Magnetic viscosity far and close to equilibrium in the superparamagnetic alloy

We report on the simultaneous measurements of the magnetic viscosity and of the ac linear magnetic susceptibility for a sample of Cu₉₀Co₁₀. The magnetic viscosity S=(1/M₀) [dM/d In(t)], with M the sample magnetization and M₀ the magnetization value at the beginning of the logarithmic relaxation, is measured by a standard method that involves the application to the sample of a large field step after saturation. The ac susceptibility is measured both directly at very low (≈microersteds) ac field amplitudes and from the thermal magnetization noise in the sample, the two values always being found in quantitative agreement. The imaginary susceptibility is found to be a frequency independent value χ″₀(T) at low frequencies, a behavior that translates, when expressed in the time domain, to logarithmic relaxation (dM/d In t) = (2/π)χ″₀(T)H₀, of the magnetization after a step excitation H₀. Both S and χ″₀(T) are found to be proportional to the temperature for temperatures ≤4.2 K. In addition, S and χ″₀(T) are found to be of the same order of magnitude. We discuss how these results are in agreement with the idea that in both kinds of experiments one is probing an energy barrier height distribution and that, in addition, the size of the distribution involved in the "critical state," prepared by relaxation experiments, is of the same order of magnitude as that involved in the quasiequilibrium state probed by the ac susceptibility measurements.