Pigmentary hematite carries important signals in paleomagnetic and paleoenvironmental studies. However, weak magnetism and the assumption that it has high magnetic coercivity prevents prevents routine identification of the size distribution of pigmentary hematite, especially for fine particle sizes. We present a strategy for estimating joint hematite particle volume and microcoercivity (f (V, Hk0)) distributions from low-temperature demagnetization curves and thermal fluctuation tomography (TFT) of pigmentary hematite in bulk samples of Triassic-Jurassic Inuyama red chert, Japan. The coercivity of the pigmentary hematite increases exponentially with decreasing temperature, following a modified Kneller’s law, where microcoercivity has a wide but approximately symmetric distribution in logarithmic space from ~1 tesla to tens of tesla. All of the red chert samples contain stable single domain (SSD) hematite with 35 - 160 nm diameter; a significant superparamagnetic (SP) hematite population with sizes down to several nanometers also occurs in Jurassic samples. The SP/SSD threshold size is estimated to be 8 - 18 nm in these samples. The fine particle size of the pigmentary hematite is evident in its low median unblocking temperature (194 °C to 529 °C) and, thus, this hematite may contribute to all four paleomagnetic components identified in published thermal magnetization studies of the Inuyama red chert. In this work, uniaxial anisotropy and magnetization switching via coherent rotation are assumed. Uniaxial anisotropy is often dominant in fine-grained hematite, although the dominant anisotropy type should be evaluated before using TFT. This approach is applicable to studies that require knowledge of coercivity and size distributions of hematite pigments.

Thermal demagnetization furnaces are routine facilities that underpin countless paleomagnetic studies by allowing the progressive removal of naturally acquired magnetic remanence or the imparting of well controlled laboratory magnetization. The ideal thermal demagnetizer should maintain “zero” magnetic field during thermal treatments. However, magnetic field noise, including residual magnetic fields of material used to construct the furnace and induced fields caused by the heating current in the furnace are always present. As technology advances allowing the measurement of ever weaker magnetic remanences, it is essential that high-performance demagnetization furnaces are developed to reduce these sources of magnetic field noise. By combining efficient demagnetization of shielding and a new structure of heating wire, we have developed a new demagnetization furnace with low magnetic field noise. Repeated progressive thermal demagnetization experiments using specimens that were previously completely thermally demagnetized above their Curie temperature were carried out to explore the effects of fields within various types of furnace during demagnetization. These experiments confirm that magnetic field noise in various furnaces can have an observable and detrimental impact on demagnetization behavior and that this is reduced with our new design. The new heating element design and procedure for reducing magnetic field noises represent a significant improvement in the design of thermal demagnetizers and allows for extremely weak specimens to be successfully measured.

The theory for recording of thermally blocked remanences predicts a quasi-linear relationship between low fields like the Earth’s in which rocks cool and acquire a magnetization. This serves as the foundation for estimating ancient magnetic field strengths. Addressing long-standing questions concerning Earth’s magnetic field require a global paleointensity dataset, but recovering the ancient field strength is complicated because the theory only pertains to uniformly magnetized particles. A key requirement of a paleointensity experiment is that a magnetization blocked at a given temperature should be unblocked by zero-field reheating to the same temperature. However, failure of this requirement occurs frequently and the causes and consequences of failure are poorly understood. Recent experiments demonstrate that the remanence in many samples typical of those used in paleointensity experiments is unstable, and exhibits an ”aging’ effect in which the unblocking temperature spectrum changes over only a few years resulting in non-ideal experimental behavior. While a fresh remanenence may conform to the requirement of equality of blocking and unblocking temperatures, aged remanences may not. Blocking temperature spectra can be unstable (fragile), which precludes reproduction of the conditions under which the original magnetization was acquired. This limits our ability to acquire accurate and precise ancient magnetic field strength estimates because differences between known and estimated fields can be significant (up to 10 μT) for individual specimens, with a low field bias. Fragility of unblocking temperature spectra appears to be related to grain size and may be related to features observed in first-order reversal curves.

Magnetic hysteresis loops are an important tool in theoretical and applied rock magnetism with applications to paleointensities, paleoenvironmental analysis, and tectonic studies, among many others. Hence, information derived from these data is amongst the most ubiquitous rock magnetic data used by the Earth science community. Despite their prevalence, there are no general guidelines to aid scientists in obtaining the best possible data and no widely available software to allow the efficient analysis of hysteresis loop data using the most advanced and appropriate methods. Here we provide an outline of detrimental factors and simple approaches to measuring better hysteresis loops as well as introducing a new MATLAB software package called Hysteresis Loop analysis box (HystLab) for processing and analyzing loop data. This graphical user interface software is capable of reading the wide range of data formats that are generated by the multiple types of equipment typically used to measure hysteresis loops. HystLab provides an easy-to-use interface allowing users to visualize their data and perform advanced processing, including loop centering, drift correction, linear and approach to saturation high-field slope corrections, as well as loop fitting to improve the results from noisy specimens. A large number of hysteresis loop properties and statistics are calculated by HystLab and can be exported to text files for further analysis. All plots generated by HystLab are customizable and user preferences can be saved for future use. In addition, all plots can be exported to encapsulated postscript (EPS) files that are publication ready with little or no adjustment, greatly enhancing workflow productivity when processing and analyzing large data sets. HystLab is freely available for download at https://github.com/greigpaterson/HystLab and in combination with our simple measurement guide should help the paleo- and rock magnetic communities get the most from their hysteresis data.