Mixed Configuration Ground State in Iron(Ii) Phthalocyanine

We calculate the angular dependence of the x-ray linear and circular dichroism at the L2,3 edges of a-Fe(II) Phthalocyanine (FePc) thin films using a ligand-field model with full configuration interaction. We find the best agreement with the experimental spectra for a mixed ground state of E-3(g) (a(1g)(2)e(g)(3)b(2g)(1)) and B-3(2g) (a(1g)(1)e(g)(4)b(2g)(1)) with the two configurations coupled by the spin-orbit interaction. The E-3(g) (b) and B-3(2g) states have easy-axis and easy-plane anisotropies, respectively. Our model accounts for an easy-plane magnetic anisotropy and the measured magnitudes of the in-plane orbital and spin moments. The proximity in energy of the two configurations allows a switching of the magnetic anisotropy from easy plane to easy axis with a small change in the crystal field, as recently observed for FePc adsorbed on an oxidized Cu surface. We also discuss the possibility of a quintet ground state ((5)A(1g) is 250 meV above the ground state) with planar anisotropy by manipulation of the Fe-C bond length by depositing the complex on a substrate that is subjected to a mechanical strain.