Abstract
We show how symmetry considerations and ab initio calculations within the framework of the density-functional theory can be used in the analysis of the appearance and orientation of magnetic hyperfine fields. This approach, in which the incorporation of spin-orbit coupling is essential, provides information about hyperfine fields also for the sites where the Heisenberg exchange field is zero. This is of crucial importance in the interpretation of magnetic hyperfine-field measurements. We apply this theoretical scheme and perturbed-angular-correlation experiments to study the magnetic structure of