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, only the 17 vol% sample displays critical behavior close to its spin-glass transition temperature. The values estimated for the critical exponents are $z\nu = 10.5 \pm 2$ and $\beta = 1.1 \pm 0.2$. For the 5 vol% sample deviations from critical behavior are observed near the assumed transition temperature, which can be explained by the distribution of single-particle relaxation times, blocking of large particles, or particle agglomeration in this sample.