Spin Manipulation in Functional Materials: Study from first Principles

Abstract

The thesis investigates an intriguing phenomenon, namely the Spin Crossover (SCO) that has recently been observed in many functional materials. A detailed theoretical investigation of SCO phenomena in newly synthesized materials has been carried out by employing first-principles density functional theory +U calculations. The spin state switching of a class of square-planar magnetic molecules and their interactions to the metal surfaces has been investigated. The SCO triggered by an electric polarization was observed in the perovskite Sr2CoO3F (SCOF) system. In a hybrid perovskite heterostructure, where SCOF is sandwiched between two ferroelectric BaTiO3 (BTO) layers, the spin state of the Co atom in SCOF can be switched systematically from a high-spin to a low-spin by altering the polarization direction of the BTO with respect to SCOF. A giant magnetoelectric coupling has also been observed in this system. Pressure-driven SCO has been observed in Hofmann clathrate, namely (Fe{OS(CH3)2}2{Ag(CN)2}2), while applying the hydrostatic pressure. The study shows that under a relatively low isotropic hydrostatic pressure, the complex exhibits a reversible spin switching. The investigation reveals that the system undergoes a structural phase transition when the pressure is anisotropic. The transition pressure for the spin-state transition and structural transformation has been estimated from firstprinciples calculations. In the final stage of this work, the spin crossover in metalorganic molecules and their interactions with magnetic metal substrates have been investigated. The structural, electronic, and magnetic properties of Ni-quinonoid and Ni-dinuclear molecules have been studied upon adsorption on Co(001) substrate. The study shows that these molecules undergo a spin state switching when they adsorbed on the Co(001) surface. The exchange couplings between the magnetic centers are carefully investigated. Further, the spin state and magnetic anisotropy energy of Ni atom in the Ni-dinuclear molecule adsorbed on a Co(001) substrate has tailored by using adatom as an axial ligand to the central transition metal (TM) atoms in the molecule.