Spin-orbit enabled unconventional Stoner magnetism

The Stoner instability remains a cornerstone for understanding metallic ferromagnets. This instability captures the interplay of Coulomb repulsion, Pauli exclusion, and two-fold fermionic spin degeneracy. In materials with spin-orbit coupling, fermionic spin is generalized to a two-fold degenerate pseudospin. Here we identify a new fermionic pseudospin with a symmetry that forbids it to couple to a Zeeman field. This spinless pseudospin exists in five non-symmorphic space groups and appears at the Brillouin zone (BZ) boundary. With Coulomb repulsion, this spinless pseudospin gives rise to Stoner instabilities into magnetic states that are not usual ferromagnets. These spinless-pseudospin ferromagnets break time-reversal symmetry, have a vanishing magnetization, and are generally non-collinear. The non-collinear magnetization enables a scalar spin chirality-driven Berry curvature. In addition, much like altermagnets, these pseudospin ferromagnets exhibit energy band spin-splittings that vanish by symmetry along lines in the BZ, allowing for drumhead surface states. We discuss candidate materials.