From Nano to Micro: Evolution of Magnetic Domain Structures in Multidomain Magnetite
From Nano to Micro: Evolution of Magnetic Domain Structures in Multidomain Magnetite
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DOI：
10.1029/2019gc008319
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发表时间：
2019-06-01
影响因子：
3.5
通讯作者：
Muxworthy, Adrian R.
中科院分区：
文献类型：
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作者：
Nagy, Lesleis;Williams, Wyn;Muxworthy, Adrian R.
古磁场磁性记录的可靠性在很大程度上取决于天然样品的磁性矿物学。剩磁长期稳定性的理论估计仅限于单畴（SD）状态，但微磁模型最近表明，所谓的单涡旋（SV）畴结构可以具有比SD晶粒更高的稳定性。在更大的颗粒（？10 μ m的磁铁矿）的多畴（MD）状态占主导地位，使大的均匀的磁畴由狭窄的畴壁分开。在本文中，我们使用一个并行化的微磁有限元模型，提供数百万元的分辨率，使我们能够，第一次，检查磁结构的演变从一个统一的状态，通过SV状态到发展的畴壁指示MD状态。对于一个立方八面体颗粒的磁铁矿，我们确定明确的畴壁在颗粒小到类似3 μ m的畴壁宽度等于预期在大MD颗粒，因此，我们把SV到MD过渡在类似3 μ m的磁铁矿，并期望良好定义的，稳定的，SV结构的存在，直到至少类似于1 μ m时，减少晶粒尺寸。减小尺寸进一步显示了对域结构的历史的关键依赖性，特别是SV状态，其通过所谓的不稳定区过渡，导致最近观察到的硬对准SV状态，其继续展开到SD但保持硬对准。
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 mu m 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 similar to 3 mu m with domain wall widths equal to that expected in large MD grains; we therefore put the SV to MD transition at similar to 3 mu m for magnetite and expect well-defined, and stable, SV structures to be present until at least similar to 1 mu m 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.