Liquid–Solid Hybrid Magneto-Mechanical Force Sensor

High-sensitivity force sensors enable the precise and efficient control of actuators to handle and characterize fragile objects. However, strain gauges and piezoelectric elements combined with flexible polymers or microelectromechanical cantilevers are incompatible with monolithic integration of semiconductor circuitry, including sense amplifiers that can maximize sensing performance while minimizing production costs. Here, we develop force sensors made of magnetic tunnel junctions (MTJs) in contact with a magnetic fluid (MF), where the MTJs are fabricated using a semiconductor-compatible mass-production process, and the MF-MTJ sensors are formed simply by adding a droplet of MF on the MTJ devices. While the ordinary strain-induced resistance change is absent in the MTJs, the interplay of mechanics and magnetics in the MF allows the sensor to detect mechanical loads smaller than 100 μN via the tunneling magnetoresistance effect. No degradation in magneto-mechanical properties was detected over the course of 10⁴ cyclic loading tests on these MF-MTJs.