Nanocrystalline magnetite thin films were deposited on glass substrates using simplified SILAR method. Variations in structural, optical and magnetic properties were studied with respect to annealing temperature. XRD results reveals the nanocrystalline film formation of cubic structured magnetite (Fe3O4), whose crystallite size increases with annealing. SEM images shows the spherical morphological agglomeration which varies with annealing. Transmission in UV–Vis radiation increases with annealing of films and maximum % transmission was observed for near IR range. VSM result reveals the magnetically soft nature of the deposited films with increasing Ms, Hc values on annealing.

Studies on the simplified SILAR deposited magnetite (Fe3O4) thin films annealed at different temperatures

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.

Optimisation of Fe3O4 thin films and nanostructures for atom trapping applications

We have fabricated ∼143 nm Fe3O4 thin films on Si(100) substrates at 450 ◦ C and then annealed them at the same temperature for 30, 60 and 90 min under a vacuum of 10 −6 torr with pulsed laser deposition. We studied the effects on the structural and magnetic properties of Fe3O4 thin films. The films have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometry (VSM). XRD studies showed pure single phase spinel cubic structure of Fe3O4 with a preferential (111) orientation, independent of substrate orientation at 90 min annealing. Higher magnetization was obtained up to 60 min annealing due to Fe phase but at 90 min, we obtained reduced magnetization of 335 emu/cc. This is attributed to the formation of antiphase boundaries between substrate and film.

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C. Y. Park

Papers

Effect of annealing time on structural and magnetic properties of laser ablated oriented Fe3O4 thin films deposited on Si(100)