Using Terahertz Waves to Identify the Presence of Goethite via Antiferromagnetic Resonance

Virtually every corrosion detection method reports only the presence of a material phase that denotes probable corrosion, not its spectral signature. A signature specific to the type of iron oxide corrosion product would not only confirm the presence of corrosion but also provide insight into the environment of its formation. To identify the unique spectral signature of a commonly occurring corrosion product, goethite (α-FeOOH), we performed high-resolution terahertz (THz) absorption loss measurements on a polycrystalline mineral sample of goethite, scanning from 0.045 to 1.5 THz. We report two distinct temperature-dependent absorption peaks that extend from 4.2 to 425 K. By combining X-ray diffraction and magnetic characterization on this large crystallite-sized goethite sample, we derived a Neél transition temperature of 435 K, below which the sample is antiferromagnetic. We interpret these absorption peaks as magnon transitions of the antiferromagnetic resonances, allowing precise identification of goethite, a common iron corrosion product and geological mineral, via two terahertz absorption peaks over this temperature range. This measurement technique has the potential for detecting iron-bearing oxides originating from corrosion occurring underneath layers of polymeric products and other protective coatings that can be easily penetrated by electromagnetic waves with frequencies on the order of 1 THz. Furthermore, the combined X-ray and magnetic characterization of this sample, which had a large crystallite size, also improved the previously established relationship between the Néel transition temperature and the inverse mean crystallite dimension in the [111] direction. Our results provide end-case peaks which, compared with goethite samples of smaller crystallite size and purity, will enable the extension of this non-destructive evaluation technique to real corrosion applications.