Optimisation of Fe3O4 thin films and nanostructures for atom trapping applications

Thin films and nanostructures of Fe3O4 have been investigated and analysed with the aim of being used for an exciting application where the stray magnetic field from domain walls in nanowires are used to trap ultra-cold atoms. In this thesis, polycrystalline Fe3O4 thin films have been successfully grown on Si using reactive dc magnetron sputtering from a Fe target. It has been shown that by using different growth temperatures produces different iron oxide phases. Fe3O4 was grown at substrate temperatures of 200 – 500 ºC with a mixed iron oxide phase of Fe3O4 & α-Fe2O3 produced at RT & 100 ºC. In contrast to the polycrystalline, granular films on Si, Fe3O4 thin films grown on MgO (100) have been shown to be textured with a smooth surface. However, multiple different surface morphologies have been found for the Fe3O4/MgO films. The coercivity of the Fe3O4 films has a negative trend with the film thickness for both substrates, whereas the grain size (Fe3O4/Si) has a positive trend with both film thickness and growth temperature. Following an annealing treatment of Fe3O4 thin films at 250 ºC the composition of the film was altered to become a mixed iron oxide of Fe3O4 & γ-Fe2O3. Dramatic changes in coercivity have been observed, with a large increase seen for the films on Si and negligible change seen for the films on MgO. However, a significant decrease in coercivity has been seen for a smooth film on MgO and the increase in coercivity on Si is dependent on the presence of Fe3O4 regions amongst the mixed iron oxide. Nanostructures of Fe3O4 were created from thin films on Si & MgO, with the nanorings remaining in a saturated state after an in-plane magnetic field is removed. Applying a small negative field to the nanorings on MgO produces a magnetic structure similar to that seen in nanostructures with a 4-fold magnetocrystalline anisotropy.