We determine the magnetic ground state of the FePc molecule on Au-supported thin films based on the observed values of orbital anisotropy and spectroscopic x-ray magnetic circular dichroism (XMCD) measurements at the Fe $K$ and $L$ edges. Starting from ab initio molecular orbital multiplet calculations for the isolated molecule, we diagonalize the spin-orbit interaction in the subspace spanned by the three lowest spin triplet states of ${}^3{A}_{2g}$ and ${}^3{E}_g$ symmetry in the presence of a saturating magnetic field at a polar angle $\theta$ with respect to the normal to the plane of the film, plus an external perturbation representing the effect of the molecules in the stack on the FePc molecule under consideration. We find that the orbital moment of the ground state strongly depends on the magnetic field direction in agreement with the sum rule analysis of the $L_{23}$-edge XMCD data. We calculate integrals over the XMCD spectra at the Fe $K$ and $L_{23}$ edges as used in the sum rules and explicitly show that they agree with the expectation values of the orbital moment and effective spin moment of the ground state. On the basis of this analysis, we can rule out alternative candidates proposed in the literature.

• Received 17 June 2017
• Revised 23 March 2018

DOI:https://doi.org/10.1103/PhysRevB.97.155139