Research: Mineral Magnetism from the Atomic to the Planetary Scale
Earth scientists employ a variety of tools to study the large-scale structure and behaviour of the Earth. As we seek to extend our knowledge of the structure and behaviour of other planets, magnetism emerges as one of the most powerful tools at our disposal. Magnetic fields are readily detected remotely, and magnetic surveys are among the first measurements to be made during planetary exploration. Magnetic anomalies can be observed on a range of length scales; from satellite surveys at altitudes of many hundreds of kilometres, to ground magnetic surveys from surface rovers. Anomalies are generated by the induced and/or remanent magnetisation of magnetic minerals in the crust, and provide a wealth of information about the igneous and tectonic processes that have helped shape the planet.
The magnetic properties and behaviour of minerals are profoundly influenced by phase transformations and their resulting microstructures (Fig. 1). My research demonstrates that nanoscale microstructures are extremely common in magnetic minerals and that they have a huge impact on their macroscopic magnetic properties. These microstructures not only determine the intensity and stability of macroscopic magnetism recorded in rocks thereby controlling the fidelity of paleomagnetic recordings at the global scale but are extremely important in an industrial context, paving the way for the next generation of high-density magnetic recording media.
I use a wide range of techniques to study the magnetic properties of minerals:
Electron holography is a transmission electron microscopy (TEM) technique that yields a two-dimensional vector map of magnetic flux with nanometre resolution (Fig. 2). The technique is capable of imaging the magnetisation state within individual magnetic particles and the magnetostatic interaction fields between neighbouring particles: two factors that play a central role in the interaction between magnetism and microstructure.
Neutrons have a magnetic moment, and therefore provide an excellent tool for the study of magnetic materials. I use neutron diffraction to study the magnetic structure of natural and synthetic Fe-Ti oxides as a function of temperature and applied field.
Monte Carlo Simulations
I use atomistic simulations of magnetic and cation ordering in minerals to provide new insight into the magnetic properties of nanoscale microstructures. These simulations contributed to the discovery of 'lamellar magnetism' a new theory of how nanoscale intergrowths of ilmenite and hematite generate strong and extremely stable remanent magnetic anomalies at the planetary scale.
Micromagnetic simulations are used to provide insight into the magnetic domain structure of minerals at length scales much larger than the atomistic simulations described above. The technique is particularly suited to the study of pseudo-single-domain particles with complex vortex-like magnetic states caused by the fine balance between exchange, anisotropy, and demagnetising energies.
First-Order-Reversal-Curve (FORC) diagrams are a very effective method of quantifying the macroscopic properties and interactions within complex intergrowths. The FORC diagram defines a 2-dimensionaldistribution of switching fields and interaction fields within the sample (Fig. 3). I have developed a suite of software tools FORCinel for the generation and analysis of FORC diagrams.
Older Publications by Dr Richard Harrison