K. Ridier, G. A. Craig, F. Damay, T. Fennell, M. Murrie, and G. Chaboussant.
We report a neutron-scattering investigation of the spin-crossover compound [Fe(ptz)6](BF4)2, which undergoes an abrupt thermal spin transition from high spin (HS), S=2, to low spin (LS), S=0, around 135 K. The HS magnetic state can be restored at low temperature under blue/green light irradiation. We have developed a specially designed optical setup for neutron scattering to address the magnetic properties of the light-induced HS state. By using neutron diffraction, we demonstrate that significant HS/LS ratios (of up to 60%) can be obtained with this experimental setup on a sample volume considered large (400 mg), while a complete recovery of the LS state is achieved using near-infrared light. Finally, with inelastic neutron scattering (INS) we have observed magnetic transitions arising from the photo-induced metastable HS S=2 state split by crystal-field and spin-orbit coupling. We interpret the INS data assuming a spin-only model with a zero-field splitting of the S=2 ground state. The obtained parameters are D≈−1.28±0.03 meV and |E|≈0.08±0.03 meV. The present results show that in situ magnetic inelastic neutron-scattering investigations on a broad range of photomagnetic materials are now possible.