Highly Tunable Intrinsic Exchange Bias from Interfacial Reconstruction in Epitaxial NixCoyFe3-x-yO4(111)/{\alpha}-Al2O3(0001) Thin Films

Intrinsic exchange bias up to 12.6 kOe is observed in NixCoyFe3-x-yO4(111)/{\alpha}-Al2O3(0001) (0<=x+y<=3) epitaxial thin films where 0.15<=y<=2. An interfacial layer of rock-salt structure emerges between NixCoyFe3-x-yO4 thin films and {\alpha}-Al2O3 substrates and is proposed as the antiferromagnetic layer unidirectionally coupled with ferrimagnetic NixCoyFe3-x-yO4. In NiCo2O4(111)/{\alpha}-Al2O3(0001) films, results of reflection high energy electron diffraction, X-ray photoelectron spectroscopy, X-ray reflectometry, and polarized neutron reflectometry support that the interfacial layer is antiferromagnetic NixCo1-xO (0.32<=x<=0.49) of rock-salt structure; the interfacial layer and exchange bias can be controlled by growth oxygen pressure revealing the key role of oxygen in the mechanism of the interfacial reconstruction. This work establishes a family of intrinsic exchange bias materials with great tunability by stoichiometry and growth parameters and emphasizes the strategy of interface engineering in controlling material functionalities.