Size-driven magnetic transitions in monodisperse MnO nanocrystals.

We report the observation of weak ferromagnetism up to T(C) approximately 250 K and a spin-glass-like behavior at temperatures below T(SG) approximately 30 K in nanoscale MnO particles. T(SG) is considerably lower and T(C) is much higher than the Néel temperature (T(N)=122 K) of bulk MnO. While the dominant low temperature behavior (below 30 K) may be attributed to the effects studied in this system before, such as uncompensated surface spins in antiferromagnetic particles, no manganese oxides have been observed with the Curie temperature as high as 250 K. We explain the magnetic ordering below T(C) as due to indirect exchange of the type observed in dilute magnetic oxides, which can be mediated by crystal defects andor surfaces.