Thermoremanence, anhysteretic remanence and susceptibility of submicron magnetites: Nonlinear field dependence and variation with grain size

摘要: We have measured initial susceptibility χ0 and the dependence of anhysteretic remanent magnetization (ARM) thermoremanent (TRM) on applied field for seven samples magnetite, with mean grain sizes from 40 to 540 nm. TRM acquisition is nonlinear in geomagnetically relevant weak fields, contrary usual assumption made paleointensity determination. Over 40–540 nm range, varies particle shape but only weakly size d can be used correct varying magnetite concentrations sediment cores. ARM strongly dependent over same range best parameter monitoring variations natural samples. similar variations, as d−1 d≤1 μm, suggesting a common source this pseudo-single-domain (PSD) size. However, 10–20 times more intense than grains around 0.2 μm The microstate seems two-domain structure, whereas may vortex which has never before been convincingly demonstrated by magnetic measurements. Independent evidence comes theoretical fits data. In moderate strong 215–540 magnetites explained theory, not. smaller (d≤0.1 μm), PSD theory predicts that carried single-domain (SD) moments entire resides (≈1 per grain) 20–40 weaker. These remanence levels match those (metastable) SD states, respectively. therefore propose 0.1–0.5 remain metastable or states following revert ground state when field-cycled, example, alternating demagnetization.