Fully relativistic noncollinear magnetism in spin-density-functional theory: Application to USb by means of the fully relativistic spin-polarized LAPW method

A fully relativistic formulation for the electronic structure of noncollinear magnets is derived from a generalization of the relativistic spin-density-functional theory (RSDFT), and is adopted on the basis of the fully relativistic spin-polarized linearized augmented-plane-wave (LAPW) method in the local spin-density approximation. All electrons are treated in a noncollinear magnetic configuration by the spin-polarized coupled Dirac equations derived from the RSDFT. The relativistic spin-orbit interaction produces an orbital moment through spin polarization due to the internal and/or external magnetic fields. The directions of spin and orbital moments in each atomic site can be determined as independent vectors from the self-consistent calculations. The magnetic moment is obtained from a combination of the spin and orbital moment. The relativistic noncollinear spin-polarized LAPW method is applied, as an example, to USb with the triple-k magnetic structure, and the electronic structure is shown together with the magnetic moments.