Unraveling Spin Density Wave Order in Layered Nickelates La₃Ni₂O₇ and La₂PrNi₂O₇ via Neutron Diffraction

Summary

The discovery of pressure-induced superconductivity in two- and three-layer Ruddlesden-Popper nickelates has generated significant interest in these materials as a platform for unconventional superconductivity. While their ground state exhibits magnetism, a direct determination of their magnetic structure remains elusive. Understanding this aspect is crucial, as magnetism may play a role in the pairing mechanism of superconductivity in these compounds. We resolve the magnetic structures of the bilayer (2222) polymorphs of La₃Ni₂O₇ and La₂PrNi₂O₇ using neutron powder diffraction (NPD) and muon-spin rotation/relaxation (muSR). Magnetic neutron scattering appears below approximately 150 K in both compounds and is observed at the (qx, 1/2, 0) position, with qx = 0 and 1/2 for La₃Ni₂O₇ and qx = 0 for La₂PrNi₂O₇. Within a single layer, alternating low (0.05 - 0.075 muB) and high (0.66 muB) magnetic moment stripes form. These layers stack antiferromagnetically along the c-direction to form bilayers. The presence of two propagation vectors (qx = 0 and 1/2) in undoped La₃Ni₂O₇ suggests the coexistence of two magnetic stacking polymorphs within a single crystallographic phase. The muSR spectra further confirm these magnetic structures. Our findings provide a detailed understanding of the magnetic ground state in bilayer nickelates, offering insights into possible precursor states that may influence the emergence of superconductivity in these materials.

Materials

• La₃Ni₂O₇
• La₂PrNi₂O₇

Methods

• neutron powder diffraction (NPD)
• muon-spin rotation/relaxation (muSR)

Keywords

• spin density wave
• magnetic ground state
• magnetic stacking polymorphs

Highlights

• We resolve the magnetic structures of the bilayer (2222) polymorphs of La₃Ni₂O₇ and La₂PrNi₂O₇ using neutron powder diffraction and muSR.
• Our findings provide a detailed understanding of the magnetic ground state in bilayer nickelates, offering insights into possible precursor states that may influence the emergence of superconductivity.

Conclusions

• Magnetic neutron scattering appears below approximately 150 K in both compounds.
• Within a single layer, alternating low (0.05 - 0.075 muB) and high (0.66 muB) magnetic moment stripes form.
• These layers stack antiferromagnetically along the c-direction to form bilayers.
• The presence of two propagation vectors (qx = 0 and 1/2) in undoped La₃Ni₂O₇ suggests the coexistence of two magnetic stacking polymorphs within a single crystallographic phase.
• The muSR spectra further confirm these magnetic structures.

Main claims

• Spin density wave (SDW) order in bilayer nickelates consists of alternating low- and high-moment stripes stacked antiferromagnetically.
  • Evidence: Alternating low- (0.05–0.075 µB) and high- (0.66 µB) magnetic moment stripes form a single layer; the bilayers are formed through antiferromagnetic stacking of single layers along the c-direction
• Two magnetic stacking polymorphs coexist in undoped La₃Ni₂O₇, while La₂PrNi₂O₇ exhibits only one.
  • Evidence: The presence of two propagation vectors (qx = 0 and 1/2) in undoped La₃Ni₂O₇ suggests the coexistence of two magnetic stacking polymorphs within a single crystallographic phase,Pr-substituted sample shows only qx=0 reflections

Workflow

• sample preparation — Nearly stoichiometric bilayer nickelate samples were obtained for neutron and muon studies.
  • Materials: NiO; La₂O₃; Pr₆O₁₁
  • Methods: solid-state reaction; mechanochemical ball-milling; pelleting; annealing in oxygen flow
  • Observations: polycrystalline samples of La₃Ni₂O₇ and La₂PrNi₂O₇ with 2222 polymorph; oxygen content near stoichiometric (δ ≈ 0.01–0.04)
• neutron powder diffraction (NPD) — Long-range spin density wave order is detected via neutron diffraction in bilayer nickelates.
  • Materials: polycrystalline La₃Ni₂O₇ (≈10 g); La₂PrNi₂O₇ (≈11 g)
  • Methods: DMC cold-neutron diffractometer (λ=3.82 Å); HRPT thermal neutron diffractometer (λ=1.89 Å)
  • Observations: five additional magnetic reflections appear below 150 K in La₃Ni₂O₇; indexed with q1=(0,1/2,0) and q2=(1/2,1/2,0) in Cmcm basis; La₂PrNi₂O₇ shows only q1 reflection
• muon-spin rotation/relaxation (µSR) — Bulk magnetic order with two distinct internal field components is confirmed by µSR.
  • Materials: same polycrystalline samples
  • Methods: weak transverse field (WTF) µSR at B=5 mT; zero-field (ZF) µSR; GPS and Flame spectrometers at PSI
  • Observations: initial asymmetry drops from ≈0.27 to ≈0.02 below 150K; internal fields B_int,1 ≈ 145 mT (fast), B_int,2 ≈ 10 mT (slow); magnetic fraction >90% at low T
• magnetic structure analysis — The magnetic structure consists of stripe-like spin density wave order with alternating moments, and two stacking variants coexist in undoped La₃Ni₂O₇.
  • Materials: diffraction and µSR data
  • Methods: representation and magnetic symmetry analysis (JANA₂₀₀₆/2020, ISODISTORT); dipole field simulations (MUESR code)
  • Observations: model: alternating low-moment (0.05–0.075 µB) and high-moment (0.66 µB) stripes; antiferromagnetic stacking along c-axis; two stacking polymorphs for La₃Ni₂O₇ (q1 and q2), single for La₂PrNi₂O₇ (q1); dipole simulations match experimental internal fields