MAGNETIC BLOCKING TEMPERATURES OF MAGNETITE CALCULATED WITH A 3-DIMENSIONAL MICROMAGNETIC MODEL

We present an analysis of thermal stability of magnetic remanence in f ine grains of magnetite (grain size d = 15-120 nm). In order to model incoherent transitions between single-domain (SD) and pseudo-sing;le-d omain (PSD) magnetization configurations, we employ a three-dimensiona l constrained minimization method proposed by Enkin and Williams [1994 ]. Using this approach, one can track in detail the transition from on e local energy minimum state into another by constraining the magnetiz ation vectors of appropriate cells in a discrete model. For each parti cle, we obtain the energy barriers E-B(T) from 25 degrees to 578 degre es C. Magnetic blocking temperatures (T-B) are calculated by integrati ng E-B(T) for two extreme cooling schedules representing laboratory an d geological timescales. The computed blocking temperatures for labora tory timescales are in excellent agreement with the experimentally det ermined blocking temperatures for magnetite by Dunlop [1973b]. The res ults of our computations are summarized as relaxation time versus bloc king temperature curves, which deviate from the curves of Pullaiah et al. [1975] for particles with grain sizes in the SD-PSD transition reg ion. A consequence of the dependence of T-B on timescale is that some PSD size particles are blocked in vortex states on geologic timescales but are blocked in the SD state on laboratory timescales. Paleointens ity determinations with the Thellier method on such samples can theref ore underestimate the paleofield. The superparamagnetic to SD threshol d size d(S) is determined as 50 nm for cubic grains, whereas a small a spect ratio of q = 1.1 is sufficient to depress d(S) to 27 nm. SD part icles of magnetite with small shape anisotropy and cubic grains with 5 8 nm equal to or less than d equal to or less than 72 nm are reliable carriers of paleomagnetic information.