We investigated the use of magnetite, a type of iron oxide with low environmental impact, as a mid-infrared detector. Mid-infrared rays with a wavelength of 10.6 μm were irradiated to magnetite thin films at regular intervals using a chopper, and changes in electrical resistance were recorded. For comparison, thin films comprising platinum (Pt) nanoparticles dispersed in magnetite were prepared and evaluated in the same way. The results showed that the average decrease in electrical resistance of magnetite corresponding to the mid-infrared irradiation period was 3.50 a.u., the average response time was 0.12 s, and the signal-to-noise ratio was 2.25. Therefore, magnetite can be used as a mid-infrared detector. Magnetite with Pt nanoparticles exhibited a two-orders-of-magnitude decrease in electrical resistance compared to that without Pt nanoparticles. This contributes to lower device power consumption. Its sensitivity is comparable to that of magnetite without Pt nanoparticles, with a signal-to-noise ratio of 1.66. Based on our results, magnetite could contribute to the realization and widespread use of mid-infrared sensors with low environmental impact in the future.

In nondestructive evaluation of concrete structures, the far-infrared region, including terahertz waves, which can penetrate concrete and measure the amount of corrosion in the internal steel, has attracted much attention. Magnetite has the potential to be used as a far-infrared detection device that meets the requirements for nondestructive evaluation devices, such as room temperature operation and portability, while also having a low environmental impact. In this study, the sensitivity of magnetite thin films with different concentrations of Pt to electromagnetic waves at a wavelength of 10.6 μm was evaluated and compared: a nanocomposite with Pt nanocrystals dispersed in magnetite thin films was prepared by radio frequency sputtering, electrodes were prepared by a photoresist process, and the resistance variation was recorded after irradiation with 10.6 μm pulse electromagnetic waves. As a result, it was experimentally confirmed that the peak of response was the maximum at the amount of Pt added, where the electrical resistivity reached 12,000 µΩcm, and the S/N ratio was the maximum at the amount of Pt added, where the electrical resistivity reached 14,000 µΩcm. This indicates that Pt-doped magnetite with a Pt content of 14,000 µΩcm electrical resistivity is suitable as a far-infrared detector element material.