Dimensionality is known to play an important role in many compounds for which
ultrathin layers can behave very differently from the bulk. This is especially true
for the paramagnetic metal LaNiO3, which can become insulating and
magnetic when only a few monolayers thick. We show here that an induced
antiferromagnetic order can be stabilized in the [111] direction by
interfacial coupling to the insulating ferromagnet LaMnO3, and used to
generate interlayer magnetic coupling of a nature that depends on the exact number
of LaNiO3 monolayers. For 7-monolayer-thick
LaNiO3/LaMnO3 superlattices, negative and positive
exchange bias, as well as antiferromagnetic interlayer coupling are observed in
different temperature windows. All three behaviours are explained based on the
emergence of a (¼,¼,¼)-wavevector antiferromagnetic structure
in LaNiO3 and the presence of interface asymmetry with LaMnO3.
This dimensionality-induced magnetic order can be used to tailor a broad range of
magnetic properties in well-designed superlattice-based devices.