Magnetic texture modulated superconductivity in superconductor/ferromagnet shells of semiconductor nanowires

In a one-dimensional ferromagnet-superconductor nanowire, magnetism can suppress superconductivity except where the Zeeman field is suppressed, for example domain wall superconductivity (DWS) near magnetic domain walls or multi-domain-averaged superconductivity (MDAS) in multi-domain states where the net magnetization over the coherence length averages to nearly zero. Here we study full-shell InAs/EuS/Al nanowires using scanning SQUID magnetometry and transport, and find superconductivity in the Al shell only when the EuS is in a multi-domain state, consistent with both DWS and MDAS, and absent in the saturated single-domain state. Scanning SQUID magnetometry further shows that the EuS magnetic texture is position dependent and reconfigurable by small changes in external magnetic field, including moving a well-defined domain wall at $\approx$5.5 $μ$m/mT with sub-mT fields, implying that any associated localized superconducting region would likewise be movable. Such magnetic texture controlled superconductivity along a nanowire may be useful for topological qubits, Andreev spin qubits, superconducting logic, and memory devices.

💡 Research Summary

In this work the authors investigate how magnetic texture controls superconductivity in full‑shell InAs/EuS/Al nanowires, a platform that combines strong spin‑orbit coupling, superconducting proximity, and ferromagnetism in a one‑dimensional geometry. Theory predicts that a Zeeman field generated by the ferromagnetic EuS layer suppresses Cooper pairing in the Al shell, but superconductivity can survive where the Zeeman field averages to zero over the superconducting coherence length ξ_Al≈200 nm. Two scenarios are considered: (i) domain‑wall superconductivity (DWS), where a magnetic domain wall locally cancels the Zeeman field, and (ii) multi‑domain‑averaged superconductivity (MDAS), where many sub‑ξ domains produce a net zero magnetization.

The authors fabricate nanowires with a 100–250 nm InAs core, fully coated on all six facets by a 3–10 nm EuS ferromagnetic layer and a 4–10 nm Al superconducting shell. Scanning SQUID magnetometry performed at 4.2 K maps the out‑of‑plane magnetic flux with ~1 µm resolution while sweeping an axial magnetic field H_a through a full hysteresis loop (0 → +8 mT → –8 mT → 0). At zero field the SQUID images reveal multiple positive and negative lobes along the wire, indicating a multi‑domain state. At +8 mT the wire is driven into a single domain, which persists as the field returns to zero, visible as a pair of opposite lobes at the wire ends. When the field is swept negative, a new domain nucleates near –0.8 mT, a clear domain wall appears around –2.4 mT, and the wall moves along the wire at ≈5.5 µm per mT. Near the coercive field H_c≈–3.5 mT the two domains become comparable in size and a ≈3.7 µm region of near‑zero SQUID signal appears, consistent with either an extended domain wall or a dense collection of nanoscale domains. The magnetic moment extracted from the images reproduces a conventional hysteresis loop, confirming that the EuS shell undergoes a well‑defined coercive transition.

Transport measurements are carried out at 30 mK on four‑probe segments of the same nanowires. Differential resistance dV/dI versus bias current I shows a pronounced low‑bias dip and finite‑bias peaks only when the EuS shell is in the multi‑domain configuration. The superconducting window opens at a nucleation field H_n≈–13 mT (or +10 mT on the opposite sweep) and closes at an annihilation field H_ann≈±18 mT, matching the coercive field observed in SQUID data. The superconducting features are symmetric with respect to field sweep direction, albeit with a small hysteresis, and disappear once the EuS shell reaches a saturated single‑domain state, where the exchange field in Al is maximal and suppresses superconductivity.

Angle‑dependent experiments, where the applied field is rotated by an angle φ relative to the wire axis, reveal that the coercive field window shifts to higher values and broadens with increasing φ. SQUID images at φ=0.03π, 0.36π, and 0.5π show progressively more complex domain patterns, while transport data on a second segment display a superconducting region that expands from a narrow field interval at small φ to almost the entire field range at φ≈0.5π. Micromagnetic simulations of H_c(φ) reproduce the observed angular dependence, reinforcing the link between the EuS magnetic texture and the emergence of superconductivity.

The authors conclude that both DWS and MDAS are realized in their system. Near the coercive field, a movable domain wall creates a localized superconducting channel (DWS) that can be shifted by sub‑mT field changes with micron precision. In broader field ranges, the EuS shell fragments into many sub‑ξ domains, yielding a globally averaged zero Zeeman field and thus MDAS. This magnetic‑texture‑controlled superconductivity opens pathways for engineering topological superconductivity (e.g., Majorana bound states), Andreev spin qubits, superconducting logic elements, and non‑volatile memory where the position of a superconducting segment can be written and erased by tiny magnetic fields. The full‑shell geometry, providing a uniform exchange field across the Al layer, is highlighted as a robust platform for future hybrid quantum devices.