  The combination of charge and spin degrees of freedom with electronic
correlations in condensed matter systems leads to a rich array of phenomena,
such as magnetism, superconductivity, and novel conduction mechanisms. While
such phenomena are observed in bulk materials, a richer array of behaviors
becomes possible when these degrees of freedom are controlled in atomically
layered heterostructures, where one can constrain dimensionality and impose
interfacial boundary conditions. Here, we unlock a host of unique, hidden
electronic and magnetic phase transitions in NdNiO$_3$ while approaching the
two-dimensional (2D) limit, resulting from the differing influences of
dimensional confinement and interfacial coupling. Most notably, we discover a
new phase in fully 2D, single layer NdNiO$_3$, in which all signatures of the
bulk magnetic and charge ordering are found to vanish. In addition, for quasi
two-dimensional layers down to a thickness of two unit cells, bulk-type
ordering persists but separates from the onset of insulating behavior in a
manner distinct from that found in the bulk or thin film nickelates. Using
resonant x-ray spectroscopies, first-principles theory, and model calculations,
we propose that the single layer phase suppression results from a new mechanism
of interfacial electronic reconstruction based on ionicity differences across
the interface, while the phase separation in multi-layer NdNiO$_3$ emerges due
to enhanced 2D fluctuations. These findings provide insights into the
intertwined mechanisms of charge and spin ordering in strongly correlated
systems in reduced dimensions and illustrate the ability to use atomic layering
to access hidden phases.
