THERMOREMANENCE ANHYSTERETIC REMANENCE AND SUSCEPTIBILITY OF SUBMICRON MAGNETITES - NONLINEAR FIELD-DEPENDENCE AND VARIATION WITH GRAIN-SIZE

We have measured initial susceptibility chi(o) and the dependence of a nhysteretic remanent magnetization (ARM) and thermoremanent magnetizat ion (TRM) on applied field for seven samples of magnetite, with mean g rain sizes from 40 to 540 nm. TRM acquisition is nonlinear in geomagne tically relevant weak fields, contrary to the usual assumption made in paleointensity determination. Over the 40-540 nm range, chi(o) varies with particle shape but only weakly with particle size d and can be u sed to correct for varying magnetite concentrations in sediment cores. ARM is strongly size dependent over the same range and is the best pa rameter for monitoring grain size variations in natural samples. ARM a nd TRM have similar variations, as d(-1) for d less than or equal to 1 mu m, suggesting a common source for this pseudo-single-domain (PSD) dependence on grain size. However, TRM is 10-20 times more intense tha n ARM in grains around 0.2 mu m in size. The TRM microstate seems to b e a two-domain structure, whereas the ARM microstate may be a vortex s tructure, which has never before been convincingly demonstrated by mag netic measurements. Independent evidence comes from theoretical fits t o TRM and ARM field dependence data. In moderate and strong fields, TR M in 215-540 nm magnetites is explained by two-domain theory, but ARM is not. In smaller grains (d less than or equal to 0.1 mu m), a PSD th eory predicts that TRM is carried by single-domain (SD) moments of ent ire, grains but ARM resides in moments (approximate to 1 per grain) 20 -40 times weaker. These remanence levels match those of (metastable) S D and vortex states, respectively. We therefore propose that magnetite grains in the 0.1-0.5 mu m size range can remain in metastable SD or two-domain states following acquisition of TRM but revert to a vortex ground state when field-cycled, for example, in ARM acquisition or alt ernating field demagnetization.