Abstract
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 $^{3}A_{2g}$ and $^{3}E_{g}$ symmetry, we diagonalize the Hamiltonian made of the anisotropic spin-orbit interaction and the ligand field splitting $\mathrm{\ensuremath{\Delta}}$, defined as the energy difference between $^{3}E_{g}$ and $^{3}A_{2g}$. We find that the ground state switches from a $^{3}E_{g}$-like state with large orbital moment and out-of-plane easy axis for $\mathrm{\ensuremath{\Delta}}l\ensuremath{-}60$ meV to a $^{3}A_{2g}$-like singlet state with in-plane easy axis for $\mathrm{\ensuremath{\Delta}}g\ensuremath{-}60$ meV. The analysis of the first excited states in the two regimes explains the zero-field splitting data reported for $\ensuremath{\beta}$-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 $\mathrm{\ensuremath{\Delta}}=93$ meV compares remarkably well with the experimental data of $\ensuremath{\beta}$-FePc in the whole available temperature range of 1--300 K.
Highlights
Due to the importance of the Fe phthalocyanine (FePc) molecule for possible applications in nanodevices and spintronics, a quantum model of its magnetic ground state is of the utmost interest
There is, converging theoretical evidence obtained with different methods that, neglecting spinorbit interaction, the ground state of an isolated FePc molecule is an orbital singlet of A2g symmetry followed by two orbitally degenerate states of Eg symmetry lying about 0.1 eV higher with spin S = 1 [1,2,3,4,5]
Based on the analysis of the x-ray magnetic circular dichroism (XMCD) spectroscopic data at the Fe K and L edges and recent theoretical investigations [1,2,3,4,5], we have presented a model of the magnetic ground state and low-lying excited states of the FePc molecule in which an orbital singlet of A2g symmetry and two orbitally degenerate multiplets of Eg symmetry separated by 93 meV are mixed by a spin-orbit interaction of the order of 50 meV
Summary
Due to the importance of the Fe phthalocyanine (FePc) molecule for possible applications in nanodevices and spintronics, a quantum model of its magnetic ground state is of the utmost interest. It is important to investigate the consequences of the model suggested above in comparison with experimental evidence available for the isolated molecule, namely, the temperature dependence of the paramagnetic susceptibility in the range of 1–300 K obtained by Dale et al [6], Barraclough et al [7] and Labarta et al [8] in crystal powder samples of FePc in the β phase [9]. In this phase the molecular arrangement of the FePc molecules in a zigzag fashion with a large distance between metal ions results in weak mag-. This is important in view of technological applications, where FePc molecules will be subject to external perturbation, especially at interfaces
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