Orbital-Selective Coexistence of Interlayer Spin-Singlet Formation and SDW Order with Anomalous Spin Reconfiguration in Bilayer Nickelate La ₃ Ni ₂

The spin structure of the spin density wave (SDW) order in the bilayer nickelate La ₃ Ni ₂ O ₇ has been investigated using site-selective ¹⁷ O-NMR measurements on the inner apical O(1), outer apical O(2), and planar O(3,4) sites. Below T _SDW (= 150 K), the peak of all planar O(3,4) sites significantly broadens due to the emergence of a finite internal magnetic field, whereas O(2) sites remain with no (or a negligibly small) internal field. These results are consistent with commensurate SDW order with a single spin–spinless (or large-tiny spin) stripe. As for the O(1) sites that bridge the NiO ₂ planes, the internal field is nearly canceled below T _SDW , indicating an antiparallel spin configuration between adjacent planes. However, below T _A (∼ 115 K), the spectrum of the O(1) site disappears even though the in-plane SDW order remains robust, implying that the antiparallel spin configuration through the Ni–O(1)–Ni bond is not particularly stable below T _A , despite the expected strong interlayer spin coupling between the NiO ₂ planes. Above all, we emphasize that the local spin susceptibility is extremely small at the O(2) site that has a strong covalency with the [Formula: see text] orbital, indicating a well-developed interlayer spin-singlet formation in the Ni-[Formula: see text] orbitals bridging the NiO ₂ planes. These findings shed new light on the interlayer spin-singlet formation and the anomalous spin reconfiguration through the Ni–O(1)–Ni bonding orbitals connecting the NiO ₂ planes, which characterize the orbital-selective nature of the bilayer nickelate La ₃ Ni ₂ O ₇ .