We have investigated the details of zero-field-cooled magnetization of the superparamagnetic ultrathin Fe films from the temperature dependence of reversible and irreversible magnetization. We provide the interpretation of the magnetic field dependence of peak temperature. At the low growth temperature around 323 K, the behavior of peak temperature against the field is expressed by the Néel model. On the other hand, the peak temperature increases with increasing field for the films grown at high temperature around 773 K. The increase of peak temperature is explained by considering the reversible magnetization as a dominant mechanism.
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