Unusual magnetic behavior in ferrite hollow nanospheres
  We report unusual magnetic behavior in iron oxide hollow nanospheres of 9.3 $nm$ in diameter. The large fraction of atoms existing at the inner and outer surfaces gives rise to a high magnetic disorder. The overall magnetic behavior can be explained considering the coexistence of a soft superparamagnetic phase and a hard phase corresponding to the highly frustrated cluster-glass like phase at the surface regions. 
E. Lima Jr, J. M. Vargas, R. D. Zysler, H. R. Rechenberg, J. Arbiol,   G. F. Goya, A. Ibarra, M. R. Ibarra