Day plots of mixtures of superparamagnetic, single‐domain, pseudosingle‐domain, and multidomain magnetites

Abstract

We test how well a few hysteresis parameters (saturation remanence Mrs, coercive force Hc and remanent coercivity Hcr) serve to determine the proportions of end‐members in binary mixtures. Our end‐members are six magnetites whose grain sizes are within the superparamagnetic (SP), stable single‐domain (SD, three samples), pseudo‐SD (PSD), and multidomain (MD) ranges (Carter‐Stiglitz et al., 2001). The three SD magnetites have contrasting origins and properties: (1) bacterial magnetite crystals of a single size and coercivity, arranged in chains; (2) natural volcanic magnetites with a narrow distribution of coercivities; and (3) synthetic magnetites precipitated in glass, with a broader coercivity distribution. Our parameter mixing theory assumes linear magnetization curves of the end‐members between zero field and the largest coercive force Hc (that of the SD phase, if present). Similarly remanent hysteresis curves should be linear up to the maximum remanent coercive force Hcr. Three of our mixtures (SP plus bacterial SD, PSD plus bacterial SD, MD plus volcanic SD) had acceptable agreement between predicted and measured dependences of Hc, Hcr and the curve of Mrs/Ms versus. Hcr/Hc (Day plot) on end‐member concentrations. A nonlinear approximation to remanent hysteresis curves gave a reasonable fit to MD plus glass SD results. In this case, Hcr/Hc for the most MD‐rich mixture is larger than Hcr/Hc of either end‐member. Such behavior is characteristic of bimodal mixtures in which Hcr is largely determined by the hard (SD) phase and Hc by the soft (MD) phase. The only mixture that could not be modeled by linear or nonlinear parameter theory was MD plus bacterial SD. The bacterial SD hysteresis loop descends almost vertically at −Hc because of the extremely narrow range of particle sizes and coercivities. In general, linear and nonlinear parameter mixing models are adequate if only an approximate fit to real data is needed. An inversion method using complete magnetization curves as end‐member basis functions is preferable as an unmixing technique. However, comparison of measured data to type curves, for example, on a Day plot, gives a quick indication of what end‐member phases might be involved in the mix and provides additional insight before beginning an inversion.