Micrormagnetic modelling of naturally occurring mineral systems.

地球的大部分地质历史，以及我们对发生在地球深处的物理过程的理解，都是通过研究地球表面形成的岩石所产生的地磁场记录而解开的。例如，如果没有这些信息，我们就不会意识到地磁场已经反转的事实，也不会对地核内发生的物理和化学过程有多少了解。大陆板块运动的制图几乎完全依赖于对岩石-磁记录的详细解释，而编纂地磁倒转的频率和周期则提供了一种磁地层测年工具，广泛应用于石油工业和地质研究。不幸的是，磁记录特性高度依赖于晶粒尺寸，并且只有对于最小的晶粒，具有近乎均匀的磁化称为单畴（SD）晶粒，我们才能完全理解它们记录磁场的能力。岩石中自然产生的磁性物质实际上远非理想的磁性记录器，因为一般来说，它们包含各种磁性矿物学，并且具有各种各样的颗粒几何形状和大小。通常，岩石记录的大多数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。这样的粒子被称为伪单畴（PSD）颗粒。尽管这些颗粒对岩石磁残留的贡献占主导地位，但人们对它们的磁记录保真度知之甚少。该项目旨在通过建立一个数值模型来解决这个问题，该模型能够检查PSD颗粒的磁性，特别是在自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现存的两个最令人困惑的问题。首先，我们将模拟彼此距离足够近以至于相互作用的磁性颗粒，就像紧密间隔的条形磁性颗粒可能相互作用一样。其次，我们将检查PSD颗粒在地质时间尺度上保持地磁场作为时间和温度函数的准确记录的能力。为了实现这些目标，我们必须有一个能够具有高空间分辨率的数值模型，以准确地解释不均匀的区域结构，这些结构不仅发生在单个颗粒中，而且发生在相互作用的颗粒簇中。对于任何特定的磁性结构，我们还需要能够通过估计阻止热波动破坏其磁剩余物的能垒来确定它们的热稳定性。这样的模型不能在任何合理的时间范围内使用单个计算机处理器运行（一个解决方案需要数周的计算时间）。相反，我们必须通过将计算分散到许多处理器来提高计算效率，特别是对于需要最多计算的静磁场（相互作用）场。该项目将开发这样一个并行的微磁模型，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.