Paramagnetically driven superconducting re-entrance in Eu-doped infinite layer nickelates

The breakthrough discovery of superconductivity in infinite-layer nickelates, and subsequently in several superconducting nickelates with more complex layered structures, capped a search spanning more than two decades and opened an entirely new field of research. Significant efforts aim to increase the critical temperature, to determine the electronic structure of the system, the underlying pairing mechanism, and the similarities between this system and cuprates  - Ni¹⁺ in infinite-layer nickelates being isoelectronic to Cu²⁺ in high-T_c cuprates. Here, we explore the unique role of magnetic rare earth ions in superconducting Eu-doped NdNiO₂. We show that the field-induced re-entrant superconductivity which we evidence in this compound is the result of a delicate balance between the competing effects of the Eu²⁺ and Nd³⁺ ions. Our analyses of the extraordinary Hall effect and modeling of the superconducting critical fields demonstrate that the influence of these ions on magneto-transport is only felt when they are polarized by a magnetic field.