Spatially resolved variations in reflectivity across iron oxide thin films

Chris S Kelley,Sarah M Thompson,Daniel Gilks,James Sizeland,Leonardo Lari,Vlado K Lazarov,Kosuke Matsuzaki,Stéphane Lefrançois,Gianfelice Cinque,Paul Dumas

doi:10.1016/j.jmmm.2017.04.004

Abstract

The spin polarising properties of the iron oxide magnetite (Fe3O4) make it attractive for use in spintronic devices, but its sensitivity to compositional and structural variations make it challenging to prepare reliably. Infrared microspectroscopy and modelling are used to determine the spatial variation in the chemical composition of three thin films of iron oxide; one prepared by pulsed laser deposition (PLD), one by molecular beam epitaxy (MBE) deposition of iron whilst simultaneously flowing oxygen into the chamber and one by flowing oxygen only once deposition is complete. The technique is easily able to distinguish between films which contain metallic iron and different iron oxide phases as well as spatial variations in composition across the films. The film grown by post-oxidising iron is spatially uniform but not fully oxidised, the film grown by simultaneously oxidising iron showed spatial variation in oxide composition while the film grown by PLD was spatially uniform magnetite.

Highlights

The key components of a spintronic device are the spin polarisers and analysers; an important candidate material is the iron oxide magnetite, Fe3O4, as it is predicted to be 100% spin polarised at the Fermi energy [1,2,3]
Chemical variations across three iron oxide thin films were measured by collecting reflectivity spectra, obtained at the SOLEIL synchrotron, in an attempt to determine the composition of the samples and observe any spatial variations in reflectivity, which could be linked back to the film structure
One film each was produced by post-oxidation, simultaneous oxidation and pulsed laser deposition (PLD)

Summary

Introduction

The key components of a spintronic device are the spin polarisers and analysers; an important candidate material is the iron oxide magnetite, Fe3O4, as it is predicted to be 100% spin polarised at the Fermi energy [1,2,3]. Fe3O4 is very sensitive to small variations in chemical composition which can significantly change in the magnetic properties of the material. This leads to a reduction in spin polarisation [4,5] and eventual device failure. It is sensitive to contaminant oxide phases [6], and one of the key challenges in the development of Fe3O4 based devices is uniform thin film growth. Resolved infrared (IR) reflection microspectroscopy, supported with modelling, has been demonstrated to be an effective non-destructive method of determining the variation of chemical and magnetic properties across Fe3O4 thin films [7,8,9]. Three deposition methods commonly used to grow Fe3O4 thin films were used to prepare three sets of films, IR microspectroscopy was used to measure the variation in oxide composition across the films and models were developed to determine the relative fractions of different oxides present across the samples

Modelling the infrared spectra of iron oxides

Experimental techniques

Estimating the ratio of magnetite to maghemite

Estimating the ratio of iron oxide to iron

Findings

Conclusions