High-temperature and high-magnetic-field-induced reentrant superconductivity has been discovered in the infinite-layer nickelate ${\mathrm{Sm}}_{1-x-y}{\text{Eu}}_x{\text{Ca}}_y{\text{NiO}}_2$ (SECNO). Infinite-layer nickelates are the closest known analogs of high-$T_c$ 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. Magnetic freezing occurs at a significantly higher temperature than in other nickelate compounds, and a net magnetization of $55\pm 10\mathrm{kA}{\mathrm{m}}^{-1}$ emerges and remains largely unchanged across the superconducting transition. The magnitude and temperature dependence of this net magnetization are inconsistent with a paramagnetic response, and instead point towards an as-yet unidentified magnetic ordering of Eu moments. These observations suggest a close connection between the magnetic state and the unique superconducting physics exhibited by SECNO, providing a potential mechanism for Jaccarino-Peter screening in high magnetic fields.

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• Magnetism
• Superconductivity
• Nickelates
• Thin films