Dear Experts,
I have performed DFT+U calculations in VASP for a bulk heterostructure (ISIF=3). After optimization, I performed magnetic anisotropy calculations with SOC included in SCF runs for spin orientations along different crystallographic directions. I used a high-k mesh of 15×15×1 k-points to calculate the magnetic anisotropy energies. I obtained the following results:
100 = -270.34617219 eV (ΔE = 0.1251 meV)
010 =-270.34615575 eV (ΔE = 0.14155 meV)
001 = -270.34624761 eV (ΔE = 0.04969 meV)
110 = -270.34629730 eV (ΔE = 0 meV, lowest)
Now, I have some confusion regarding the obtained results:
1. Which direction should be considered the easy axis?
2. Since the differences are extremely small (<0.15 meV), should I simply take 110 as the easy axis (lowest energy), or treat 001 as nearly degenerate? The model Hamiltonian construction and the rest of the things depend on the easy axis in my system.
3. More generally, how should such small anisotropy energies be interpreted in practice? Are they meaningful, or within the numerical noise of DFT+U calculations?
I would be really thankful for suggestions and for clarifying my doubts.