Magnetic relaxation in Fe-(SiO2) granular films

Magnetic relaxation of granular Fe-(${\mathrm{SiO}}_2$) solids with low metal-volume fraction has been studied by employing dc SQUID (superconducting quantum interference device) magnetometry and Mössbauer spectroscopy with the characteristic measuring time differing by nine orders of magnitude. The blocking temperatures ($T_B$), extrapolated to zero external field as measured by the two techniques maintain a constant ratio of 0.35 independent of samples. Arrhenius law was found adequate in describing the relaxation process, with a well-defined relaxation-time constant of ${\tau }_0$≃${10}^{\mathrm{-}13}$ sec. A cusp in the susceptibility is observed at $T_B$, below which irreversible difference between the field-cooled and zero-field-cooled magnetizations occurs. Despite this spin-glass-like behavior, granular magnetic systems are found to be fundamentally different from spin glasses.