Relaxation in spin glasses at weak magnetic fields

The low-field time-dependent magnetic susceptibility of the amorphous metallic spin glass (${\mathrm{Fe}}_{0.15}$ ${\mathrm{Ni}}_{0.85}$) ${}_{75}{}^{}{}_{}{}^{}\mathrm{P}_{16}^{}{}_{}{}^{}$ ${\mathrm{B}}_6$ ${\mathrm{A}1\mathrm{}}_3$ has been studied in a superconducting-quantum-interference-device magnetometer. Both ac susceptibility and zero-field-cooled susceptibility measurements have been utilized to cover more than ten decades in time, 3×${10}^{\mathrm{-}6}$–${10}^5$ sec. At short observation times the equilibrium relaxation is observed, while at long observation times the influence of the aging process dominates the relaxation. The aging behavior is shown to persist through the spin-glass freezing temperature and it remains to have a vital influence on the relaxation until the spin system has been allowed to age for times comparable to the longest relaxation time of the system. The functional form of the time-dependent susceptibility is also illuminated.