MAGNETIC VISCOSITY FAR AND CLOSE TO EQUILIBRIUM IN THE SUPERPARAMAGNETIC ALLOY

We report on the simultaneous measurements of the magnetic viscosity a nd of the ac linear magnetic suceptibility for a sample of Cu90Co10. T he magnetic viscosity S=(1/M(0))[dM/dln(t)], with M the sample magneti zation and M(0) the magnetization value at the beginning of the logari thmic relaxation, is measured by a standard method that involves the a pplication to the sample of a large field step after saturation. The a c susceptibility is measured both directly at very low (approximate to 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 in dependent value chi(0)(n)(T) at low frequencies, a behavior that trans lates, when expressed in the time domain, to logarithmic relaxation (d M/d lnt) = (2/pi)(chi 0)(n)(T)H-0, of the magnetization after a step e xcitation H-0. Both S and (n)(chi 0)(T) are found to be of the same or der of magnitude. We discuss how these results are in agreement with t he idea that in both kinds of experiments one is probing an energy bar rier height distribution and that, in addition, the size of the distri bution involved in the ''critical state'', prepared by relaxation expe riments, is of the same order of magnitude as that involved in the qua siequilibrium state probed by the ac susceptibility measurements. (C) American Instituted of Physics.