Unraveling spin density wave order in layered nickelates La₃Ni₂O₇ and La₂PrNi₂O₇ via neutron diffraction

The observation of pressure-induced superconductivity in two- and three-layer Ruddlesden-Popper nickelates has spurred intense interest in these materials as a platform for exploring unconventional superconductivity. While the ground state of these systems has been shown to exhibit magnetism, the direct determination of their magnetic structure remains elusive. This is a crucial aspect, as magnetism may play a role in the pairing mechanism of superconductivity in these materials. 

In this study, we resolve the magnetic structures in the bilayer (2222) polymorphs of La₃Ni₂O₇ and La₂PrNi₂O₇ compounds, using a combination of complementary techniques, namely, neutron powder diffraction and muon-spin rotation/relaxation (μSR). Magnetic neutron scattering in both samples emerges below∼150 K and is observed at the (q_x ,1/2 , 0) position, with q_x = 0 and 1/2 for La3Ni2O7 and q_x = 0 for La₂PrNi₂O₇. Alternating low-magnetic-moment (0.05–0.075μ_B) and high-magnetic-moment (0.66μ_B) stripes form a single layer; the bilayers are formed through antiferromagnetic stacking of single layers along the out-of-plane direction. The magnetic scattering with two propagation vectors q_x = 0 and 1/2 in the undoped La₃Ni₂O₇ is attributed to two magnetic stacking polymorphs within a single crystallographic phase. The magnetic structures are substantiated by the μSR spectra. 

These findings provide a detailed understanding of the magnetic ground state in bilayer nickelates, offering crucial insights into the possible precursor states that may influence the emergence of superconductivity in these materials.