C. Carvallo

Micromagnetic modeling of first-order reversal curve (FORC) diagrams for single-domain and pseudo-single-domain magnetite

First-order reversal curve (FORC) diagrams have been experimentally shown to be a better way of discriminating domain states in a sample compared to the straightforward use of major hysteresis loops. In order to better understand the fundamental behavior of assemblages of single-domain (SD) grains, we used a micromagnetic model witha conjugate gradient algorith m to calculate FORC diagrams for isolated grains of magnetite as well as for arrays of grains. In the case of individual elongated grains, we found that the FORC diagram consists of a single peak centered on the coercive force Hc if the grain is SD. For a pseudo-single-domain (PSD) grain with vortex structure, we observe multiple peaks on the FORC diagram. The modeling of arrays of elongated SD particles reveals two distinct types of patterns depending on the spacing between particles. In a 2U2U2 array of particles, a secondary branchon the reversal curves appears if the spacing between particles is less than about twice the particle length. This feature translates into the appearance of one negative and three positive peaks on the FORC diagram. In the case of a 3U3U3 array of particles, we again observe several secondary branches when the spacing between grains is less than about twice the particle length, leading to the appearance of multiple peaks on the FORC diagram. Splitting of the central peak on the Hu axis when particles interact could explain the vertical spread of FORC distributions of natural interacting samples as an effect of superposition of multiple peaks caused by the random orientation and distributions of particle spacing and switching fields of a large number of grains. The presence of symmetric peaks on a FORC diagram can be an indicator of the presence of either small PSD grains or magnetic interactions in an ensemble of grains.

Archeomagnetism of potsherds from Grand Banks, Ontario: a test of low paleointensities in Ontario around A.D. 1000

An archeomagnetic study was carried out on 24 pottery fragments from Grand Banks, southern Ontario. 14 C dating on maize found in the same pit as the potsherds gives an age in the range A.D. 990^1160 (modal age A.D. 1030). Hysteresis measurements indicate that the natural remanent magnetization is carried by pseudosingle-domain magnetite grains. Paleointensity experiments were done on 63 samples using a Thellier-type double-heating method. Half of the samples were heated in helium and the other half in air. Hysteresis measurements on samples after heating to various temperatures reveal only slight mineralogical changes during heating steps. Thirty reliable results give a paleofield intensity of 42.0 ˛ 7.4 WT, and a virtual axial dipole moment of 7.0 ˛ 1.3U10 22 Am 2 . This result is in agreement with the low virtual axial dipole moment values given by studies on material from Ontario compared to the values from the southwestern USA in the interval A.D. 900^1400. fl 2001 Elsevier Science B.V. All rights reserved.