First-principles study of the electronic and magnetic properties of Fe, Co, and Ni dimers adsorbed on polycyclic-aromatic-hydrocarbon molecules as well as the laser chirp effect on the ultrafast spin dynamics

We investigate the geometric, electronic, and magnetic properties of ${\mathrm{TM}}_{2}(\mathrm{PAH})$ complexes [transition metal (TM) = Fe, Co, Ni; $\text{polycyclic-aromatic}\phantom{\rule{4.pt}{0ex}}\text{hydrocarbon}\phantom{\rule{4.pt}{0ex}}\text{(PAH)}={\mathrm{C}}_{10}{\mathrm{H}}_{8}, {\mathrm{C}}_{16}{\mathrm{H}}_{10}, {\mathrm{C}}_{24}{\mathrm{H}}_{12}, {\mathrm{C}}_{32}{\mathrm{H}}_{14}]$ as well as the optically induced ultrafast spin dynamics from a first-principles study. Geometrically, the magnetic dimer of each of the complexes [except for ${\mathrm{Ni}}_{2}({\mathrm{C}}_{16}{\mathrm{H}}_{10}$)] turns out to prefer to adsorb above the hollow site of the outer carbon ring of the PAH molecule. The PAH-size effect and TM-element effect on the energy levels and spin localizations are analyzed. It is found that for the structures with the same PAH molecules, the level bands generally get wider from Fe via Co to Ni, while the number of spin-localized states overall decreases in the same order. For the structures with the same TM species [except for the two complexes ${\mathrm{Ni}}_{2}({\mathrm{C}}_{16}{\mathrm{H}}_{10}$) and ${\mathrm{Fe}}_{2}({\mathrm{C}}_{32}{\mathrm{H}}_{14}$)], the low-lying levels are hardly affected by the size of the PAH molecules. Among all the calculated levels for each complex [except for ${\mathrm{Ni}}_{2}({\mathrm{C}}_{16}{\mathrm{H}}_{10}$)], there are always more states with spin localized on the remote magnetic center than those localized on the near center. Driven by nonchirped subpicosecond laser pulses, a series of ultrafast spin-flip and spin-transfer scenarios on these structures are predicted and analyzed, based on which the laser chirp effect is further explored and some rules of thumb about the chirp tolerance and sensitivity are obtained. The results demonstrated in this paper are believed to enrich our understanding of the size- and structure-dependent electronic and magnetic properties of the carbon-based magnetic molecular structures, and further to promote the relevant experimental realization of the spin dynamics and their potential applications in future molecular spin devices.