Time decay of the thermoremanent magnetization in spin-glasses as a function of the time spent in the field-cooled state

We have measured the time decay of the thermoremanent magnetization (TRM) in Ag: Mn as a function of the wait time (tw) that the sample spent in the field-cooled state before the field was removed. When the results are analyzed in terms of a stretched exponential, TRM(t)=0exp[-C(t)1-n(1-n)], we find that the effect of tw can be empirically characterized as an exponential decrease of the relaxation frequency with increasing wait time: =0exp(-twt0), where 0=3.3×10-4 sec-1 and t0=650 sec. This fact has two implications, First, even though the field-cooled magnetization is constant in time, the field-cooled state is not in equilibrium for finite tw. And second, if tw then the field-cooled spin-glass will have a permanent magnetic moment.