Ac-susceptibility study in rare earth substituted magnetite ferrofluids

It is shown that variation of the third order ac-susceptibility as a function of measuring field and frequency lead to distinguish between superparamagnetic and spin glass like ordering in the rare earth substituted magnetic ferrofluids. We observe the divergent behavior of the peak values of the third order susceptibility, as a function of measuring field and frequency, tends to zero is consistent with theoretical prediction for the ground state for a spin glass like system. This behavior is further substantiated by a linear dependence of log-log plots of peak of the third order susceptibility as a function of the measuring field and frequency.