Graphene-modulated interfacial exchange coupling across organic molecular/ferromagnet spin interfaces

Magnetic molecules on ferromagnetic metallic substrates have been widely explored to exploit the potential for molecular magnetic storage and spintronics applications. Recent advances in these hybrid interfaces integrated with two-dimensional materials have been proposed as a flexible platform for realizing new spin-related effects. Herein, the impact of inserting graphene on the electronic and magnetic properties of a family of transition metal phthalocyanines (TMPcs, TM = Cr, Mn, Fe, Co, and Cu) deposited on ferromagnetic Ni(111) surfaces have been systematically rationalized by density functional theory analysis. Our calculations reveal that the magnetic exchange interaction across the molecule-substrate interfaces can be significantly mediated by the introduction of a graphene interlayer. Interestingly, these TMPcs exhibit ferromagnetic coupling with the Ni substrate. However, the strength of this coupling is reduced in the presence of a graphene decoupling layer, with the exception of CoPc. In the case of CoPc, the original ferromagnetic coupling with Ni(111) can be altered to antiferromagnetic when a graphene interlayer is introduced. By analyzing the different channels of communication involved in the spin interaction between the molecule and the magnetic substrate, we attribute these significant differences to the varied influences on the exchange interaction caused by the intermediary graphene layer. The presence of the inserted graphene layer may block the direct exchange interaction between the TMPc molecule and substrate, while the indirect superexchange interaction facilitated by the nitrogen atoms of the organic ligands is only reduced. Our study thus demonstrates that the inserted graphene can serve as an optimal intermediary layer for mediating the magnetic couplings across the molecule-substrate interfaces while allowing effective spin communication between them. These findings provide important insights into relevant experiments and offer a promising strategy to control the magnetic exchange interactions via utilizing graphene at metal-molecule interfaces.