The origin of the antiferromagnetic behaviour of the charge-transfer compound (HMTTF)[Ni(mnt)2]

The charge-transfer (HMTTF)[Ni(mnt)2] material (1) was the first mixed-stack compound reported to present ferromagnetic (FM) interactions between the donor (D) and acceptor (A) units within each mixed-stack. Despite the presence of a dominant FM interaction, its magnetic susceptibility curve, χ(T), corresponded to that of an antiferromagnetic (AFM) compound at low temperatures, a fact that was tentatively explained in terms of a FM-to-AFM magnetic transition. In this work, the First-Principles Bottom-Up procedure has been applied to rationalize the magnetic properties of 1. The results presented herein indicate the presence of up to five sizeable JAB magnetic exchange interactions (26.0, -1.7, 1.9, 0.1, and -99.3 cm(-1)) that create a complex 2D magnetic network. This magnetic topology not only produces a calculated χ(T) curve that is in excellent agreement with the experimental measurements, but also allows us to understand the magnetic behaviour of 1, in the full range of temperatures, solely in terms of the different Boltzmann population of diamagnetic and paramagnetic spin states as the temperature changes. Our data thus provide a plausible explanation for the AFM behaviour of 1 at low temperatures that does not involve a phase transition.