The classical flipping ratio technique applied to non classical magnetic materials: Molecule-based and Photoswitchable magnetic compounds

Applications of the classical flipping ratio technique to the determination of the spin density by polarised neutron diffraction (PND) are described. In the domain of molecule-based magnetic compounds, such investigations provide direct information on the nature and the origin of the intraand inter-molecular magnetic interactions. For instance, the visualisation of the spin distribution in the crystal cell and in the molecule enables to demonstrate the interaction pathways between the magnetic centres (transition metals ions, organic radicals) and to investigate the role of the spin delocalisation and spin polarisation effects in the interaction mechanisms. Moreover, the experimental spin density in large molecular clusters permits to elucidate the nature of the magnetic ground state that results from the competition between antiferromagnetic intramolecular interactions. Photomagnetism in molecule-based compounds receives a growing interest, because of the possible future applications of photo-switchable magnetic materials. Spin crossover compounds containing an octahedrally coordinated Fe ion present a low spin diamagnetic (S = 0) ground state which can be switched, under light illumination with a suitable light wavelength, to a high spin paramagnetic (S = 2) metastable state having an extremely long lifetime at low temperature. The magnetisation density in a photoexcited magnetic state was determined for the first time for the spin crossover complex [Fe(ptz)6](BF4)2 (ptz = 1-propyltetrazole).