Magnetic Characterization of Geologic Materials with First-Order Reversal Curves

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

“…Harrison and Lascu ( 2014) demonstrated that a negative region with slope at −45° away from the main peak is characteristic of FORC distributions for particle systems controlled by cubic anisotropy. This finding has been confirmed in numerical FORC simulations of randomly oriented particles with cubic anisotropy (Egli, 2021;Valdez-Grijalva & Muxworthy, 2019). Harrison et al (2019) used the micromagnetic FORCulator tool of Harrison and Lascu (2014) to simulate FORC diagrams for magnetostatically interacting and noninteracting SD particles.…”

“…The value of FORC diagrams is generally considered to lie in diagnosing domain state, and in providing measures of the coercivity and magnetostatic interaction field distributions of samples (e.g., Roberts et al, 2000Roberts et al, , 2014. As demonstrated in several studies (e.g., Egli, 2021;Harrison et al, 2019;Harrison & Lascu, 2014;Valdez-Grijalva et al, 2018;Valdez-Grijalva & Muxworthy, 2019), FORC diagrams also contain rich information about magnetocrystalline anisotropy type that is linked directly to mineralogy. This adds an important level of diagnostic information available from FORC diagrams.…”

“…Such magnetically viscous hematite particles are common in nature (e.g., Collinson, 1969;Creer, 1961;Jiang et al, 2015;Pike et al, 2001;Stokking & Tauxe, 1990), along with stable SD particles (e.g., Figures 5c and 5e). The FORC manifestations of thermal activation associated with such particles are explained by Pike et al, (2001); the key manifestation is an asymmetrical positive feature near the origin, with a dominantly vertical distribution along the lower B u axis and a small response in the upper FORC half-plane (see also Berndt et al, 2018 andEgli, 2021).…”

“…Thermally assisted switching or exchange interactions among intergrown or touching particles (e.g., Figure 4e) can also cause FORC distribution smearing into the upper half plane. For example, thermal activation can cause premature switching that leads to vertical spreading of FORC distributions above B u = 0 (e.g., Berndt et al, 2018;Egli, 2006Egli, , 2021Pike et al, 2001). Simulations with additional magnetization mechanisms are needed to understand the cause of spreading in the upper FORC half plane for particle systems with multiaxial anisotropy.…”

Rapid light carbon releases and increased aridity linked to Karoo–Ferrar magmatism during the early Toarcian oceanic anoxic event

“…The maximum applied field was 1.2 T. IRM acquisition curves were unmixed into several components based on cumulative log-Gaussian (CLG) functions with the Kruiver et al 96 software or on skewed generalized (log) Gaussian functions with the Max UnMix program (Maxbauer et al 97 ) to isolate the contributions of magnetite and hematite (denoted here as SIRM of component 1–2 and component 3, respectively). One sample from MC3 was further characterized with high-resolution FORC measurements performed with a Lake Shore 8600 Vibrating Sample Magnetometer at the material magnetism laboratory of the Central Institute of Meteorology and Geodynamics in Austria (13 stacks of 851 curves each, acquired in regular field steps of 0.35 mT) and processed with VARIFORC 98 , 99 .…”