If we consider only the chemical structure, there is 3 four-fold rotational symmetry axis through the FeI-N-FeI axis, which means it’s the PAS for the electrical field gradient.
If we now take into account the magnetic structure (ferro, along z), then we only have one 4-fold rotational axis, along z. We can then say it’s the z-axis of our PAS and that eta=0. We don’t know yet the 2 other axes of our PAS.