Exchange Interactions Induce Multiple Magnetic Relaxation Channels in Single Molecule Magnets

In neutral double-decker phthalocyaninato (Pc) terbium(III) complexes, [TbPc2]0, the single itinerant electron is delocalized across both Pc ligands. Intramolecular magnetic interactions between the itinerant electron and the localized 4f electrons of the Tb(III) center introduce coupling of Tb ion total angular magnetic moment with radical spin magnetic moment (i.e. JTb − Srd coupling) across all J-sublevels, complicating the electronic and magnetic structure. Even though such (tunable) interactions are found to be moderate to weak in nature for the [TbPc2]0 complexes, the impact is quite significant. It converts the system from non-Kramer ionic [TbPc2]−/+ state into [TbPc2]0 Kramer’s system. The parallel and anti-parallel coupling of the radical spins split all the J-sublevels of terbium into two, and the extent of such “exchange splitting” is directly associated with the strength of the exchange interactions between the radical spin in the ligand and central terbium. In this work, employing multireference ab-initio calculations, we have deciphered the role of the exchange-coupled states in the demagnetization process and the single-molecule magnet (SMM) behavior. The demagnetization or magnetic relaxation process generally occurs through an excited state, associating quantum tunneling of magnetization (QTM) or the Orbach process that involves a spin-flip process. Our study reveals multiple relaxation pathways due to exchange-coupled states, opening up a profound possibility in engineering the single-molecule magnet properties in any exchange-coupled systems.