Single-stripe magnetic order and bilayer spin dynamics in single-crystalline La3Ni2O7

High-temperature superconductivity in pressurized and thin-film nickelates has generated intense interest, yet magnetism in their ambient-pressure parent phases remains poorly understood, despite its potentially crucial role in pairing. Progress has been limited by the lack of large single crystals suitable for neutron spectroscopy. Here we overcome this limitation and map the spin order and excitations of single-crystalline La3Ni2O7. We find single-stripe magnetic order with a finite spin gap, anisotropic in-plane excitations and pronounced bilayer modulation revealing antiferromagnetic interlayer coupling. The dispersion is captured by a bilayer Heisenberg-type model with strong interlayer exchange and competing in-plane couplings. Absolute normalization reveals that, although the spin-wave bandwidth is only about 25% of that in cuprates, the local dynamic susceptibility at comparable energies is enhanced, yielding a comparable total fluctuating moment. These results highlight intense mid-energy spin excitations rooted in substantial electronic correlations, establishing a magnetic framework distinct from cuprates and relevant to superconductivity in nickelates.