If we don’t consider the spin, we have three perpendicular 4-fold rotational axis which go through the N atom and the Fe-IIa and Fe-IIb atoms. Any of this three axis can be chosen as the PAS of the system.
But if all spins are pointing up, then the symmetry is reduced. If considering spins, only a 4-fold rotational axis is present, which is found in the middle of the cell, and cross the N atom as well as the Fe-IIb atoms. This will be the PAS, and rotations in the perpendicular directions of this axis won’t be symmetric, due to the spin of the Fe atoms.
We can also say that if there are not spins, ή will be 0, and the electric field gradient tensor will be 0 as well, but if there are spins, ή will be 0 but not the EFG.