Magnetic Domain State and Anisotropy in Hematite (α‐Fe₂O₃) From First‐Order Reversal Curve Diagrams

Abstract

Hematite (α-Fe 2 O 3 ) is a naturally abundant and important magnetic mineral in paleomagnetism, rock magnetism, and environmental magnetism. Although its magnetic properties have been investigated extensively, first-order reversal curve (FORC) diagrams (Pike et al., 1999;Roberts et al., 2000) have yet to be studied in detail for hematite even though FORC diagrams are now a standard tool for characterizing magnetic domain states and magnetostatic interactions. FORC diagrams for hematite have been reported widely (

Interpretation of Anhysteretic Remanent Magnetization Carriers in Magnetofossil‐Rich Marine Sediments

“…A central ridge and negative region in the lower quadrant are evident from FORC quantile contours for EM1 (Figure 10a‐1), which is typical of single‐stranded magnetofossil chains. The positive bulge in the lower quadrant at B c = 100 mT is not typical of magnetofossils and might be an unmixing artifact, or part of a HC contribution (e.g., Roberts et al., 2021). The EM1 backfield coercivity distribution is bimodal with peaks at ∼45 and ∼100 mT (blue curves, Figure 10c‐1).…”

Remagnetization Under Hydrothermal Alteration of South Tibetan Paleocene Lavas: Maghemitization, Hematization, and Grain Size Reduction of (Titano)magnetite

“…Samples with similar amounts of titanomagnetite and maghemite have PSD‐like FORC diagrams, whereas samples with a much larger amount of maghemite than titanomagnetite have SD‐like FORC diagrams (e.g., Roberts et al., 2014). Type IV samples are characterized by mixed kidney‐shaped and ridge‐type FORC diagrams (Figures 5k and 5l), indicating that SD hematite particles are of mixed uniaxial and triaxial magnetocrystalline anisotropy (Harrison et al., 2019; Roberts et al., 2021). SD maghemite likely contributes to the region with low B c in the FORC distributions (e.g., Muxworthy et al., 2005).…”

“…For slightly altered type I samples, the dominant magnetic carrier is MD titanomagnetite with accessory magnetic SD maghemite. A small amount of hematite was also identified by Mössbauer spectroscopy (Figures 8a and 11b), but its contribution to the remanence is negligible because the saturation magnetization of hematite is 1/200 that of magnetite (e.g., Roberts et al., 2021, and references therein). For moderately altered type II samples, MD titanomagnetite and SD maghemite are the dominant magnetic carriers, while SD hematite is the minor magnetic carrier (Figure 11c).…”

“…In contrast, type II samples show more triangular distributions centered around higher coercivities, with negative indentations oriented 135° (most clear in panel 5e but evident as well from the shape of the lower lobes in panels 5d, 5f, and 5g) diagnostic of vortex state (PSD-behavior) grains, and more prominent high coercivity distributions that extend to >200 mT indicative of harder SD grains (Figure 5). We note that presence of uniaxial hematite may contribute to the high coercivity distributions along the B c axes (Roberts et al, 2021). Types I and II samples also show a nearly linear trend between M rs /M s and B c values on a Néel, or squareness plot (Tauxe et al, 2002), indicating a trend of grain size reduction from types I to type II samples (Figure 6c).…”

Enhanced specific loss power of hematite–chitosan nanohybrid synthesized by hydrothermal method

“…When the particles are coated with nonmagnetic chitosan the long-range dipolar interactions are eliminated [14]. Roberts et al [59] demonstrated that the single domain threshold size of hematite is about 25-30 nm. According to this study, we can predict that most of the particles after coating are of a single domain, although some multi-domain particles of larger size exist.…”