Exchange torque in noncollinear spin-density-functional theory with a semilocal exchange functional

We propose a novel energy functional for spin density functional theory based on the short-range expansion of the noncollinear exchange hole.

Our exchange functional is directly derived for noncollinear magnetism, is U(1) and SU(2) gauge invariant and contains a non-zero exchange torque.

We demonstrate the performance of the functional on a frustrated antiferromagnetic chromium cluster, where the obtained exchange torque is compared to the one coming from the far more expensive Slater potential and the optimized effective potential for exact-exchange.

This functional opens the door to the understanding of important questions like the role of the exchange-correlation torque on the magnetic properties.

Potential applications of this new functional range from equilibrium studies of unconventional spin structures like skyrmions and spin-wave dispersion, to the ab initio study of demagnetization in light-driven solids and, more generally, magnetic materials driven out of equilibrium.