From Nano to Micro: Evolution of Magnetic Domain Structures in Multidomain Magnetite. (22nd June 2019)

• Record Type: Journal Article
• Authors: Nagy, Lesleis; Williams, Wyn; Tauxe, Lisa; Muxworthy, Adrian R.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 20:Number 6(2019)
• Page Start: 2907
• Page End: 2918
• Publication Date: 2019-06-22
• Subjects: single vortex -- multidomain -- domain state evolution -- magnetite -- Bloch wall; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2019GC008319
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: Reliability of magnetic recordings of the ancient magnetic field is strongly dependent on the magnetic mineralogy of natural samples. Theoretical estimates of long‐term stability of remanence were 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 μm 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 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 cuboctahedral grain of magnetite, we identify clear domain walls in grains as small as ∼3 μm with domain wall widths equal to that expected in large MD grains; we therefore put the SV to MD transition at ∼3 μm for magnetite and expect well‐defined, and stable, SV structures to be present until at least ∼1 μm 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. Key Points: Single‐vortex domain states extend to at least 1, 000 nm for cuboctahedral and spherical magnetite grains Single‐vortex to multidomain transition seen as vortex core elongation along hard crystalline axes Models show domain wall widths of about 100 nm, close to that predicted for 71 degrees Bloch‐like walls