  Two-dimensional magnetic materials attract a lot of attention since they
potentially exhibit new magnetic properties due to, e.g., strongly enhanced
spin fluctuations. However, the suppression of the long-range magnetic order in
two dimensions due to long-wavelength spin fluctuations, as suggested by the
Mermin-Wagner theorem, has been questioned for finite-size laboratory samples.
Here we study the magnetic properties of a dimensional crossover in
superlattices composed of the antiferromagnetic LaFeO$_3$ and SrTiO$_3$ that,
thanks to their large lateral size, allowed examination using a sensitive
magnetic probe - muon spin rotation spectroscopy. We show that the iron
electronic moments in superlattices with 3 and 2 monolayers of LaFeO$_3$
exhibit a static antiferromagnetic order. In contrast, in the superlattices
with single LaFeO$_3$ monolayer, the moments do not order and fluctuate to the
lowest measured temperature as expected from the Mermin-Wagner theorem. Our
work shows how dimensionality can be used to tune the magnetic properties of
ultrathin films.
