Spin-glass-like transition in a highly concentrated Fe-C nanoparticle system

A highly concentrated (17 vol%)Fe-C nano particle system, with a narrow siz e distribution d = 5.4 +/- 0.4 nm, has been investigated using magnetic AC susceptibility measurements covering a wide range of frequencies (17-170 Hz ). A dynamic scaling analysis gives evidence for a phase transition to a lo w-temperature spin-glass-like phase. The critical exponents associated with the transition are zv = 10.5 +/- 2 and beta = 1.1 +/- 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 are due to collective behavior. This crite rion can only be fulfilled for a highly concentrated nano particle sample w ith a narrow size distribution. (C) 2001 Elsevier Science B.V. All rights r eserved.