Extracting DMI and J_ani for highly canted non colinear magnetic state
Siddharth Pandya
Dear HeXu and the TB2J development team,
I am working on a material with a strongly noncollinear magnetic ground state, where some magnetic moments within the unit cell are canted by 70–90° from the ferromagnetic axis. I am using the standard TB2J workflow , rotating structures with TB2J_rotate, running seven VASP+Wannier90 calculations, and merging with TB2J_merge ,to extract the full exchange tensor including DMI and symmetric anisotropic exchange J_ani.
I am finding that the merged DMI magnitudes are unphysically large, with |D|/|J_iso| ratios exceeding 1.0 for a 3d transition metal system where I would expect values in the range 0.05–0.2. TB2J itself emits a warning that the reconstruction matrix is close to singular. Shell-by-shell validation shows large leave-one-out errors and poor consistency between the six rotated calculations after back-rotation to a common frame, particularly for DMI and J_ani. The J_iso extracted from the noncollinear reference state is also significantly underestimated compared to a collinear FM reference calculation, which I attribute to the LKAG projection problem for strongly noncollinear spin configurations.
My specific questions are:
- Is the standard TB2J_rotate and TB2J_merge workflow valid for strongly noncollinear reference states, or does the LKAG formalism break down when the local spin quantization axes differ by 70–90° between sites?
- What is the recommended procedure for extracting reliable DMI and J_ani parameters in this regime? Is a spin spiral approach more appropriate, and if so, does TB2J support noncollinear spin spiral Hamiltonians as input?
- Is there a way to assess whether the near-singular reconstruction matrix is a fundamental limitation of the reference state or a numerical issue that can be resolved with a different set of rotation axes?
Any guidance would be greatly appreciated.