Manipulation of Spin Crossover Phenomenon in an Fe (II) Molecular Complex and Application to Molecular Spintronics

Molecules with a large local magnetic moment have attracted considerable attention for application in spintronic devices. One candidate of a suitable device goes to the spin crossover molecule, where these 3d transition metal compounds are able to exhibit a robust spin state transition between distinct states. By proper design, the spintronic devices fabricated via spin crossover molecular thin films could achieve novel functionality while retaining flexibility and other traits based on its “organic” nature. Controlling the spin state transition is a key factor of these possibilities. This thesis work investigates the manipulation of the spin state transition in [Fe{H2B(pz)2}2(bipy)] molecular system. The key question is: how can we effectively control the spin state of molecules, and how does the control inspire the future device design? Chapter 1 provides the necessary background knowledge for molecular spintronics and spin crossover molecular systems. Chapter 2 explains relevant experimental techniques involved in the thesis project. Chapter 3 focuses on the bistability and coordination dependence of the spin crossover system. The dielectric substrate will have an influence on the local environment of the spin state occupancy, and a surface to bulk difference in the thin film system indicates the presence of the cooperative effect. Chapter 4 demonstrates the variation of spin state locking effects of the molecular additives to the [Fe{H2B(pz)2}2(bipy)] molecule, and an observation of re-entrant spin state transition. Chapter 5 discusses the isothermal voltage switching of the molecular spin state. Upon forming a heterostructure of [Fe{H2B(pz)2}2(bipy)] with organic ferroelectrics, it is applicable to achieve a room temperature nonvolatile switching of the spin state via external electric fields. This sheds light on the fact that fabricating the molecular thin film spintronics device is indeed possible. Chapter 6 presents an investigation of the energy landscape of the spin state transition. The magnetic field could lower the energy barrier between spin states for the molecular spin crossover thin film fabricated on top of magnetic substrates. Advisor: Peter A. Dowbe