Colossal intrinsic exchange bias from interfacial reconstruction in epitaxial CoFe2O4/Al2O3 thin films

We have studied the epitaxial ${\mathrm{CoFe}}₂{\mathrm{O}}₄$ (111) films grown on ${\mathrm{Al}}₂{\mathrm{O}}₃$ (0001) substrates of different thickness at various temperature and discovered colossal intrinsic exchange bias up to 7 \ifmmode\pm\else\textpm\fi{} 2 kOe. X-ray and electron diffraction clearly indicate an interfacial layer about 2 nm of different crystal structure from the ``bulk'' part of the ${\mathrm{CoFe}}<sub>2</sub>{\mathrm{O}}<sub>4</sub>$ film. The thickness dependence of the exchange bias suggests a hidden antiferromagnetic composition in the interfacial layer that couples to the ferrimagnetic ``bulk'' part of the ${\mathrm{CoFe}}₂{\mathrm{O}}₄$ film as the origin of the exchange bias. Considering the structural, magnetic, and electronic structure, CoO has been identified as the most likely candidate of the antiferromagnetic composition in the interfacial layer. This work suggests a path for enhancing intrinsic exchange bias using combination of film and substrate of large structural differences, highlighting the role of interfacial atomic and electronic reconstructions.