Magnetic Moment Fragmentation in an All-in-All-out Pyrochlore $\mathrm{Nd_2Sn_2O_7}$

Abstract: We report single crystal neutron spectroscopy and bulk characterization on hydrothermally grown $\mathrm{Nd_2Sn_2O_7}$, revealing magnetic moment fragmentation embedded within the all-in-all-out ordered state. The spectra reveal a nearly flat band with pinch-point momentum dependence accompanied by dispersive branches that produce half-moon features across multiple Brillouin zones. These defining signatures are captured quantitatively by a minimal dipolar-octupolar spin Hamiltonian, demonstrating excellent agreement between experiment and theory. The higher flat-mode energy helps account for the absence of dynamical interference in prior $\mathrm{\mu SR}$ studies, while the lack of any photon-like excitation imposes strict constraints on the proposed Coulombic antiferromagnet scenario. Our results extend moment fragmentation to $\mathrm{Nd_2Sn_2O_7}$ and identify it as a clean, tractable platform for quantitative exploration of emergent gauge field physics in frustrated magnets.