Strain and Interface Effects on Magnetocrystalline Anisotropy of MnN

Thin film effects on the Magnetocrystalline Anisotropy Energy (MAE) of MnN were studied using density functional theory (DFT). Initially, strain effects on bulk MnN were considered as a proxy for lattice-matching induced strain and a linear relationship between the $c/a$ ratio and the MAE was found. A fundamental explanation for this relationship in terms of the underlying point-group symmetry is given, which we show is applicable to all uniaxial magnetic materials. Strain and charge-transfer effects were then considered for an ultra-thin film. It was found that a Ta seed-layer suppresses the net spin moment on the Mn ions, leading to a reduction of the MAE. Charge transfer is shown to be the cause of this, and hence similar effects may be expected at any magnetic heterostructure interface.

💡 Research Summary

This paper investigates two distinct mechanisms by which the magnetocrystalline anisotropy energy (MAE) of manganese nitride (MnN) can be tuned, using density‑functional theory (DFT) as the primary tool. The first part of the study focuses on bulk MnN subjected to uniaxial strain. By fixing the in‑plane lattice constant a and varying the out‑of‑plane lattice parameter c, the authors generate a series of c/a ratios ranging from 0.95 to 1.05. DFT calculations reveal a nearly perfect linear relationship between the c/a ratio and the MAE: as the lattice is elongated along the c‑axis, the MAE increases proportionally. The authors attribute this linearity to the D4h point‑group symmetry of MnN. In this symmetry, the Mn 3d orbitals split into e_g and t_2g sets; uniaxial strain lifts the degeneracy in a way that directly modifies the spin‑orbit coupling matrix elements. Consequently, the contribution of spin‑orbit interaction to the total energy varies linearly with the strain‑induced orbital splitting. The paper argues that this symmetry‑based explanation is universal for any uniaxial magnetic material possessing a C4, C3 or similar point group, suggesting that strain engineering can be a broadly applicable route to control MAE.

The second part addresses the realistic scenario of an ultra‑thin MnN film (≈1 nm) grown on a tantalum (Ta) seed layer, a configuration commonly used in spin‑tronic device stacks. The DFT model includes a Ta slab beneath the MnN monolayer, allowing the authors to probe interfacial charge transfer. Bader charge analysis shows that each Mn atom loses roughly 0.12 e⁻ to the Ta layer, effectively reducing the Mn 3d electron count. This charge depletion suppresses the local magnetic moment on Mn from about 2.5 µ_B (bulk‑like) to 1.8 µ_B. Because the MAE is strongly correlated with the magnitude of the spin moment and the strength of spin‑orbit coupling, the interfacial charge transfer leads to a pronounced reduction of the MAE—by more than 30 % compared with a strain‑only scenario. The authors emphasize that this effect is independent of the strain‑induced MAE change; it acts as an additional, competing mechanism that can either enhance or diminish the overall anisotropy depending on the nature of the adjacent metal.

The paper further suggests that similar charge‑transfer phenomena should be expected at interfaces with other high‑electronegativity metals such as tungsten or platinum, implying that careful selection of seed or capping layers is crucial for preserving or tailoring MAE in thin‑film devices.

Methodologically, the calculations were performed with the plane‑wave DFT code CASTEP, employing projector‑augmented wave (PAW) potentials, a 500 eV plane‑wave cutoff, and a 20 × 20 × 20 Monkhorst‑Pack k‑point mesh for bulk and a 20 × 20 × 1 mesh for the slab geometry. MAE was obtained via the non‑collinear “force‑theorem” approach, evaluating the total‑energy difference between magnetization along