Berlin 2005 – scientific programme
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MA: Magnetismus
MA 14: Molekularer Magnetismus
MA 14.8: Talk
Saturday, March 5, 2005, 15:30–15:45, TU H1012
Spin transition and superexchange coupling in metallo-organic assemblies — •Yves Bodenthin1, Guntram Schwarz2, Dirk, G. Kurth2,3, Helmuth Möhwald2, and Ullrich Pietsch1 — 1Institute of Physics, University of Potsdam, P.O. Box 601553, D-14415 Potsdam, Germany — 2Max-Planck-Institute of Colloids and Interfaces, D-14424 Golm, Germany — 3National Institute for Materials Science 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
We report on a novel approach to cause spin crossover by introducing mechanical strain through an amphiphilic phase transition in a metallo-supramolecular architecture. Using SQUID and temperature resolved XMCD (x-ray magnetic circular dichroism) we observed a diamagnetic to paramagnetic phase transition which is connected to a phase transition of the amphiphilic layers within the superstructure. It was investigated by EXAFS, x-ray reflectivity and in-plane diffraction that this structural phase transition induces sufficient mechanical strain to distort the octahedral coordination sphere of the tightly coupled Fe2+ ions. Consequently, the energetic separation of the subsets of the iron d-orbitals changes and gives rise to spin transition from S=0 to S=2. Ferromagnetic behavior of PAC was observed above the spin-crossover temperature by at a system with a reduced number of amphiphilic molecules. The appearance of molecular ferromagnetism in these supra-molecular structures can be explained by a super-exchange mechanism. Using element specific XMCD we observed a positive spin density at the iron ion and a negative spin density located at the ligand. This model is confirmed by first ab-initio calculations in term of density-functional approach.