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. 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.
FeO, FeO and FeO share a common oxygen close packed plane leading to good epitaxial growth of one phase upon another along the direction. FeO has been shown to have a sizeable magnetoresistance () at room temperature whilst orientated layers show none at this temperature. The study thus focusses on oriented oxide layers as base layers for further epitaxial growth (eg -FeO).
The Fe stacking sequence of FeO 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 FeO thin films were grown on two subtrates, Pt(111) and AlO(0001). Two different processes were used for deposition on AlO: A) standard sputtering source and B) using an oxygen plasma source. The oxygen plasma source gives higher quality epitaxial films on alumina substrates. The three samples produced are named FeO/Pt, FeO/AlOA and FeO/AlOB for samples grown on Pt(111), AlO(0001) with the oxygen plasma source and AlO(0001) with a normal source, respectively.
The sample FeO/Pt was grown, using iron enriched with the Fe isotope, to a thickness of . Samples FeO/AlOA and FeO/AlOB were grown each with non-enriched iron to a nominal thickness of .
CEMS was used to determine the magnetic hyperfine interactions, moment orientation, and the growth quality of the film.
Dr John Bland, 15/03/2003