Speaker
Dr
Heike Herper
(Uppsala university, Dept. of Physics and Astronomy)
Description
The ongoing miniaturization of magnetic and electronic devices requires the search for new materials to avoid the size limits of conventional technology. Hybrid systems consisting of paramagnetic molecules and metallic substrates are promising candidates. Especially porphyrin and phthalocyanine molecules with transition metal center have attracted interest because of their tunable magnetic properties.
We have performed density functional theory (DFT) calculations for Fe centered macrocycles on different substrates to model these systems and to understand the underlying mechanisms on the atomic scale.
In agreement with recent x-ray absorption measurements we observe that the magnetic coupling between FePc molecules and a ferromagnetic (FM) Co(001) substrate can be switched from FM to antiferromagnetic by adding an oxygen adlayer to the substrate. Our ab initio investigations have shown that this is accompanied by a strong reduction of the coupling strength and a change in the coupling mechanism from a direct coupling to a 180º super-exchange between Fe and Co [1,2].
The magnetic moment and the spin-state could be tailored using ligands. In our case Cl and O ligands have been attached to Fe porphyrin (FeP) adsorbed on a nonmagnetic substrate. Both types of ligands lead to a weakening of the molecule substrate hybridization and an increase of the Fe spin moment [3]. The change of the magnetic moment is directly related to an increase of the Fe-N bond length. An even more efficient manipulation of the spin state has been predicted for FeP adsorbed on a graphene sheet with a divacancy[4].
In all DFT calculations an effective Hubbard term has been included for the 3d-orbitals of the Fe center to open the gap between the highest occupied and lowest unoccupied molecular orbital in the minority spin channel. In order to incorporate the long-ranged dispersion forces in the molecular system, Grimme's 2nd method has been used. It turned out that van der Waals interactions not only influence the adsorption distance but play an important role in view of the orientation of the molecule on the substrate [2].
[1] D. Klar et al., Phys. Rev. B 88, 224424 (2013)
[2] H. C. Herper et al., accepted for publ. in Phys. Rev. B 89 (2014)
[3] H. C. Herper et al. , Phys. Rev. B 87, 174425 (2013)
[4] S. Bhandary et al. Phys. Rev. Lett. 107, 257202 (2011)
Primary author
Dr
Heike Herper
(Uppsala university, Dept. of Physics and Astronomy)
Co-authors
Dr
Barbara Brena
(Uppsala University, Dept. of Physics and Astronomy)
Prof.
Biplab Sanyal
(Uppsala University, Dept. of Physics and Astronomy)
Prof.
Olle Eriksson
(Uppsala University, Dept. of Physics and Astronomy)
Mr
Sumanta Bhandary
(Uppsala University, Dept. of Physics and Astronomy)
