  Perovskite oxide heterostructures offer an important path forward for
stabilizing and controlling low-dimensional magnetism. One of the guiding
design principles for these materials systems is octahedral connectivity. In
superlattices composed of perovskites with different crystal symmetries,
variation of the relative ratio of the constituent layers as well as the
individual layer thicknesses gives rise to non-equilibrium crystal symmetries
that, in turn, lead to unprecedented control of interfacial ferromagnetism. We
have found that in superlattices of CaMnO$_3$ (CMO) and LaNiO$_3$ (LNO),
interfacial ferromagnetism can be modulated by a factor of three depending on
LNO and CMO layer thicknesses as well as their relative ratio. Such an effect
is only possible due to the non-equilibrium crystal symmetries at the
interfaces and can be understood in terms of the anisotropy of the exchange
interactions and modifications in the interfacial Ni-O-Mn and Mn-O-Mn bond
angles and lengths with increasing LNO layer thickness. These results
demonstrate the potential of engineering non-equilibrium crystal symmetries in
designing ferromagnetism.
