Significant magnetic exchange interactions have been observed in infinite-layer nickelates $R$NiO$_2$ ($R$ = La, Pr, Nd), which exhibit unconventional superconductivity upon hole doping. Despite their structural and Fermi surface similarities to cuprates, infinite-layer nickelates possess a larger charge transfer gap, which influences their magnetic exchange interactions via oxygen. In this work, we performed $^{17}$O nuclear magnetic resonance (NMR) measurements on LaNiO$_2$ and Sr-doped LaNiO$_2$, revealing glassy spin dynamics originating from Ni-O planes. This indicates that infinite-layer nickelates are in proximity to magnetic ordering and that magnetic correlations play a crucial role in their physics. More importantly, our analysis of the Knight shift and hyperfine coupling of $^{17}$O nuclei revealed that the Ni-Ni superexchange interaction, mediated by the $\sigma$ bond between the Ni-$d_{x^2-y^2}$ and O-$p$ orbitals, is one order of magnitude weaker than that in cuprates. This alone cannot account for the total magnetic exchange interaction observed in nickelates. First-principles many-body calculations indicate that an interstitial $s$ orbital near the Fermi level, coupled with the Ni-$d_{3z^2-r^2}$ orbital, significantly enhances the superexchange interaction. This contrasts with cuprates, where magnetic interactions are predominantly governed by Cu-$d_{x^2-y^2}$ superexchange via oxygen. Our findings provide new insights into the distinct magnetic interactions in infinite-layer nickelates and their potential role in unconventional superconductivity.

Reference: arXiv:2505.09476