Abstract:
This paper reports the alternating field demagnetization characteristics of glass-ceramic magnetite assemblages carrying weak-field thermoremanent magnetization (TRM), weak-field anhysteretic remanent magnetization (ARM), and saturation remanence (Jrs). Average grain sizes vary from less than 0.1 μm to approximately 100 μm, and hysteresis parameters indicate that these assemblages encompass single-domain (SD) through truly multidomain (MD) behavior. In all assemblages, weak-field TRM and weak-field ARM are more stable to alternating field demagnetization than is (Jrs). This response is especially remarkable in the 100 μm assemblage, which otherwise displays truly MD behavior. Although the SD samples pass the Lowrie-Fuller test for SD behavior, calculations presented here show that populations of noninteracting, uniaxial SD grains should behave in just the opposite sense to that reported originally by Lowrie and Fuller. This discrepancy could indicate that SD, glass-ceramic magnetite populations are more affected by magnetic interactions than would be expected for magnetite crystals that nucleated individually from a silicate matrix. This interpretation is supported by the SD assemblages failing the `Cisowski' test: that is, the curves for acquisition and AF demagnetization of (Jrs) intersect well below the 50% mark. However, a second and intriguing explanation of the SD-like results obtained from all samples is that alternating field demagnetization characteristics reflect a strong dependence of local energy minimum domain state, and its associated stability, on the state of magnetization.
