Bistable Magnetic Materials Based on Spin State Switching and Single Molecule Magnets

Abstract

Molecular Magnets have intrigued researchers because they can enhance the data storage density, act as molecular switches and sensors, and have prospects in spintronics and quantum computing. They encompass various systems, including spin crossover (SCO) systems, single molecule magnets (SMMs), electron transfer systems, coordination polymers, valence tautomers, grids, helicates, cages, etc. Of these exciting topics, my Ph.D. thesis is centered on Stimuli-responsive Spin Crossover (SCO) Systems and Lanthanide Single Molecule Magnets (SMMs). Spin Crossover (SCO) systems have garnered significant interest in the research community due to their ability to respond to various stimuli, such as temperature changes, light, pressure and pH. These stimuli can induce changes in their spin states, which in turn alter their observable physical properties. Transition metal complexes with 3d4-7 electronic configurations often exhibit this behaviour in an octahedral coordination environment, particularly when the octahedral splitting energy (Δo) is comparable to the pairing energy (P). This makes SCO systems promising candidates for multifunctional bistable materials. As a result, they hold potential applications in areas like molecular switches, sensors, and spintronics. On the other hand, Single Molecule Magnets (SMMs) bridge the gap between classical and quantum realms. As molecular-scale magnets, SMMs do not require long-range magnetic ordering like classical magnets. Researchers are working to optimise the blocking temperature, the temperature below which SMMS retain their magnetisation. Thus, making them suitable for use as qubits in quantum computing and spintronics. Achieving a high-spin ground state and high uniaxial anisotropy is crucial in these systems, as it enhances the energy barrier between bistable magnetic spin states, thus increasing the stability of data storage. These topics are discussed in detail in Chapter 1. In Chapter 2, I investigated various Mn(III)-based Spin Crossover (SCO) systems, focusing on their magneto-structural properties, along with spectroscopic and electrochemical analyses. Our approach involved tuning the spin transition temperature by modifying ligands and altering counter anions. As a d4 system, Mn(III) features a Jahn-Teller active metal center, which undergoes notable structural changes during spin crossover at different temperatures. By carefully adjusting ligand field strength, orientation, and cooperativity, Mn(III) systems can transition between high-spin and low-spin states in response to temperature variations. We have successfully used lanthanide (III) ions, namely Dy(III), Nd(III), Ho(III), Er(III), Tb(III), to prepare several Single Molecule Magnets (SMM). Mononuclear Dy(III) complexes have been explored in chapter 3 to unravel its zero field SMM behaviour and luminescent thermometric properties. In chapter 4, we were able to study the magnetic and luminescent behaviour of complexes of lesser known lanthanides like Ho(III) and Nd(III). Finally, we have also explored the prospects of Co(II) spin crossover and tried to tune the nuclearity and dimensionality in chapter 5. We investigated how counter anions and ligand substitutions influence the Spin Crossover (SCO) behaviour of Mn(III) mononuclear systems by modulating the cooperativity within these systems. Additionally, we aimed to tune the slow relaxation of magnetization in Ln(III)-based Single Molecule Magnets (SMMs) by optimizing the anisotropy through strategic ligand design. With precise modifications, these stable molecular systems hold great promise for applications in molecular switches and molecular memory devices.