Ferrites with (inverse) spinel structure display a large variety of electronic and magnetic properties making some of them interesting for potential applications in spintronics. We investigate the thermally induced interdiffusion of Ni2+ ions out of NiO into Fe3O4 ultrathin films resulting in off-stoichiometric nickelferrite-like thin layers. We synthesized epitaxial Fe3O4/NiO bilayers on Nb-doped SrTiO3(001) substrates by means of reactive molecular beam epitaxy. Subsequently, we performed an annealing cycle comprising three steps at temperatures of 400\,∘C, 600\,∘C, and 800\,∘C under an oxygen background atmosphere. We studied the changes of the chemical and electronic properties as result of each annealing step with help of hard x-ray photoelectron spectroscopy and found a rather homogenous distribution of Ni and Fe cations throughout the entire film after the overall annealing cycle. For one sample we observed a cationic distribution close to that of the spinel ferrite NiFe2O4. Further evidence comes from low energy electron diffraction patterns indicating a spinel type structure at the surface after annealing. Site and element specific hysteresis loops performed by x-ray magnetic circular dichroism uncovered the antiferrimagnetic alignment between the octahedral coordinated Ni2+ and Fe3+ ions and the Fe3+ in tetrahedral coordination. We find a quite low coercive field of 0.02\,T, indicating a rather low defect concentration within the thin ferrite films.
@article{arxiv.1602.05773,
title = {From Fe$_3$O$_4$/NiO bilayers to NiFe$_2$O$_4$-like thin films through Ni interdiffusion},
author = {O. Kuschel and R. Buß and W. Spiess and T. Schemme and J. Wöllermann and K. Balinski and T. Kuschel and A. T. N'Diaye and J. Wollschläger and K. Kuepper},
journal= {arXiv preprint arXiv:1602.05773},
year = {2016}
}