a The aim of this paper is to demonstrate the possibility of expressing molecular magnetic phenomena on hybrid nano-particles. The advantage of such composite materials is their wonderful versatility, in terms of choice of molecular compounds to be grafted on, and in terms of possibilities for integration in devices. As a first example, we designed a composite made of silica nanoparticles as cores on which Fe(II) coordination complexes displaying spin-crossover (SCO) phenomenon have been grafted in three steps involving the silanization of the SiO 2 core, the anchoring of bis(1-methylimidazolyl)-type ligands and at last the coordination of octahedral Fe(II) complexes at the surface of the nanoparticles. This step-by-step procedure enables an adequate coverage of the Fe(II) complexes at the surface of the nanoparticles that allows for the appearance of the spin-transition (ST) behavior at a level detectable by SQUID magnetometry. These tailor-made particles can thus be considered as nanometric switchable units.