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Recent micromagnetic simulations predict that particles have an abnormal region of low magnetic stability for grain sizes near the domain state transitions from single domain to single vortex. The implications of this zone are not yet fully understood, but if the size range of this window changes as a function of applied field and temperature then it is likely to contribute to observational uncertainties in paleomagnetic recordings. The narrow size range of this zone means that its effects are difficult to identify when integrated across the much wider distribution of grain sizes present in a sample. Here we report a method of examining the unstable region using low temperature oxidation of magnetite. The rock magnetic experiments on partially oxidized particles with a median diameter close to the unstable zone are reinterpreted using micromagnetic simulations of a multi‐layer core‐shell structure. The predicted magnetic properties exhibit significantly improved agreement with experimental data than that of the previously reported single core‐shell coupled geometry. The observed changes in remanence and coercivity are associated with predicted domain structure changes near a proposed magnetic unstable zone (∼80‐∼120 nm), and so provide the first experimental indication of the existence of such a region. We also demonstrate that magnetic remanence in particles with grain sizes outside this size range were stable to maghemitization. The research provides ideas for the experimental study of the “magnetic unstable zone” and is significant to the correct interpretation of magnetic recordings. Finally, the partially oxidized magnetite particles are able to record paleomagnetic signals.
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- 2 Finished
1/11/18 → 30/06/22