High-sensitive mechanical response in metal–insulator nanogranular films with large gauge factor

Metal strain gauges have been employed for strain and stress measurements for more than 50 years, given their low cost and ease of implementation. However, they face challenges in achieving high sensitivity and miniaturization from the millimeter to the micrometer scale due to their small gauge factor (≃ 2) and low electrical resistivity (≤ 1.1 μΩ∙m). Here, we discovered that metal-insulator nanogranular films can become a novel high-sensitive and micro-scale strain gauge. A Co-(Mg-F) nanogranular film, which comprises metal Co nanogranules and an insulating MgF₂ matrix, exhibited a gauge factor 5 times larger and an electrical resistivity 10⁷ times higher than those of practical metal strain gauges. These superior properties originate from the electronic tunneling conduction between the nanogranules, as opposed to the metallic conduction occurring in metal strain gauges. The large gauge factor of nanogranular films is given by the deformation in the intergranular spacing, which causes the modulation of electron tunneling, and thus, the resistivity.