MAGNETIC ANISOTROPY AND EXCHANGE INTERACTIONS AT MOLECULE SUBSTRATE SPIN INTERFACES

Abstract

In the last couple of decades, molecular spintronics has emerged, exploiting the spin of molecules to design magneto-electronic devices. Recently, the development of understanding the fundamental processes at the hybrid interface of the magnetic molecules and various magnetic/non-magnetic substrates has paved the path toward interface-assisted molecular spintronics. The nature of the magnetic exchange in- teractions between different magnetic centers and the inherent magnetic anisotropy of an individual magnetic center are the two vital magnetic properties that provide turnability. Despite the fact that various experimental reports in conjunction with ab initio quantum mechanical calculations have advanced the evolution in this field, the interface-mediated control of those magnetic parameters remains an open area of research. This thesis explores the effect of structural dynamics on the magnetic properties of the molecules at the molecule/substrate interfaces. The porphyrins and other tetrapyrrole macrocycles possess various functional properties and have already been marked as essential components in molecular spin- tronics and single molecular devices. We have studied a series of tetra-coordinated iron (II) porphyrins with various substitutions at the peripheral macrocyclic rings, applying first-principle calculations. The spin-state-dependent nature of the chem- ical bonds and spin polarization on the N-atoms play a crucial role in the vibra- tional dynamics of the central Fe-atom. Theoretical nuclear resonance vibrational spectroscopy (NRVS) is adopted to investigate the vibrational dynamics, Fe-atom- associated spectral peaks, and their spin-dependent features. We find that NRVS studies and iron pre-K-edge X-ray absorption analysis convincingly provides infor- mation on the spin state, which could be utilized to establish a new methodology to identify the spin states without adopting any magnetic probes. On the other hand, studying magnetic properties, especially the magnetic anisotropy of iron–porphyrin complexes employing multiconfigurational methods, is quite challenging due to many strongly correlated electrons in nearly degenerate orbitals. However, a pre- requisite for observing the magnetic anisotropy and slow magnetization relaxation, i.e., the zero-field splitting parameter, D, was experimentally observed decades ago for halide-based axially ligated penta- coordinate Fe(III)–porphyrins. In these com- plexes, the signs of D were primarily reported as positive, while in a few cases, in- conclusive signs of the D parameter were also mentioned. We confirm the positive D values by deciphering the electronic structure of these penta-coordinated com- plexes employing the complete active space self-consistent field method (CASSCF) and N- electron valence second-order perturbation theory (NEVPT2). However, a negative D value is highly desired to observe the single-molecule magnet properties ixwithout an external magnetic field, which we observed in the Fe(II)–porphyrin com- plexes with axial imidazole ligands instead of halide ligands. The detailed analysis of the multireference wave functions unravels the role of axial ligands in determin- ing the sign and magnitude of the D parameters. Molecular spintronics has witnessed a spike of interest in utilizing organometal- lic compounds of the sandwiched metallocene complexes due to their unique struc- tural features. The typical chromocene molecule possesses a magnetic easy axis aligned with the principal axis and significant zero-field splitting. Although the magnetic center of the molecule doesn’t directly interact with the substrate, con- siderable change in the magnetic properties is observed when deposited on a sub- strate. The surface adsorption of the molecule takes place by the involvement of π- electrons of the cyclopentadienyl ligand in forming the interfacial states. These hy- bridized interfacial states, in turn, significantly change the chromium center’s elec- tronic structure and affect the molecule’s magnetic anisotropy on a non-magnetic substrate. On the other hand, the ligand-mediated superexchange couples the molec- ular spin ferromagnetically with the ferromagnetic substrate, and the magnetic aniso- tropy of the molecule is controlled by the anisotropy of the substrate itself. Finally, adopting ab initio molecular dynamics simulations (AIMD) based on Born- Oppenheimer molecular dynamics (BOMD), the on-surface dynamics of Iron porphyrin (FePor) molecule on ferromagnetic Co substrates and non-magnetic Au substrate have been explored at finite temperatures. The magnetic exchange in- teractions of the molecule with the magnetic substrates are highly dependent on the extent of adsorption and consequent thermal fluctuation of the molecule. On the contrary, the hopping of the iron(II) porphyrin molecule due to the weaker ph- ysisorption becomes more prominent when a non-magnetic Au(111) substrate is present. Consequently, the magnetic anisotropy of the molecule is found to be switched between in-plane and out-of-plane directions depending on the adsorp- tion site of the molecule. The work demonstrated in this thesis contributes to un- derstanding the dynamic nature of the spin interfaces, which will eventually help design novel molecular setups for spintronics applications.