Researchers propose that the puzzling magnetic behavior seen in layered Ruddlesden–Popper nickelates comes from moving electrons rather than fixed atomic spins. In two multilayer nickelates, La3Ni2O7 and La4Ni3O10, the team shows that electrons organize into a spin‑density wave (SDW). A spin‑density wave is a repeating pattern of spin polarization that travels with the electrons, not a pattern of localized magnetic moments stuck to atoms.

The key idea is a mirror symmetry of the NiO2 layers. That symmetry splits the low‑energy electronic states into mirror‑even and mirror‑odd groups. The authors find that parts of the electron states in opposite mirror sectors line up, or “nest,” when shifted by a particular wavevector. This interband nesting between mirror‑opposite bands drives an SDW that is selective to the mirror sectors. In the trilayer compound La4Ni3O10 the same SDW also encourages a secondary charge density wave, giving linked spin and charge patterns.

To reach these conclusions the team built a low‑energy electronic model guided by existing photoemission data. They used a mean‑field Hartree‑Fock calculation to obtain the SDW ground state and then computed the collective spin excitations with the random‑phase approximation (RPA). Their calculated spin spectra reproduce key features seen in resonant inelastic x‑ray scattering (RIXS) and neutron scattering experiments, including a spin‑wave‑like dispersion and the relative intensity of two excitation branches.

A notable result is that the ordered magnetic moments in their itinerant SDW are small — on the order of 0.1 Bohr magnetons per site — as reported in experiments. This supports the view that magnetism here is itinerant (carried by mobile electrons) rather than from large local magnetic moments on atoms. The itinerant picture also resolves a puzzle: previous fits that treated the materials as local magnets required unrealistically large interlayer exchange interactions. In the new model the high‑energy mode is a collective interband excitation, not a direct measure of local interlayer coupling.