This work is a three-dimensional micromagnetic study of the magnetic properties and structures of magnetite. Large magnetite crystals in the size range 10 - 50 μm were modelled using a relatively low resolution model. The vortex configuration was found to be stable for this size range. A flower type structure was also obtained with large, uniformly magnetised domains lying approximately in magnetocrystalline easy directions as in figure 6.1. A global optimisation algorithm called simulated annealing (SA) was used to minimise the total free magnetic energy and hence find stable structures for crystals in the sub-micron size range. The resolution of the model was limited by computer constraints when using SA for optimisation. The structures obtained were interpolated to a higher resolution and input as the initial configuration for a fast local optimisation technique called the conjugate-gradient method. This combination of techniques allowed high resolution models in the ground state configuration to be obtained. Below 0.06μm, nearly uniform structures magnetised in the magnetocrystalline easy directions were obtained. Between 0.06 μm and 1.0μm, only the vortex configuration was obtained. These are the lowest energy states found by any method to date. A modified SA algorithm was used to introduce thermal fluctuations into a micromagnetic model, and hence to determine blocking temperatures of grains up to 0.1 μm in size. The model gave results consistent with Néel's single domain thermoremanent magnetisation theory for grains up to 0.065 μm. Between this size and 0.07 μm a reduction in blocking temperature was observed to take place for cubic grains.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:662914 |
| Date | January 1993 |
| Creators | Thomson, Leslie Campbell |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/13127 |
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