  We determine theoretically the effect of spin-orbit coupling on the magnetic
excitation spectrum of itinerant multi-orbital systems, with specific
application to iron-based superconductors. Our microscopic model includes a
realistic ten-band kinetic Hamiltonian, atomic spin-orbit coupling, and
multi-orbital Hubbard interactions. Our results highlight the remarkable
variability of the resulting magnetic anisotropy despite constant spin-orbit
coupling. At the same time, the magnetic anisotropy exhibits robust universal
behavior upon changes in the bandstructure corresponding to different materials
of iron-based superconductors. A natural explanation of the observed
universality emerges when considering optimal nesting as a resonance
phenomenon. Our theory is also of relevance to other itinerant system with
spin-orbit coupling and nesting tendencies in the bandstructure.
