Perpendicular Magnetic Anisotropy Preserved by Orbital Oscillation in Strained Tetragonal Fe₄N/BiFeO₃ Bilayers

Orbital performances are important for inducing and manipulating the perpendicular magnetic anisotropy (PMA) in spintronic devices. Herewith, the orbital-mediated PMA in highly spin-polarized Fe₄N are investigated in strained tetragonal Fe₄N/BiFeO₃(001) heterostructures with the Fe_AFe_B/Fe-O₂ termination using the first-principles calculations. Different from the d₂ = d_xz + d_yz + d_z favored PMA in previously reported Fe film, for all the Fe₄N atomic layers at the biaxial strain of S, all d orbitals (i.e., d₁ = d_xy + d_x and d₂) make contributions to the PMA at S = 0% and in-plane magnetic anisotropy (IMA) at S = -2 and 2%. Specifically, the d₁-d₂ orbital oscillation preserves (or favors) the PMA in 0% strained Fe₄N, where the stronger MAE contribution alternates between d₁ and d₂ in adjacent Fe₄N layers. However, at S = -2 and 2%, the whole Fe₄N shows IMA with stable d₁ and d₂ contributions. Moreover, the PMA in the unstrained Fe₄N can be transformed into the IMA by a strain of -2% with a high spin polarization, where Fe₄N/BiFeO₃ interfacial effects are crucial. The PMA preserved by the controllably orbital oscillation in highly spin-polarized Fe₄N paves a way for developing novel spintronic devices.