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The effects of screw dislocations on the magnetic properties of magnetite

The values of the hysteresis parameters produced by micromagnetic models are usually lower than those found experimentally, and in the case of grains which have been subjected to stress the simulated values can be as much as an order of magnitude too low. It has been suggested that the presence of crystalline defects within these stressed grains may be responsible for raising the coercivity and saturation remanence values in comparison with unstressed grains. Grains of magnetite containing a regular array of screw dislocations are modelled for different grain sizes and different dislocation densities using a micro-magnetic model which considers the exchange, demagnetising, magnetoelastic, magnetocrystalline anisotropy and external field interactions. It is found that the values obtained from the new model for coercivity and saturation remanence rise with increasing dislocation density in line with experimental results. Models containing dislocations are found to have the magnetic properties of defect-free grains of smaller grain size, again in line with experiment. The unblocking temperature of a 1μm grain of magnetite in zero applied field is found to decrease in an approximately linear manner with increasing dislocation density. This thesis shows that by considering the magnetoelastic energy arising from the presence of screw dislocations the discrepancy between the hysteresis parameters of magnetite obtained theoretically and experimentally may be resolved.

Identiferoai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:659939
Date January 1998
CreatorsNewcombe, Lee
PublisherUniversity of Edinburgh
Source SetsEthos UK
Detected LanguageEnglish
TypeElectronic Thesis or Dissertation
Sourcehttp://hdl.handle.net/1842/15500

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