Thermomagnetic recording fidelity of nanometer-sized iron and implications for planetary magnetism
Thermomagnetic recording fidelity of nanometer-sized iron and implications for planetary magnetism
复制标题
DOI：
10.1073/pnas.1810797116
复制
发表时间：
2019-02-05
影响因子：
11.1
通讯作者：
Ferreira, Idenildo
中科院分区：
文献类型：
--
作者：
Nagy, Lesleis;Williams, Wyn;Ferreira, Idenildo
古地磁观测提供了太阳系演化过程中磁场强度的宝贵证据。这些信息对原行星盘快速吸积的物理过程提供了重要的限制。为此，磁记录必须是稳定的，并且能够抵抗数十亿年的热事件和粘性采集造成的磁套印。到目前为止，对携带剩余物的磁畴结构缺乏全面的了解，阻碍了对几乎所有古磁样品记录保真度的不确定性的准确估计。最近的计算进步允许详细分析铁颗粒中的磁畴结构作为晶粒形态，尺寸和温度的函数。我们的研究结果表明，均匀磁化的等维铁颗粒不能提供稳定的记录，相反，含有单涡畴结构的大晶粒具有非常大的剩余物和高的热稳定性，两者随着晶粒尺寸的增大而迅速增加。我们得出了与磁热稳定性和时间稳定性相关的曲线，证明立方体（>35 nm）和球体（>55 nm）可能能够保存太阳系形成时的磁记录。此外，我们模拟了各种粒度分布的古磁退磁曲线，发现除非样品在超顺磁粒度边界上以晶粒为主，否则大部分剩余物将在高温下（类似于100℃的居里点）阻塞。我们的结论是，假设在取样、储存或实验室测量过程中没有化学或磁蚀变，铁和卡玛石（低镍含量FeNi）颗粒几乎是理想的天然记录器。
Paleomagnetic observations provide valuable evidence of the strength of magnetic fields present during evolution of the Solar System. Such information provides important constraints on physical processes responsible for rapid accretion of the protoplanetesimal disk. For this purpose, magnetic recordings must be stable and resist magnetic overprints from thermal events and viscous acquisition over many billions of years. A lack of comprehensive understanding of magnetic domain structures carrying remanence has, until now, prevented accurate estimates of the uncertainty of recording fidelity in almost all paleomagnetic samples. Recent computational advances allow detailed analysis of magnetic domain structures in iron particles as a function of grain morphology, size, and temperature. Our results show that uniformly magnetized equidimensional iron particles do not provide stable recordings, but instead larger grains containing single-vortex domain structures have very large remanences and high thermal stability-both increasing rapidly with grain size. We derive curves relating magnetic thermal and temporal stability demonstrating that cubes (>35 nm) and spheres (>55 nm) are likely capable of preserving magnetic recordings from the formation of the Solar System. Additionally, we model paleomagnetic demagnetization curves for a variety of grain size distributions and find that unless a sample is dominated by grains at the superparamagnetic size boundary, the majority of remanence will block at high temperatures (similar to 100 degrees C of Curie point). We conclude that iron and kamacite (low Ni content FeNi) particles are almost ideal natural recorders, assuming that there is no chemical or magnetic alteration during sampling, storage, or laboratory measurement.