Net Magnetization and Inhomogeneous Magnetic Order in a High-Tc Nickelate Superconductor

High-temperature and high-magnetic-field-induced re-entrant superconductivity has been discovered in the infinite-layer nickelate $\mathrm{S}{\mathrm{m}}_{1-\mathrm{x}-\mathrm{y}}\mathrm{E}{\mathrm{u}}_{\mathrm{x}}\mathrm{C}{\mathrm{a}}_{\mathrm{y}}\mathrm{N}\mathrm{i}{\mathrm{O}}_2$ (SECNO). Infinite-layer nickelates are the closest known analogues of high-${\mathrm{T}}_c$ cuprate superconductors, yet they host distinct magnetic ground states. Using low-energy muon spin relaxation and polarized neutron reflectometry, we reveal the magnetic order in SECNO. We find that magnetic freezing occurs at a higher-temperature than in other nickelate compounds, and that a substantial net magnetization of 55 $\mathrm{k}\mathrm{A}{\mathrm{m}}^{-1}$ $\pm 10\mathrm{k}\mathrm{A}{\mathrm{m}}^{-1}$ emerges and remains largely unchanged across the superconducting transition. The magnetism in SECNO is disordered and nonuniform.