Авторы: Nagy L., Williams W., Tauxe L., Muxworthy A

2019 г.

GGG

Reliability of magnetic recordings of the ancient magnetic eld are strongly dependent on the magnetic mineralogy of natural samples. Long term stability was recently thought to be restricted to single-domain (SD) states, but micromagnetic models have recently demonstrated that the so-called single vortex (SV) domain structure can have even higher stability that SD grains. In larger grains (~10 microm in magnetite) the multidomain (MD) state dominates, so that large uniform magnetic domains are separated by narrow domain walls. In this paper we use a parallelized micromagnetic nite element model to provide resolutions of many millions of elements allowing us, for the rst time, to examine the evolution of magnetic structure from a uniform state, through the SV state up to the development of the domain walls indicative of MD states. For a cub-octahedral grain of magnetite, we identify clear domain walls in grains as small as ~3 microm with domain wall widths equal to that expected in large MD grains; we therefore put the SV to MD transition at ~3 microm for magnetite, and expect well de ned, and stable, SV structures to be present until at least ~1 microm when reducing the grain size. Reducing the size further shows critical dependence on the history of domain structures, particularly with SV states that transition through a so-called `unstable zone leading to the recently observed hard-aligned SV states that proceed to unwind to SD yet remain hard aligned.

Файл: 955.pdf