Abstract
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 $\ensuremath{\theta}$ 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.
Highlights
Since their discovery and the subsequent systematic studies of their molecular structure, phthalocyanines have been the subject of special research interest because of their multiple applications such as dyes, catalysts, and coatings
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 3A2g and 3Eg symmetry in the presence of a saturating magnetic field at a polar angle θ 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 L23-edge x-ray magnetic circular dichroism (XMCD) data
Summary
Since their discovery and the subsequent systematic studies of their molecular structure, phthalocyanines have been the subject of special research interest because of their multiple applications such as dyes, catalysts, and coatings. While accounting for the large orbital moment anisotropy of around a factor of two, the interpretation of these data could not answer the crucial question of the ground electronic configuration Quite naturally, these findings prompted a further investigation of the Fe K edge XMCD, since this signal is known to be sensitive only to the orbital moment [10]. These findings prompted a further investigation of the Fe K edge XMCD, since this signal is known to be sensitive only to the orbital moment [10] Based on these measurements and on ab initio molecular orbital multiplet calculations of the isolated FePc molecule, we have proposed a model for the magnetic ground state of the FePc film that explains the Fe K edge XMCD data and reproduces the observed values of the orbital moments in the perpendicular and planar directions [11]. The expression of the ground state was introduced as an Ansatz and no attempt was given to derive it in a quantum mechanical way
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