Introduction

In recent years, high values of magnetoresistance (MR) have been obtained in ferromagnetic tunnel junctions and spin valve structures involving exchange-biasing antiferromagnetic metal oxide layers.[52] In particular magnetite has been suggested as a promising material for magnetoresistive sensors because of its significant spin polarisation at the Fermi level at room temperature. Magnetite's half-metallic nature could, in principle, lead to MR tending towards infinity.[53]

Fe$_2$O$_3$, Fe$_3$O$_4$ and FeO share a common oxygen close packed plane leading to good epitaxial growth of one phase upon another along the $\left[111\right]$ direction.[54] Fe$_3$O$_4(111)$ has been shown to have a sizeable magnetoresistance ($\sim 7\%$) at room temperature whilst $\left[100\right]$ orientated layers show none at this temperature.[55] The study thus focusses on $\left[111\right]$ oriented oxide layers as base layers for further epitaxial growth (eg $\alpha$-Fe$_2$O$_3$).

The Fe stacking sequence of Fe$_3$O$_4$ in the (111) direction is comprised of alternating antiferromagnetically coupled octahedrally and tetrahedrally coordinated sites. Hence each layer along the (111) direction has a well defined magnetisation direction in the plane of the sample.

Heteroepitaxial Fe$_3$O$_4(111)$ thin films were grown on two subtrates, Pt(111) and Al$_2$O$_3$(0001). Two different processes were used for deposition on Al$_2$O$_3$: A) standard sputtering source and B) using an oxygen plasma source. The oxygen plasma source gives higher quality epitaxial films on alumina substrates.[56] The three samples produced are named Fe$_3$O$_4$/Pt, Fe$_3$O$_4$/Al$_2$O$_3$A and Fe$_3$O$_4$/Al$_2$O$_3$B for samples grown on Pt(111), Al$_2$O$_3$(0001) with the oxygen plasma source and Al$_2$O$_3$(0001) with a normal source, respectively.

The sample Fe$_3$O$_4$/Pt was grown, using iron enriched with the $^{57}$Fe isotope, to a thickness of $54\ensuremath{\unskip\,\mathrm{\AA{}}}$. Samples Fe$_3$O$_4$/Al$_2$O$_3$A and Fe$_3$O$_4$/Al$_2$O$_3$B were grown each with non-enriched iron to a nominal thickness of $100\ensuremath{\unskip\,\mathrm{\AA{}}}$.

CEMS was used to determine the magnetic hyperfine interactions, moment orientation, and the growth quality of the film.