On the superparamagnetic—stable single domain transition for magnetite, and frequency dependence of susceptibility

Sediments and soils often contain superparamagnetic (SP) magnetite or maghemite grains that cause a frequency dependence of low‐field susceptibility X fd which does not exceed 15 per cent/decade of frequency. Present models predict very different volume distributions for samples with the largest observed frequency dependence of susceptibility. While Stephensons' (1971) power‐law model predicts most grains to be smaller than the stable single domain (SSD) threshold, the phenomenological model of >Dearing et al . (1996) suggests that most grains are between 10 and 25 nm in diameter. Finally, the recent calculations of Eyre (1997) indicate very broad volume distributions. This study reviews the nature of the superparamagnetic–stable single domain (SP–SSD) transition. The change of AC susceptibilities with grain size (or temperature) at the SP–SSD boundary is more gradual than commonly assumed. When distributions of particle coercivities and volumes are also considered, X fd values are much smaller than those calculated by Eyre (1997). Nonetheless, X fd can be larger than 15 per cent, and a larger frequency dependence has indeed been measured for some samples. The question whether the observed limited X fd of soils and sediments is a result of a broad distribution or of a bimodal distribution, where SP and SSD grains are restricted to a certain relative abundance, can potentially be answered by susceptibility determinations at more than two frequencies and by measurements of the temperature dependence of susceptibility.