Correlation between phase composition and exchange bias in CoFe/MnN and MnN/CoFe polycrystalline films

Magnetic properties and phase composition of both MnN/CoFe (MnN at top of bilayer) and CoFe/MnN films (MnN at bottom of bilayer) bilayers through annealing at various temperatures (T-a = 300-450 degrees C) and then cooling to room temperature under the application of an external magnetic field of 1.5 kOe are compared. The exchange bias field (H-E), the magnitude of magnetic hysteresis loop shift, of the studied films is highly related to phase composition of antiferromagnetic (AF) layer. The increase of H-E with increasing T-a in the range of 300-375 degrees C possibly results from the improvement of magnetocrystalline anisotropy of AF related to the promoted crystallinity and stress relaxation of tetragonal face-centered theta-MnN phase. The reduction of H-E at higher T-a is due to the decreased volume fraction or disappearance of theta-MnN phase and the formation of impurity phases, such as Mn4N and Mn. The induction of impurity phases is possibly related to the diffusion of part of N out of MnN phase at higher T-a. Higher H-E for CoFe/MnN than MnN/CoFe at T-a = 300-375 degrees C might be attributed to larger amount and higher degree of stress relaxation for theta-MnN phase. For CoFe/MnN film annealed at 375 degrees C, the highest H-E = 562 Oe is attained, and the corresponding interfacial exchange energy of 0.47 mJ/m(2) in this study is comparable to that reported by Meinert et al. [Phys. Rev. B 92, 144408 (2015)].