Excited Spin-State Trapping in Spin Crossover Complexes on Ferroelectric Substrates

We have studied thin films of Fe(II) spin crossover complexes deposited on differently poled ferroelectric PMN-PT ([Pb(Mg_1/3Nb_2/3)O₃]_1-x[PbTiO₃]_x, x = 0.32) substrates by X-ray absorption spectroscopy. The X-ray spectra reveal the temperature-driven conversion between high-spin and low-spin states without any observable effect of the ferroelectric polarization on the spin state of the molecules down to 100 K. In the soft X-ray-induced excited spin-state trapping (SOXIESST) regime at 3 K, large differences occur between the two ferroelectric polarizations. The efficiency of X-rays in exciting the molecules to the high-spin state is more than an order of magnitude larger when the ferroelectric dipoles of the substrate are pointing toward the surface compared to the opposite polarization. We explain our findings by a modulation of the polarization-dependent efficiency of the scattering of X-ray-generated secondary electrons at the molecules. Our results provide a deep insight into the SOXIESST mechanism and suggest that such molecules could be used as detectors for electrons traveling in the substrate at energies lower than the substrate electron affinity.