Disappearance of the magnetic relaxation in spin glasses

Abstract

We have studied the relaxation of the magnetization in the spin-glass phase of Fe10Ni70P20 and CuMn0.243%. Following zero field cooling the magnetization M(H,T,t) of spin-glasses increases quasilogarithmically with time t after applying a field H at t=0: M(H,T,t)=M(H,T,t=0)+S(H,T) ln t. The quantity S(H,T) has been studied for 30 s≤t≤1200 s below Tg, temperature of the ac susceptibility peak. For both systems S(H) increases almost linearly with H for a given temperature, reaches a maximum value Sm for Hm, then decreases and tends to zero for a field HM at least four times larger than Hm. For a given field, S(T) exhibits a maximum near Tg in low field. This maximum becomes higher, sharper and is displaced to lower temperatures for increasing field values. A line Hm∝(Tg-T)γ with γ=1.5 and 1.8 for CuMn and Fe70Ni10P20, respectively, is obtained. This line has a form similar to the de Almeida-Thouless line Hc=(Tg-T)1.5. However, Hm or HM are smaller by a factor 16 or 4 than the corresponding Hc value. The data for each sample, when scaled according to S/M(t=30 s), collapse onto similar curves when plotted as a function of H/Hm. This suggests some universal behavior of the magnetic relaxation in quite different spin-glasses.