P-orbital nanomagnetism in an organic chain magnet

Abstract

A long-standing challenge in spintronics is the development of a stable, processable and tunable organic magnetic semiconductor. We reveal, through first-principles calculations, that a $p$-electron organic molecular magnet, lithium phthalocyanine (LiPc), can display surprisingly strong antiferromagnetic coupling. The strong coupling, far exceeding that observed in the widely studied transition-metal phthalocyanines, is found to be due to the delocalized spin orbital of the ligand which facilitates intermolecular interactions. The enhanced hopping between the $\ensuremath{\pi}$-conjugated orbitals is also responsible for the wide bandwidth required for high spin mobility. The interactions are a strong function of the intermolecular arrangement and increase when approaching a face-on geometry resulting in a crossover to an itinerant spin density wave ground state, which we propose as an explanation for the unusual spin susceptibility in the related $x$-LiPc phase [M. Brinkmann et al., J. Mater. Chem. 8, 675 (1998)]. This strong coupling, in conjunction with the structural flexibility of the metal phthalocyanine organic semiconductors, suggests a promising route for the fabrication of transition-metal-free, room-temperature, chain magnets for spintronic applications.