Electric field manipulation of the molecular spin state of a Fe(II) spin crossover complex

The spin crossover (SCO) phenomenon, in 3d transition metal compounds, has potential applicability in molecular spintronic devices for low-cost flexible memory. Isothermal changes of the electronic structure has now been achieved for the Fe(II) spin crossover complex [Fe{H₂B(pz)₂}₂(bipy)], where pz = tris(pyrazol-1-yl)- borohydride and bipy = 2,2’-bipyridine, by external electric fields. This isothermal voltage-controlled switching is evident in thin film bilayer structures where the molecular spin crossover film is adjacent to a molecular ferroelectric thin film (the tested examples being polyvinylidene fluoride hexafluoropropylene or croconic acid (C₅H₂O₅)). These organic ferroelectric substrates appear to lock the spin crossover molecular complex largely in the low or high spin state depending on the direction of ferroelectric polarization, in both a planar two terminal diode structure and for a transistor structure. Moreover, the spin state change is observed to be accompanied by a conductance change, where higher conductance occurs at a high spin state while lower conductance is observed for a low spin state, and is seen to be nonvolatile, i.e. the spins state is retained in the absence of an applied electric field.