Simple model of the ground state and spin-orbital excitations of free and adsorbed Fe(II) phthalocyanine molecules

We investigate the ground state and low-energy spin-orbital excitations of a single iron(II) phthalocyanine molecule in isolation and on an oxidized Cu(110) surface. Considering the subspace spanned by the three lowest spin-triplet states of A2g3 and Eg3 symmetry, we diagonalize the Hamiltonian made of the anisotropic spin-orbit interaction and the ligand field splitting Δ, defined as the energy difference between Eg3 and A2g3. We find that the ground state switches from a Eg3-like state with large orbital moment and out-of-plane easy axis for Δ−60 meV. The analysis of the first excited states in the two regimes explains the zero-field splitting data reported for β-FePc as well as for FePc molecules adsorbed on an oxidized Cu(110) surface [N. Tsukahara et al., Phys. Rev. Lett. 102, 167203 (2009)]. Importantly, the calculated magnetic susceptibility obtained with the ab initio value Δ=93 meV compares remarkably well with the experimental data of β-FePc in the whole available temperature range of 1–300 K.