Inversion of exchange bias and complex magnetization reversal in full-nitride epitaxial γ′-Fe₄N/CoN bilayers

Exchange bias has been observed in the full-nitride epitaxial γ′-Fe₄N/CoN bilayers. With the increase of temperature, the sign of exchange bias (EB) is inverse, which is independent on the cooling field and training effect. This novel behavior appears in the bilayers with different CoN and γ′-Fe₄N thicknesses. The inversion of EB sign not only occurs at low temperatures, but also takes place even at 200 K for the 10 and 12 nm thick CoN layer. With the decreased γ′-Fe₄N layer thickness, the inversion temperature of EB sign shows a roughly increased tendency. For the bilayer with a 4 nm-thick γ′-Fe₄N, the interfacial magnetization reversal presents a complex trend, which is considered as the combined actions of the disordered ferromagnetic spins and various competed magnetic structures. This new manifestation of EB has been discussed in terms of the complicated interfacial spin structures and frustration effects due to the competition between the ferromagnetic and antiferromagnetic exchange interactions at the interface. - Highlights: • Exchange bias (EB) sign reverses from negative to positive with increasing temperature in epitaxial γ′-Fe₄N/CoN bilayers. • The positive EB can be attributed to the antiferromagnetic interfacial coupling and frustrated interfacial spin structures. • The EB transition temperature is not monotonically dependent on CoN thickness t_CoN. • For a 4-nm γ′-Fe₄N, the unusual hysteresis loops are observed