Alternating field demagnetization, single‐domain‐like memory, and the Lowrie‐Fuller test of multidomain magnetite grains (0.6–356 μm)

Abstract

A fundamental question in paleomagnetism is how magnetite grains much larger than single‐domain size preserve stable remanence over millions of years. In an effort to answer this question we measured alternating field (AF) demagnetization of thermoremanent magnetization (TRM) and saturation isothermal remanent magnetization (SIRM) before and after low‐temperature demagnetization (LTD). LTD (zero‐field cycling through 120 K) unpins or nucleates domain walls, reducing the remanence of multidomain grains. We used two sets of sized crushed magnetites (0.6, 1, 3, 6, 9, 14, 20, 110, and 135 μm), one set unannealed and the other annealed, and a set of hydrothermally grown magnetites (0.8, 6.3, 25, 64, 94, and 356 μm). For all sizes, TRM and SIRM memories after LTD are much harder to AF demagnetize than the original remanences. AF demagnetization curves after LTD are flat for the first ∼10 mT. Such initial plateaus are one hallmark of single‐domain behavior. In high‐stress unannealed grains, after‐LTD response depends on grain size, larger grains demagnetizing more easily than smaller ones. In low‐stress annealed and hydrothermal magnetites, after‐LTD response is almost independent of grain size over nearly 3 decades in grain size. This size‐independent behavior could be due to grains in metastable single‐domain states in the smaller‐sized samples, but in >100 μm grains, there must be a source of single‐domain‐like AF behavior within the multidomain grains themselves. We propose an ad hoc model in which LTD triggers domain wall unpinning and nucleation events up to a coercivity threshold, producing the observed initial plateaus in AF demagnetization curves of TRM and SIRM memories. The size‐independent demagnetization behavior of memory in hydrothermal and annealed magnetites is ascribed to nucleation events above the threshold level, and the additional size‐dependent AF demagnetization of memory in high‐stress unannealed grains is explained by wall unpinning from strong stress centers. In both cases the AF properties merely mimic single‐domain behavior. Although LTD memory has single‐domain‐like AF curves and erased remanence has multidomain‐like curves in grains of all sizes, the Lowrie and Fuller [1971] test still works for the annealed samples. For memory, erased fraction and pre‐LTD remanence alike, TRM is more stable than SIRM in fine grains (1 μm), less stable in large grains (135 μm), and of comparable stability in medium‐size grains (9 and 20 μm).