A Magnetic Knob for a Superconducting Orchestra ✛

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A superconductor is like an orchestra that plays without a single wrong note, until the conductor turns a tuning knob. In this new work, that knob turned out to be the magnetic history of nickel contacts. The researchers created superconducting micromembranes from the oxides LaAlO3 and SrTiO3, where a thin layer of free electrons forms at the interface between the two materials: a two-dimensional electron gas. Ferromagnetic nickel contacts were attached to this buried layer through the sidewall, something previously thought nearly impossible.

All three devices transitioned to the superconducting state at temperatures between 100 and 200 millikelvins, just above absolute zero. Current flows through such a conductor without resistance, like water through a perfectly smooth pipe. But magnetized nickel contacts lie nearby, and their magnetic field interferes with the electrons' paired motion. It turned out that an external magnetic field can precisely compensate for this influence: the resistance dropped to zero not at zero field, but at a shift of about 45 millitesla in either direction.

The magnetic field from the contacts acts on the electrons like a crosswind on a runner. With the field directed along the plane of the membrane, the critical current was about five times larger than with a perpendicular field. The reason is simple: the stray field from the thin nickel films is weaker in the plane, so the electrons can more easily maintain their paired dance. The researchers built a model linking the external field, the magnetization of the contacts, and the effective field felt by the superconductor. The coupling coefficient was 1.69×10⁻⁷ henries per meter, which allowed them to reconstruct the magnetization loop of nickel directly from transport measurements.

Previously, it was unclear whether ferromagnetic contacts could be attached to a superconducting two-dimensional gas at a buried interface without destroying superconductivity. Now it has been shown that it is possible. Moreover, the magnetic state of the contacts acts as a tuning knob: by turning it, we change the effective field in the two-dimensional gas and control the critical current. This is the first time a superconductor and a ferromagnet coexist in a single oxide micromembrane with a side contact.

The orchestra from the opening sentence now sounds different. We are not just listening to its performance; we can turn the tuning knob, knowing that each note depends on how the knob was turned before. The next step is to learn to control two contacts independently, to obtain a ferromagnet–superconductor–ferromagnet transition and test whether triplet Cooper pairs are born there. Then superconducting spintronics will cease to be a promise and become a working tool.