  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.
