An interpreter gets distracted by every rustle in the hall, and a one-hour meeting stretches into two. So does an ordinary gyroscope: to notice the tiny displacement of a proof mass, it needs perfectly precise gaps between plates. The smaller the sensor, the harder it is to maintain that precision. Engineers have long been searching for another way to listen to motion.
In the new work, the interpreter's role is taken over by a magnetic tunnel junction (MTJ)—a sandwich of two magnetic layers with an insulator between them. When a radio-frequency current passes through it, the junction rectifies it into a DC voltage, and that voltage changes if a magnetic proof mass moves nearby.
The interpreter has finally stopped being distracted by rustles. Now it listens to the magnetic echo of motion, and this echo can be amplified without touching the hall itself. All that remains is to make its voice clearer.
The spin-torque diode sensitivity in the experiment was about 20 μV/mT, which allows detecting proof-mass displacements of fractions of a nanometer. In the fully spintronic version, the drive amplitude decreases to 150 nm, which is roughly 500 times thinner than a human hair.