  By many-body quantum-chemical calculations, we investigate the role of two
structural effects - local ligand distortions and the anisotropic Cd-ion
coordination - on the magnetic state of Cd2Os2O7, a spin S=3/2 pyrochlore. We
find that these effects strongly compete, rendering the magnetic interactions
and ordering crucially depend on these geometrical features. Without trigonal
distortions a large easy-plane magnetic anisotropy develops. Their presence,
however, reverses the sign of the zero-field splitting and causes a large
easy-axis anisotropy (D = -6.8 meV), which in conjunction with the
antiferromagnetic exchange interaction (J = 6.4 meV) stabilizes an
all-in/all-out magnetic order. The competition uncovered here is a generic
feature of pyrochlore magnets.
