The “Day” diagram (Day et al., 1977, https://doi.org/10.1016/0031-9201(77)90108-X) is used widely to make inferences about the domain state of magnetic mineral assemblages. Based on theoretical and empirical arguments, the Day diagram is demarcated into stable “single domain” (SD), “pseudo single domain” (“PSD”), and “multidomain” (MD) zones. It is straightforward to make the necessary measurements for a sample and to plot results within the “domain state” framework based on the boundaries defined by Day et al. (1977, https://doi.org/10.1016/0031-9201(77)90108-X). We discuss 10 issues that limit Day diagram interpretation, including (1) magnetic mineralogy, (2) the associated magnetocrystalline anisotropy type, (3) mineral stoichiometry, (4) stress state, (5) surface oxidation, (6) magnetostatic interactions, (7) particle shape, (8) thermal relaxation, (9) magnetic particle mixtures, and (10) definitional/measurement issues. In most studies, these variables are unknowns and cannot be controlled for, so that hysteresis parameters for single bulk samples are nonunique and any data point in a Day diagram could result from infinite combinations of relevant variables. From this critical appraisal, we argue that the Day diagram is fundamentally ambiguous for domain state diagnosis. Widespread use of the Day diagram has also contributed significantly to prevalent but questionable views, including underrecognition of the importance of stable SD particles in the geological record and reinforcement of the unhelpful PSD concept and of its geological importance. Adoption of approaches that enable correct domain state diagnosis should be an urgent priority for component‐specific understanding of magnetic mineral assemblages and for quantitative rock magnetic interpretation.

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Hematite-bearing red beds are renowned for their chemical remanent magnetization (CRM). If the CRM was acquired substantially later than the sediment was formed, this severely compromises paleomagnetic records. To improve our interpretation of the natural remanent magnetization, the intricacies of the CRM acquisition process must be understood. Here, we contribute to this issue by synthesizing hematite under controlled 'Earth-like' field conditions (< ~100 µT).CRM was imparted in 90 oriented samples with varying inclinations. The final magnetic remanence carrier is hematite with traces of ferrimagnetic ferrihydrite or maghemite. When the magnetic field intensity is > ~40 µT, CRM of hematite records the field direction faithfully. However, for growth field intensities < ~40 µT, the CRM direction may deviate considerably from that of the growth field. The CRM intensity normalized by the isothermal remanent magnetization (CRM/IRM @2.5 T ) increases linearly with the intensity of growth field, implying that CRM could potentially be useful for relative paleointensity studies. The hematite CRM has a distributed unblocking temperature spectrum from ~200 to ~650 °C while hematite with a depositional remanent magnetization (DRM) has a more confined spectrum from ~600 to 680 °C. Further, the CRM thermal demagnetization curves with their concave shape are notably different from their DRM counterparts which are convex. These differences together are suggested to be a potential discriminator of CRM from DRM carried by hematite in natural red beds, and of significance for the interpretation of paleomagnetic studies on red beds.