RIXS with pi-polarized incident photons; momentum-dependent scans along (H,0) and (0,0,L); grazing incidence geometry
  • Observations: dispersive spin excitations in LNO/SLAO and LNO/LAO; suppressed spin excitations and broadened dd excitations in LNO/STO
• analysis — Quantitative analysis shows compressive strain enhances interlayer exchange Jz, while tensile strain severely suppresses it.
  • Materials: SpinW code
  • Methods: damped harmonic oscillator (DHO) fitting of RIXS spectra; classical Heisenberg model fitting of magnon dispersion; comparison of magnon energies and spectral weights
  • Observations: LNO/SLAO magnon bandwidth ≈10 meV larger than bulk; LNO/STO magnon energies reduced by >30 meV, spectral weight suppressed >80%
• interpretation — Strain-tuned interlayer antiferromagnetic superexchange Jz is a key factor for superconductivity, consistent with spin-fluctuation-mediated pairing theories.
  • Methods: correlation with superconductivity observations; comparison with theoretical pairing scenarios
  • Observations: superconductivity only in compressively strained films; enhanced Jz in LNO/SLAO, collapsed magnetic correlations in LNO/STO