Determining ancient magnetic field strengths from the Earth and Solar System

古岩石和陨石中的古地磁记录是回答地球和行星科学中一些最基本的问题的关键，这些问题包括核心和地球发电机的演化、板块构造和古地理以及太阳系的形成。最近，我们对岩石如何记录地磁场的基本理解受到了挑战。到目前为止，古磁学家认为理想的记录器是那些既均匀磁化又热稳定的粒子，并将其称为单磁区(SD)颗粒。然而，人们早就认识到，大多数颗粒形状的SD范围尺寸范围(表示为其等效体积球体的直径)非常窄，仅存在于尺寸约为30至80 nm的颗粒中。大多数古地磁样品以较大的颗粒为主，含有不均匀的磁畴结构，称为伪单畴(PSD)，这反映了其磁性已知的模糊性。PSD颗粒的磁化和表面上的高稳定性的起源在很大程度上仍然是一个谜；为SD行为设计的古地磁方案通常有非常高的失败率(高达90%)。这些高故障率通常归因于PSD颗粒的存在。数值微磁模型和纳米磁成像的出现，使我们有能力理解这些复杂的系统。来自PI、COI和命名为PDRA的合作的一项引人注目的最新发现表明，这些高度磁化的PSD颗粒通常以单一的磁旋区状态存在，这种状态具有高的磁信号，并且记录稳定性甚至比被均匀磁化的理想的SD颗粒要高得多。随着数值模拟的这些最新进展，我们现在可以构建一个包含在真实古地磁样品中的数百万磁性颗粒的完整模型。这将通过开发一个微磁数据库来实现，该数据库包含作为颗粒尺寸、形状、温度和外部磁场强度的函数的完整的磁畴结构和磁性特征，用于磁铁矿中最古地磁有效尺寸范围为30-1000纳米的颗粒。这个数据库将形成一个独特的资源，我们可以从中挖掘数据：1)提供对PSD领域状态特征的全面和基本的新理解。这是至关重要的，以便提供一种手段，将实验室的岩石磁学观测与控制样品的热稳定性和磁阻挡温度联系起来，以便能够准确地记录数百万年的地球磁场。2)重建古地磁样品的记录保真度。对于任何颗粒大小的分布，我们将能够预测样品通过模拟冷却从其居里温度以上获得自然剩余磁化的能力。然后可以确定实验室磁场中随后的部分再磁化和冷却速度的影响，以确定实验过程对我们提取古代地磁场的正确值的能力的影响。3)建立了获得可靠古烈度值的非加热法。目前古强度测定方法的一个主要缺点是，磁性矿物在实验室再加热过程中经常发生化学变化。通过建立域态热和磁场稳定性(阻塞温度和矫顽力)之间的关系，将有可能为用室温等温或无滞后磁化测量代替实验室加热提供理论基础。

Palaeomagnetic recordings in ancient rocks and meteorites hold the key to answering some of the most fundamental questions in Earth and Planetary Sciences including the evolution of the Core and geodynamo, plate tectonics and palaeogeography, and the formation of the Solar System. Recently, our fundamental understanding of how rocks record the geomagnetic field has been challenged. Hitherto palaeomagnetists regarded ideal recorders as those particles that are both uniformly magnetized and thermally stable, and referred to these as single-domain (SD) grains. However, it has long been recognised that the SD range size range for most grain shapes (expressed as the diameter of their equivalent volume sphere) is extremely narrow, only existing in particles between approximately 30 to 80 nm in size. Most palaeomagnetic samples are dominated by larger grains that contain non-uniform magnetic domain structures referred to a pseudo-single-domain (PSD), which reflects the ambiguity with which their magnetic properties are known. The origin of PSD grains' magnetization and apparently high stability has remained largely a mystery; palaeomagnetic protocols designed for SD behaviour often have very high failure rates (as much as 90 %). These high failure rates are then usually attributed to the presence of PSD grains. The advent of numerical micromagnetic modelling and nanometric magnetic imaging, has given us the ability to understand these complex systems. A remarkable recent discovery, from a collaboration between the PI, CoI and named PDRA, who showed that these highly magnetised PSD grains usually exist in a single magnetic vortex domain state, which has both a high magnetic signal and a much higher recording stability than even the 'ideal' uniformly magnetised SD particles. With these recent developments in numerical modeling, we are now in position to construct a full model of the many millions of magnetic particles contained within a real palaeomagnetic sample. This will be achieved through the development of a micromagnetic database that contains the full domain structure and magnetic characteristics as a function of grain size, shape, temperature and external field strength, for particles in the most palaeomagnetic significant size range of 30 - 1000 nm in magnetite. This database will form a unique resource from which we can mine the data to: 1) Provide a comprehensive and fundamental new understanding of PSD domain state characteristics. This is essential in order to provide a means of linking laboratory rock-magnetic observations to the thermal stability and magnetic blocking temperatures that control a samples ability to retain an accurate recording of the geomagnetic field over many millions of years. 2) Reconstruct the recording fidelity of palaeomagnetic samples. For any distribution of grain sizes we will be able to predict the ability of a sample to acquire a natural remanent magnetisation through a simulated cooling from above its Curie temperature. The effect of subsequent partial remagnetization in laboratory magnetic fields and cooling rates can then be determined to determine the impact of the experimental process on our ability to extract the correct value of the ancient geomagnetic field. 3) Establish a non-heating method of obtaining reliable palaeointensity values. A major drawback in current methods of paleointensity determinations is that the magnetic minerals often undergo chemical alteration during laboratory re-heating. By establishing the relationship between domain state thermal and magnetic field stability (blocking temperatures and coercivities) it will be possible to provide a theoretical basis for replacing laboratory heating by room-temperature isothermal or anhysteretic magnetization measurements.