A density functional study of the magnetic interactions in transition metal phthalocyanines

In recent years, molecular and low-dimensional magnetic materials have drawn considerable interest within the field of spintronics, with the opportunity to use such materials to fabricate next-generation electronics. However, to be considered for use in these devices, candidates must exhibit near room temperature magnetism. With this in consideration, the family of transition metal phthalocyanines (TMPcs) has demonstrated potential in fulfilling this objective. In particular, the alpha polymorph of cobalt (II) phthalocyanine (α-CoPc) exhibits strong antiferromagnetic ordering, with a transition temperature of approximately 100K. Improvement of the coupling in such systems is restricted, however, by limited understanding of both the underlying exchange mechanisms and structure-property relationships. The work presented here aims to provide insight into these mechanisms, thereby, offering routes to the design of novel phthalocyanine systems with specific characteristics. These investigations are carried out using first-principles theoretical electronic structure methods, employing hybrid exchange density functional theory. The respective results indicate that the strong antiferromagnetic coupling in α-CoPc arises due to a dominant kinetic exchange interaction between adjacent singly occupied, dz2-derived magnetic orbitals. By mapping the dynamics of this interaction to the parameters of the Hubbard Hamiltonian, subsequent calculations indicate that, by replacing the central cobalt with its second-row counterpart, rhodium, the strength of the interaction could be significantly increased, with evaluated coupling magnitudes exceeding room temperature within α-phase crystal configurations. This work also provides a comprehensive examination of the electronic structure and emergent magnetic properties within the class of two-dimensional polymeric TMPcs. Significant variation in the exchange coupling is observed across the first-row group of these systems. Most consequently, polymeric manganese (II) phthalocyanine is predicted to exhibit strong ferromagnetic coupling, with a calculated Curie transition temperature, Tc~130K. This ferromagnetism is shown to originate in the system's electronic structure.