From nano to micro: evolution of magnetic domain structures in multi-domain magnetite

Micromagnetic models found in the paper 'From nano to micro: evolution of magnetic domain structures in multi-domain magnetite'. Reliability of magnetic recordings of the ancient magnetic field are strongly dependent on the magnetic mineralogy of natural samples. Theoretical estimates of long term stability of remanence was 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 (at approximately 10 mm in magnetite) the multidomain (MD) state dominates, so that large uniform magnetic domains are separated by narrow domain walls. In this study we use a parallelized micromagnetic finite element model to provide resolutions of many millions of elements allowing us, for the first 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 mm with domain wall widths equal to that expected in large MD grains; we therefore put the SV to MD transition at ~3 mm for magnetite, and expect well defined, and stable, SV structures to be present until at least ~1 mm 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.