Spin Frustration Drives Exchange Bias Sign Crossover in CoFe2O4–Cr2O3 Nanocomposites

A combined Raman spectroscopy and magnetometry study of CoFe₂O₄–Cr₂O₃ nanocomposites demonstrates the presence of spin–phonon interactions and exchange bias at the interface between the ferrimagnetic CoFe₂O₄ and the antiferromagnetic and magnetoelectric Cr₂O₃ phases. The pinned layer of uncompensated spins at the surface of the chromium oxide nanocrystals provides a source of unidirectional anisotropy and changes the sign of the exchange bias when transitioning from a canted to a frustrated situation, that is, as increasing the percentage of CoFe₂O₄ or decreasing the percentage of Cr₂O₃ in the composites. This assessment of the distinctive interfaces attained in the final oxide heterostructures offers a prospective route for hybrid materials which uphold a magnetoelectric effect.