  The properties of functional oxide heterostructures are strongly influenced
by the physics governing their interfaces. Modern deposition techniques allow
us to accurately engineer the interface physics through the growth of
atomically precise heterostructures. This enables minute control over the
electronic, magnetic, and structural characteristics. Here, we investigate the
magnetic properties of tailor-made superlattices employing the ferromagnetic
and insulating double perovskites RE$_2$NiMnO$_6$ (RE = La, Nd), featuring
distinct Curie temperatures. Adjusting the superlattice periodicity at the unit
cell level allows us to engineer their magnetic phase diagram. Large
periodicity superlattices conserve the individual para- to ferromagnetic
transitions of the La$_2$NiMnO$_6$ and Nd$_2$NiMnO$_6$ parent compounds. As the
superlattice periodicity is reduced, the Curie temperatures of the superlattice
constituents converge and, finally, collapse into one single transition for the
lowest period samples. This is a consequence of the magnetic order parameter
propagating across the superlattice interfaces, as supported by a minimal
Landau theory model. Further, we find that the Nd-Ni/Mn exchange interaction
can be enhanced by the superlattice interfaces. This leads to a field-induced
reversal of the Nd magnetic moments, as confirmed by synchrotron X-ray magnetic
circular dichroism measurements and supported by first-principles calculations.
Our work demonstrates how superlattice engineering can be employed to fine-tune
the magnetic properties in oxide heterostructures and broadens our
understanding of magnetic interfacial effects.
