  Doped Mott insulators exhibit some of the most intriguing quantum phases of
matter, including quantum spin-liquids, unconventional superconductors, and
non-Fermi liquid metals. Such phases often arise when itinerant electrons are
close to a Mott insulating state, and thus experience strong spatial
correlations. Proximity between different layers of van der Waals
heterostructures naturally realizes a platform for experimentally studying the
relationship between localized, correlated electrons and itinerant electrons.
Here, we explore this relationship by studying the magnetic landscape of
4Hb-TaS2, which realizes an alternate stack of a candidate spin liquid and a
superconductor. We report on a spontaneous vortex phase whose vortex density
can be trained in the normal state. We show that time reversal symmetry is
broken above Tc, indicating the presence of a magnetic phase independent of the
superconductor. Strikingly, this phase does not generate detectable magnetic
signals. We use scanning superconducting quantum interference device (SQUID)
microscopy to show that it is incompatible with ferromagnetic ordering. The
discovery of this new form of hidden magnetism illustrates how combining
superconductivity with a strongly correlated system can lead to new, unexpected
physics.
