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.

Controlled crystalline structure and surface stability of cobalt nanocrystals.

The synthesis of monodispersed 10 nm cobalt nanocrystals with controlled crystal morphology and investigation of the surface stability of these nanocrystals are described. Depending on the surfactants used, single crystalline or multiple grain nanocrystals can be reproducibly produced. The relative surface stability of these nanocrystals is analyzed using the temperature dependences of the dc magnetic susceptibility. The novel method, which allows sensitive monitoring of the surface stability, is based on the observation that, with particle oxidation, an anomalous peak appears at 8 K in zero-field-cooled magnetization measurements. It is found that the surfactant protective layer is more important for long-term stability at room temperature, while the high-temperature oxidation rate is controlled by the crystal morphology of the nanoparticles.