Calculated iron L2,3 x-ray absorption and x-ray magnetic circular dichroism of spin-crossover Fe(phen)2(NCS)2 molecules adsorbed on a Cu(001) surface

The projector augmented wave method has been used to compute the iron L-2,L-3 edges of x-ray absorption spectra (XAS) and x-ray magnetic circular dichroism (XMCD) of the spin-crossover Fe(phen)(2)(NCS)(2) molecule when adsorbed on Cu(001) surface and in the gas phase, for both the high-spin (HS) and low-spin (LS) states. The electronic structures of both HS and LS states have been calculated using the spin-polarized generalized gradient approximation for the exchange-correlation potential, and the strongly localized iron 3d states are described using Dudarev's rotationally invariant formulation of the DFT+U method. It is shown that only the iron all-electron partial waves are necessary to calculate the XAS transition matrix elements in the electric dipole approximation, as the contribution of the pseudo partial waves is compensated by the plane-wave component of the wave function. It is found that the calculated XAS and XMCD with the static core hole or the Slater transition state half hole are in less good agreement with experiment than those using the so called initial state. This disagreement is due to the reduction of the iron spin magnetic moment caused by the static screening of the core hole by the photo-electron. The L(2,)3 XAS formula is found to be directly related to the unoccupied 3d density of states (DOS), and hence the symmetry broken e(g )and the t(2)g iron DOS are used to explain the XAS and XMCD results. It is demonstrated that the dependence of the HS XMCD on the direction of incident x-ray circularly polarized light with respect to the magnetization direction can be used to determine the iron octahedron deformation, while the XMCD for various magnetization directions is directly related to the anisotropy of the orbital magnetic moment and the magnetocrystalline energy. The Thole-Carra-Van der Laan XMCD sum rules have been applied to the XMCD L-2,L-3 spectra to compute the spin and orbital magnetic moments. It is shown that the magnetic dipole moment T-z is very large due to the strong distortion of the iron octahedron and is necessary for an accurate determination of the sum rule computed spin magnetic moment.