  A highly concentrated (17 vol.%) Fe-C nano-particle system, with a narrow
size distribution $d = 5.4\pm 0.4$ nm, has been investigated using magnetic ac
susceptibility measurements covering a wide range of frequencies (17 mHz - 170
Hz). A dynamic scaling analysis gives evidence for a phase transition to a low
temperature spin-glass-like phase. The critical exponents associated with the
transition are $z\nu = 10.5 \pm 2$ and $\beta = 1.1 \pm 0.2$. The reason why
the scaling analysis works for this sample, while it may not work for other
samples exhibiting collective behavior as evidenced by aging phenomena, is that
the single particle contribution to $\chi''$ is vanishingly small for $T>T_g$
and hence all slow dynamics is due to collective behavior. This criterion can
only be fulfilled for a highly concentrated nano-particle sample with a narrow
size distribution.
