Quantifying Internal Stress and Demagnetization Effects for Natural Multidomain Magnetite and Magnetite-Ilmenite Intergrowths

Demagnetizing effects and internal stress are difficult to distinguish in natural magnetite samples, but quantitative stress estimates can provide valuable information about microstructure formation, surface oxidation, impacts, tectonic stresses, or interface properties in exsolution structures. Quantifying demagnetizing effects informs about magnetite particle shape, magnetostatic interaction, or anisotropic texture. Here, we establish an improved measurement workflow to separate demagnetizing effects from internal stress for natural magnetite. The method is based on temperature-dependent hysteresis measurements, and for natural samples require accurate estimates of Curie temperature and temperature-dependent saturation magnetization to ensure that near-end-member magnetite is the dominant magnetic mineral, and to calibrate the temperature-dependent scaled reversible work (SRW). SRW is the fundamental quantity to determine stress and demagnetizing factor. The improved SRW method is applied to three natural samples with different stress histories where it proves that large magnetite crystals in the metamorphosed Modum complex (Norway) have low internal stress (<100 MPa), while in highly exsolved magnetite-ilmenite intergrowths from Taberg (Sweden) and Bushveld (South Africa) the magnetite component is highly stressed (>230 MPa). This confirms experimentally that interface strain in complex microstructures due to spinodal decomposition and partial oxidation creates large average internal stress in the magnetite minerals. Because sister specimens have similar internal stress but noticeably (>20%) different demagnetizing factors, textural, and shape anisotropy contribute substantially to SRW in these samples.

Automated summary

This study establishes an improved method to distinguish demagnetizing effects from internal stress in natural magnetite samples based on temperature-dependent hysteresis measurements. It confirms that interface strain and partial oxidation in complex microstructures create large internal stress in magnetite minerals.