Magnetic contacts on freestanding superconducting LaAlO$_3$/SrTiO$_3$ micromembranes

In the world of materials science, two powerful forces often refuse to play well together: magnetism and superconductivity. Magnetism is the force that pulls a compass needle north or sticks a refrigerator magnet to a door, while superconductivity is a rare state where electricity flows with absolutely no resistance, allowing currents to circle forever without losing energy. For decades, scientists have been fascinated by the idea of combining these two states, hoping to create a new kind of electronic memory or a switch that could control electricity with magnetic fields. However, bringing them into the same space is notoriously difficult. Usually, the magnetic field from a magnet is so strong that it destroys the delicate superconducting state, causing the electricity to suddenly face resistance again. To make this work, researchers need a material that is superconducting but also thin enough and flexible enough to be touched by a magnet without being crushed by its field.

A team of researchers has now found a way to bring these two worlds together using a very thin, freestanding sheet of a complex material called LaAlO3/SrTiO3. This material forms a special interface where electrons can move freely, creating a two-dimensional electron gas that becomes superconducting when cooled to temperatures near absolute zero. The challenge was that this superconducting layer is buried deep inside the material, hidden beneath other layers, making it nearly impossible to attach a magnetic contact directly to it without damaging the delicate structure. By using a technique that peels off tiny, flexible membranes of this material and attaching them to a silicon base, the researchers created a setup where they could approach the hidden superconducting layer from the side. They then placed small pieces of nickel, a magnetic metal, against the edges of these membranes. What they discovered was that the superconductivity did not vanish; instead, it became incredibly sensitive to the magnetic history of the nickel, acting like a built-in sensor that could tell them exactly how the magnet was behaving.

The researchers built three tiny devices, each consisting of a curved, microscopic membrane of the oxide material resting on a silicon chip. They attached nickel contacts to the sides of these membranes, effectively sandwiching the superconducting layer between the nickel and the air. When they cooled these devices down to 30 millikelvin, a temperature just barely above absolute zero, the electricity flowing through the membrane dropped to zero resistance, confirming that the superconducting state survived the presence of the magnetic nickel. This was a significant achievement because, in many other setups, the magnetic field from the nickel would have immediately killed the superconductivity. The team then began to probe how the magnetism of the nickel influenced this superconducting state. They applied a magnetic field to the devices and measured how the electrical resistance changed as they swept the field up and down.

They found that the superconducting state did not behave the same way when the magnetic field was increasing as it did when it was decreasing. This difference, known as hysteresis, revealed that the nickel contacts were holding onto a magnetic memory. The superconducting layer was reacting not just to the external magnetic field applied by the scientists, but also to the invisible magnetic field generated by the nickel itself. The researchers noticed that the point where the material became most superconducting—the point of lowest resistance—did not happen when the external magnetic field was zero. Instead, it happened when the external field was set to a specific value of plus or minus 45 millitesla. This indicated that the nickel was generating its own magnetic field that was either helping or fighting against the external field, and the scientists had to apply that specific counter-field to cancel out the nickel's influence and let the superconductivity shine through.

To understand this interaction, the team developed a model that treated the nickel contacts as sources of an effective magnetic field. They used computer simulations to map out how the shape and size of the nickel pieces created stray magnetic fields that reached the superconducting layer. The simulations showed a direct link between how strongly the nickel was magnetized and the strength of the field felt by the superconductor. By carefully controlling the magnetic field applied to the nickel—magnetizing it in one direction, then partially reversing it without fully flipping it—the researchers could tune the effective magnetic field experienced by the superconductor. This allowed them to reconstruct the magnetic behavior of the nickel contacts just by watching how the superconducting current changed. They essentially turned the superconductor into a precise magnetometer, capable of measuring the magnetic state of the nickel with high sensitivity.

The study also revealed that the direction of the magnetic field mattered greatly. When the magnetic field was applied perpendicular to the surface of the membrane, the superconducting state was more fragile, and the switching currents were smaller. However, when the field was applied parallel to the surface, the superconducting state was much more robust, allowing currents up to 100 nanoamperes to flow. This difference suggests that the internal structure of the nickel contacts changes depending on the direction of the field. When the field is applied from the side, the nickel tends to form a uniform magnetic state that is less disruptive to the superconductor. When the field is applied from above, the nickel breaks into tiny magnetic regions, or domains, with walls between them that create messy, localized magnetic fields that disturb the superconducting electrons. This finding highlights that the geometry of the magnetic contact is just as important as the material itself.

These results open a new path for exploring how spin, a fundamental property of electrons related to magnetism, can be injected into superconductors. The researchers suggest that the unique interface of their material might allow for the creation of special types of electron pairs that can carry spin information, a key requirement for future superconducting spintronic devices. While the current setup does not yet allow for the independent control of two separate magnetic contacts needed to fully test these theories, the work proves that it is possible to integrate ferromagnetic contacts with oxide superconductors without destroying the superconducting state. By showing that the superconducting state can be tuned and controlled by the magnetic history of the contacts, the team has provided a reliable platform for future experiments. The work demonstrates that with the right design, the conflict between magnetism and superconductivity can be managed, turning a potential obstacle into a tool for controlling quantum states.