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Magnetic anisotropy and XMCD of Iron porphyrin on Cu(001) substrate
Yuki Yoshimoto and Peter Krüger
Nanomaterial Science Department,
Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan
Paramagnetic complexes such as phthalocyanine and porphyrin molecules
with transition metal atoms have strong magnetic anisotropy. As their metal centers are fairly separated from each other, they can be considered as single molecule magnets. Therefore they are interesting for fundamental research in spintronics and it is expected that they can be used for magnetic switching devices.
In this study we have analyzed the magnetic anisotropy of iron porphyrin (FeP) on Cu(001). It is known that in the case of ferromagnetic substrates such as Fe, Co, these molecules are affected by the correlation and hybridization with the substrate.[1] However, on noble metal substrates, this hybridization effect is weaker. Therefore hybridization with the ligand nitrogen atoms can become the dominant factor for the electronic state of the transition metal atom.
A previous XMCD and DFT+U calculations study has reported that iron in
FeP takes S=1 intermediate spin state and that the origin of the strongly anisotropic XMCD signal of FeP is due to the magnetic dipolar term. [2]
We have performed the simulation of XMCD by DFT+U calculations and
Charge-transfer Cluster model calculations. The cluster model is based on Ligand Field Multiplet Calculations (LFMC), and takes into account the charge fluctuation for each d-orbital (caused by the orbital mixing).
In LFMC, the ground state of this system is a mixture of two states which is caused by Spin-Orbit Coupling , and this configuration is (dz^2)^2,(dxy)^2,(dxz)^2 that is called A2 multiplet. This result agrees with the observed in-plane anisotropy which is however, overestimated. Moreover, it shows that the magnetic dipole term play only a minor role. When the hybridization with the ligand orbitals (of same symmetry as dxz, dyz, dx^2-y^2) is taken into account, the in-plane anisotropy is reduced and the shape of the spectra is improved as compared to the LFMC result. This ground state is based on A2 multiplet but it is mixed with d5 and d7 configurations. This means that the weak bonding between iron and ligand is important for the magnetism of phthalocyanine and porphyrin, while for other substrates with a different ground state multiplet, the ligand effect was found to be weak. These results imply that the effect of charge fluctuation changes significantly with the ground state multiplet.
[1] M. Bernien et al. PRL102 047202(2009)
[2] H. Herper et al. PRB87 174425(2013)