Using Terahertz Waves to Identify the Presence of Goethite via Antiferromagnetic Resonance
Edward J. Garboczi, David F. Plusquellic, Robert D. McMichael, Virgil Provenzano, Paul E. Stutzman, Jack T. Surek, Shin G. Chou, Shuangzhen Wang
Virtually every corrosion detection method reports only the presence of a material phase denoting probable corrosion, not its spectral signature. A signature specific to the type of iron oxide corrosion 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 the commonly occurring corrosion product, goethite (α-FeOOH), we performed high resolution terahertz (THz) absorption loss measurements on a polycrystalline mineral sample of goethite from 0.045 THz to 1.5 THz. From this work, we report two distinct temperature-dependent absorption peaks that extend from 4.2 K 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 antiferromagnetic resonances, allowing identification of goethite, a common iron corrosion product and geological mineral, precisely via two terahertz absorption peaks over this temperature range. This measurement technique has the potential for detecting iron- bearing oxides of corrosion underneath 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  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.
Antiferromagnetic resonance transition, one magnon transition, two-magnons, magnon sidebands, zone center, AFMR, iron corrosion, goethite, hematite, corrosion detection, non-destructive evaluation, NDE