Nanoparticles of bulk antiferromagnets often exhibit ferromagnetic behavior due to uncompensated surface spins. The peak temperature in the zero-field-cooled magnetization of $\mathrm{MnO}$—in contrast to other antiferromagnetic nanoparticles—has anomalous behavior, shifting to higher temperatures with decreasing nanoparticle size. We attribute this behavior to surface anisotropy enhanced by the specific occupancy of $3d$ levels in Mn, which produces a high-spin–low-spin transition not present in $\mathrm{NiO}$ nanoparticles.

• Received 5 February 2007

DOI:https://doi.org/10.1103/PhysRevB.75.134423