  The mechanism used by migratory birds to orientate themselves using the
geomagnetic field is still a mystery in many species. The radical pair
mechanism, in which very weak magnetic fields can influence certain types of
spin-dependent chemical reactions, leading to biologically observable signals,
has recently imposed itself as one of the most promising candidates for certain
species. This is thanks both to its extreme sensitivity and its capacity to
reproduce results from behavioral studies. Still, in order to gain a
directional sensitivity, an anisotropic mechanism is needed. Recent proposals
have explored the possibility that such an anisotropy is due to the
electron-nucleus hyperfine interaction. In this work we explore a different
possibility, in which the anisotropy is due to spin-orbit coupling between the
electron spin and its angular momentum. We will show how a
spin-orbit-coupling-based magnetic compass can have performances comparable
with the usually-studied nuclear-hyperfine based mechanism. Our results could
thus help researchers actively looking for candidate biological molecules which
may host magnetoreceptive functions, both to describe magnetoreception in birds
as well as to develop artificial chemical compass systems.
