Analysis of Strong-Field Hysteresis in High Coercivity Magnetic Minerals
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To evaluate the effect of undersaturation of magnetic hysteresis loops measured in moderate (<2 T) fields in magnetically hard minerals such as goethite or hematite, we measured room temperature hysteresis loops in a 7 T field and DC backfield demagnetization curves in fields up to 3 T using an MPMS 3 instrument. Sediments from different regions of the East European platform, mostly of Carboniferous age were used for this study. Similar experiments were also carried out for a small collection of archaeological ceramics (bricks) apparently containing a High Coercivity Low unblocking Temperature (HCLT) magnetic phase (ε-Fe2O3?). Hysteresis measurements were complemented by thermomagnetic analysis at low and high temperatures, microscopic observations, and X-ray diffraction studies. High-field magnetic hysteresis loops alone appear insufficient to definitively discriminate goethite from hematite, though there is, expectedly, a tendency that increasing goethite content leads to magnetic hardening, with coercive force reaching 1 T and coercivity of remanence 1.7 T. At the same time, ε-Fe2O3 can seemingly be distinguished from either hematite or goethite due to its high saturation magnetization. However, combining hysteresis measurements with low- and high-temperature thermomagnetic analysis provides a much better insight into the magnetic mineralogy of samples. Still, acquiring the reference data on well characterized hematite, goethite, and ε-Fe2O3 samples is highly desirable.
KeywordsMagnetic hysteresis Day-Dunlop plot Hematite Goethite HCLT phase
Natalya Salnaya (Institute of Physics of the Earth, Russian Academy of Sciences) greatly assisted in sampling at Lininka and Ragusha in 2017, and donated the samples of bricks from Yaroslavl. Bricks from Valaam Island were donated by Vladimir Karpinsky (Earth Physics Department, St. Petersburg State University). Measurements were carried out at the resource centers of the Scientific Park of St. Petersburg State University: Centre for Geo-Environmental Research and Modelling (GEOMODEL), Centre for Diagnostics of Functional Materials for Medicine, Pharmacology and Nanoelectronics, Centre for Innovative Technologies of Composite Nanomaterials, Centre for Microscopy and Microanalysis, and Centre for X-ray Diffraction Studies. The study was partially supported by the Russian Foundation for Basic Research via grants 16-05-00603a and 18-05-00626a.
This paper benefited from the reviews by Aleksey Smirnov and Mike Jackson.
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