Spin-electronic devices are poised to become part of mainstream microelectronic technology .Downsizing them, however, faces the intrinsic difficulty that as ferromagnets become smaller, it becomes more difficult to stabilize their magnetic moment. Antiferromagnets are much more stable, and thus research on antiferromagnetic spintronics has developed into a fast-growing field. Here, we provide proof of concept data that allows us to expand the area of antiferromagnetic spintronics to the hitherto elusive level of individual molecules. In contrast to all previous work on molecular spintronics, our detection scheme of the molecule's spin state does not rely on a magnetic moment. Instead, we use field-effect transistor devices constituting of an isolated, contacted single-wall carbon nanotube covalently bound to a limited number of molecular antiferromagnets incorporating four Mn(II) or Co(II) ions. Time-dependent quantum transport measurement along the functionalized nanotube show step-like transitions between several distinct current levels, which we attribute to transitions between different antiferromagnetic states of individual molecular complexes grafted on the nanotube. A statistical analysis of the switching events using factorial cumulants indicates that the cobalt complexes switch independently from each other, while a coherent superposition of the antiferromagnetic spin states of the molecules along the nanotube is observed for the manganese complexes. The long coherence time (several seconds at 100 mK) is made possible by the absence of spin and orbital momentum in the relevant states of the manganese complex, while the cobalt complex includes a significant orbital momentum contribution due to the pseudo-octahedral d$^7$ metal centers.

Spin-electronic devices are poised to become part of mainstream microelectronic technology. Downsizing them, however, faces the intrinsic difficulty that as ferromagnets become smaller, it becomes more difficult to stabilize their magnetic moment. Antiferromagnets are much more stable, and thus research on antiferromagnetic spintronics has developed into a fast-growing field. Here, we provide proof of concept data that allows us to expand the area of antiferromagnetic spintronics to the hitherto elusive level of individual molecules. In contrast to all previous work on molecular spintronics, our detection scheme of the molecule's spin state does not rely on a magnetic moment. Instead, we use the step-like transitions between several distinct current levels caused by transitions between different antiferromagnetic states of an individual molecule grafted onto a carbon nanotube. We find that in the absence of an orbital momentum the antiferromagnetic spin states of the molecules show coherent superposition.

We report on quantum transport measurements of a carbon nanotube (CNT) quantum dot that is functionalized with magnetic {Mn4} complexes. The coupling between the spin‐5/2 MnII ions within each {Mn4} complex is predominantly antiferromagnetic. Coulomb diamond measurements at T = 4 K reveal that the covalent attachment of the complexes to the CNT framework has only little influence on the carbon nanotube quantum dot. Surprisingly, a strong increase of noise is observed upon cooling the sample to T = 30 mK. Time traces of the current taken at a diamond edge reveal a random telegraph signal. We attribute this to fluctuations of molecular spin states of the attached {Mn4} complexes.