On the use of magnetic transient hysteresis in paleomagnetism for granulometry



[1] Parameters derived from magnetic hysteresis experiments have been used as granulometric indicators in paleomagnetism. In practice, discriminating the superparamagnetic (SP) fraction from the coarse grain magnetites in parameter ratio plots is often inconclusive. To overcome this ambiguity, transient hysteresis (TH) has been recently proposed. We have carried out micromagnetic simulations to provide a fundamental rationale for the use of TH for granulometry. We found that magnetic TH results from the difference of magnetization configuration between ascending and descending hysteresis loops as a result of self-demagnetization. The descending branch has a tendency to keep a more uniform (e.g., flower-like) configuration, while the ascending branch prefers a less uniform (e.g., vortex-like) structure. According to our simulations, TH increases as the grain size increases and as the aspect ratio decreases. We also carried out TH measurements for well-defined synthetic and natural samples. It is notable that TH from the simulation for samples with aspect ratio q = 1.5 agrees well with the experimental observations for annealed magnetites of smaller sizes. In general, small TH is a clear indication for the absence of complex magnetized structures. Adding TH analysis to the hysteresis loop measurements requires a minor effort yet provides strong constraint on grain size.