Micrormagnetic modelling of naturally occurring mineral systems.

地球的地质历史和我们对地球内部发生的物理过程的理解，大部分都是通过研究地球表面岩石形成的地磁场记录而解开的。如果没有这些信息，我们就不会知道，例如，地磁场已经逆转的事实，也不会了解地球核心内发生的物理和化学过程。大陆板块运动的测绘几乎完全依赖于对年代确定的岩石中的岩石磁性记录的详细解释，而编制地磁场随时间逆转的频率和周期提供了广泛用于石油工业和地质研究的磁性地层定年工具。不幸的是，磁记录性能高度依赖于晶粒尺寸，并且只有对于最小的晶粒，具有称为单畴（SD）晶粒的接近均匀的磁化，我们才完全了解它们记录磁场的能力。岩石中天然存在的磁性材料实际上远非理想的磁性记录器，因为一般来说，它们含有各种磁性矿物，并且具有各种各样的颗粒几何形状和尺寸。通常，岩石记录的大部分磁性信息将由磁化不均匀的颗粒携带，但它们对剩磁（强度或记录）的贡献几乎与SD颗粒一样高。这种颗粒被称为伪单畴（PSD）颗粒。尽管这些颗粒对岩石剩磁的贡献占主导地位，但它们的磁记录保真度却知之甚少。本项目旨在通过建立一个能够研究PSD颗粒的磁性的数值模型来解决这个问题，特别是在天然样品中常见的特定颗粒几何形状的磁化。我们将讨论岩石磁学中两个最令人困惑的问题。首先，我们将对彼此足够接近的磁性颗粒进行建模，它们相互作用，就像紧密间隔的条形磁性颗粒可能相互作用一样。其次，我们将研究PSD颗粒在地质时间尺度上准确记录地磁场作为时间和温度函数的能力。为了实现这些目标，我们必须有一个数值模型，该模型能够准确地解释不均匀的畴结构，这种结构不仅发生在单个颗粒中，而且发生在相互作用的颗粒群中。对于任何特定的磁结构，我们还需要能够通过估计防止热波动破坏其剩磁的能垒来确定它们的热稳定性。这样的模型不能使用单个计算机处理器在任何合理的时间范围内运行（一个解决方案的计算数周）。相反，我们必须通过将计算分散在许多处理器上来提高计算效率，特别是对于需要最多计算的静磁场（相互作用）。该项目将开发这样一个并行微磁模型，能够首次详细描述天然磁性矿物中地磁记录的磁保真度。

Much of the geological history of the Earth and our understanding of the physical process occurring deep within it, have been unravelled by studying the recordings of the geomagnetic field made by rocks that form on the Earth's surface. Without such information we would not be aware, for example, of the fact that the geomagnetic field has reversed, and would have little understanding of the physical and chemical processes that occur within the Earth's core. The mapping of the motions of the continental plates relies almost entirely on the detailed interpretation of rock-magnetic recordings in well dated rocks, whilst compiling the frequency and periods of geomagnetic field reversals through time provides a magneto-stratigraphic dating tool used extensively in the oil industry and in geological research. Unfortunately, magnetic recording properties are highly grain size dependent, and it is only for the smallest grains, with near uniform magnetisations called single-domain (SD) grains, that we fully understand their ability to record a magnetic field. Naturally occurring magnetic materials in rocks are in fact far from ideal magnetic recorders since, in general, they contain a variety of magnetic mineralogies and have a wide range of grains geometries and sizes. Frequently, most of the magnetic information recorded by the rocks will be carried by grains whose magnetisation is inhomogeneous, but nevertheless their contribution to the magnetic remanence (strength or recording) is almost as high as that of SD grains. Such particles are called pseudo-single-domain (PSD) grains. Despite the dominant contribution of these particles to rock-magnetic remanences, their fidelity of magnetic recoding is poorly understood.This project aims to address this problem by building a numerical model that is able to examine the magnetic properties of PSD grains and in particular the magnetisation in specific grain geometries of that are commonly found in natural samples. We will address the two most perplexing problems remaining in rock-magnetism. Firstly we will model magnetic grains that are sufficiently close to each other that they interact, in the same way as closely spaced bar magnetic might interact. Secondly we will examine the ability of PSD grains to hold an accurate recording of the geomagnetic field as a function of time and temperature, over geological timescales.In order to achieve these goals we must have a numerical model that is capable of the high spatial resolutions needed to accurately account for the inhomogeneous domain structures that occur not just in a single grain, but in a cluster of interacting grains. We also need to be able to determine their thermal stability by estimating, for any particular magnetic structure, the energy barrier preventing thermal fluctuations from destroying its magnetic remanence. Such models cannot be run using a single computer processor on any reasonable time frame (weeks of computation for one solution). Instead we must increase computational efficiency by spreading the calculations across many processors, particularly for the magnetostatic (interaction) field that requires the most computation. This project will develop such a parallelised micromagnetic model capable of detailing, for the first time, the magnetic fidelity of geomagnetic recordings in naturally occurring magnetic minerals.