  Extending the coherence lifetime of a qubit is central to the implementation
and deployment of quantum technologies, particularly in the solid-state where
various noise sources intrinsic to the material host play a limiting role.
Here, we theoretically investigate the coherent spin dynamics of a hetero-spin
system formed by a spin S=1 featuring a non-zero crystal field and in proximity
to a paramagnetic center S'=1/2. We capitalize on the singular energy level
structure of the dyad to identify pairs of levels associated to
magnetic-field-insensitive transition frequencies, and theoretically show that
the zero-quantum coherences we create between them can be remarkably
long-lived. Further, we find these coherences are selectively sensitive to
'local' - as opposed to 'global' - field fluctuations, suggesting these spin
dyads could be exploited as nanoscale gradiometers for precision magnetometry
or as probes for magnetic-noise-free electrometry and thermal sensing.
