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-Tc 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.

💡 Research Summary

The authors investigate a striking magnetic‑field‑induced re‑entrant superconductivity in Eu‑doped infinite‑layer nickelate Nd₀.₇Eu₀.₃NiO₂ thin films. High‑quality epitaxial films (≈22 unit cells) were grown on two substrates: (La₀.₃Sr₀.₇)(Al₀.₆₅Ta₀.₃₅)O₃ (LSAT) and NdGaO₃. X‑ray diffraction and STEM confirm excellent crystallinity and well‑defined interfaces. Electrical transport on LSAT‑grown samples shows a zero‑field superconducting transition with T₅₀% ≈ 9.5 K and zero resistance below ~6 K. When a magnetic field is applied perpendicular to the film plane, the usual suppression of Tc occurs up to ~3.5 T. Strikingly, at higher fields the transition temperature rises again and the transition sharpens, indicating a re‑entrance of superconductivity that persists up to the highest measured field of 30 T. A similar, though less pronounced, effect is observed for in‑plane fields, consistent with the reduced orbital pair‑breaking in that geometry.

To uncover the origin of this behavior, the authors perform Hall effect measurements on both superconducting (LSAT) and non‑superconducting (NdGaO₃) films of the same composition. The Hall resistance is expressed as R_xy = R₀B + R_sM, separating the ordinary Hall term (R₀) from the extraordinary Hall term (R_sM) that encodes the magnetization of the rare‑earth ions. In the non‑superconducting film, low‑temperature Hall curves display strong non‑linearity, reflecting a sizable extraordinary Hall contribution from Eu²⁺ (S = 7/2) and Nd³⁺ (S = 3/2) moments. The superconducting film shows analogous non‑linearities in the normal state (10–120 K) and, importantly, the extracted Hall coefficients evolve with temperature in a manner consistent with field‑polarized rare‑earth spins.

The key insight is that Eu²⁺ and Nd³⁺ exert opposite exchange fields on the conduction electrons: Eu²⁺ couples antiferromagnetically (negative exchange) while Nd³⁺ couples ferromagnetically (positive exchange). At low fields the net exchange field adds to the external field, suppressing superconductivity. As the field increases, Eu²⁺ moments become increasingly polarized opposite to the external field, partially canceling the Nd³⁺ contribution. In a narrow field window (≈3–5 T) the total effective field experienced by the Ni‑derived conduction electrons is minimized, allowing Cooper pairs to reform – a phenomenon reminiscent of the Jaccarino‑Peter effect but involving two competing magnetic sub‑lattices. This compensation persists to much higher fields, explaining the observed high‑field re‑entrant superconductivity.

Current–voltage characteristics further corroborate the picture: the critical current density J_c is ~1–2 × 10⁵ A cm⁻² at 0 T, drops sharply at 2 T (reflecting the initial suppression), and recovers at 12 T, mirroring the re‑entrance seen in resistivity. The authors also note substrate dependence: LSAT supports the superconducting phase, whereas NdGaO₃ does not, likely due to strain and subtle differences in the topotactic reduction process.

In summary, the work demonstrates that in Eu‑doped infinite‑layer nickelates the interplay of two magnetic rare‑earth ions can be tuned by an external magnetic field to produce a compensation of exchange fields, leading to a magnetic‑field‑induced re‑entrant superconducting state. This finding expands the conventional Jaccarino‑Peter paradigm to a multi‑ion system, highlights the sensitivity of nickelate superconductivity to rare‑earth magnetism, and suggests a new route for engineering superconductivity under high magnetic fields by judicious choice of magnetic dopants.