The field dependence of the ac susceptibility of a concentrated frozen ferrofluid containing ultrafine Fe-C particles of monodisperse nature has been analyzed using static scaling. For the first time, a divergent behavior of the nonlinear susceptibility of a dipole-dipole interacting system is demonstrated. From the analysis, the critical exponents gamma = 4.0 +/- 0.2 and beta = 1.2 +/- 0.1 were extracted. The results support the existence of a low temperature spin-glass-like phase in interacting magnetic nanoparticle systems. [S0031-9007(98)07527-9].
Physical Review Letters, 1998, Vol 81, Issue 18, p. 3976-3979