Interface-modulated large MAE in hcp-M2/Sin superlattice

Magnetocrystalline anisotropy (MCA) plays a crucial role in magnetic materials. Achieving high MCA has long been a challenging problem, especially for 3d transition metals. In this paper, we construct a hexagonal close-packed (hcp) M₂/Si_n (M = Co, Fe) superlattice with a space group switched by the parity of n between P-3m1 (even n) and P6₃/mmc (odd n). Via density-functional theory, we demonstrate the dependence of the electronic structure near the Fermi energy on the crystal symmetry, which induces quantum oscillations in the strength of M/Si interface coupling and magnetocrystalline anisotropy energy (MAE) with the parity of n. The local environment of the Fe atom has C_3v symmetry at odd n, giving rise to orbital degeneracy of $d_{xy}/d_{x^2-y^2}$ and $d_{xz}/d_{yz}$, leading to perpendicular MAE and enhancing the spin-orbit coupling matrix element. This mechanism yields an MAE of −8.54 meV/f.u. in hcp- Fe₂/Si₇, surpassing bulk Fe by two orders and rivaling L1₀ Fe-Pt alloys (∼1 meV/f.u.). In addition, the perpendicular MAE remains when the lattice distortion Δ(c/a) ranges from −3 % to 2 %, implying potential thermal stability if used as nanodevices. Our C₃ᵥ symmetry-preserving interfacial engineering strategy provides a heavy-element-free route to achieving highly anisotropic magnets.