Critical dynamics of an interacting magnetic nanoparticle system
  Effects of dipole-dipole interactions on the magnetic relaxation have been investigated for three Fe-C nanoparticle samples with volume concentrations of 0.06, 5 and 17 vol%. While both the 5 and 17 vol% samples exhibit collective behavior due to dipolar interactions, only the 17 vol% sample displays critical behavior close to its transition temperature. The behaviour of the 5 vol% sample can be attributed to a mixture of collective and single particle dynamics. 
M.F. Hansen, P. J\"onsson, P. Nordblad, and P. Svedlindh