The needle tip that you drag across a surface to feel its bumps has now grown an ultrasensitive "nose" for magnetic fields. Scientists have assembled a tiny superconducting loop on the end of a cantilever (a thin, flexible beam like the one in an atomic force microscope) that catches the slightest fluctuations in magnetic flux.
First, they grew a pyramid-shaped wire frame on a silicon wafer. Then they deposited superconducting niobium at an angle, so thin wires settled along the frame's edges. At the apex, where the wires converge, they cut two narrow constrictions with an ion beam — creating a SQUID: a superconducting ring with two weak links. Current flows through these links in quantum steps, and any change in the magnetic field alters that current — the device senses it.
Such a probe works even in strong magnetic fields — up to 1 tesla, like inside an MRI machine. And the frame's wires can double as a local coil: run a current through them to change the magnetic field right under the tip. Previously, superconducting sensors lost sensitivity in such fields, but this one holds its own.
The technology is not a one-off: hundreds of probes fit on a single wafer, ready for mass production. The needle tip that used to only feel the surface now sees the magnetic pattern of the material.
The superconducting loop on the probe tip can detect a change in magnetic field a billion times weaker than Earth's field.