  The universal theory of critical phase transitions describes the critical
behavior at second-order phase transitions in infinitely large systems. With
the increased contemporary interest in nanoscale materials, we investigated CoO
nanoparticles by means of neutron scattering and found how the theory of
critical phenomena breaks down in the nanoscale regime. Using CoO as a model
system, we have identified a size-dependent nanocritical temperature region
close to the antiferromagnetic phase transition where the magnetic correlation
length of the nanoparticles converges to a constant value, which is
significantly smaller than that of the saturated state found at low
temperatures. This is in clear contrast to the divergence around $T_{\rm N}$
observed for bulk systems. Our findings of nanocriticality in the magnetic
phase transition is of great importance for the understanding of phase
transitions at the nanoscale.
