Dominant factor of zero-field-cooled magnetization in discontinuous Fe films

The zero-field-cooled (ZFC) magnetization under the various field and temperature conditions has been investigated using the discontinuous Fe films. Among the various factors influencing the ZFC magnetization, it is found that either the thermal relaxation or the Langevin behavior dominates the ZFC magnetization, depending on the energy barrier distribution which is altered by the growth temperature. The peak temperature of ZFC magnetization follows the Neel-Brown model for the narrower energy barrier distribution. With broadening the energy barrier distribution for the higher growth temperature, the Langevin behavior of thermally fluctuated particles becomes dominant. The change of energy barrier distribution, namely, the dominant factor of ZFC magnetization, is explained by the broadening of size distribution and the degradation of crystallinity with increasing growth temperature. For both cases, we estimate the superparamagnetic blocking temperature T(B) and obtain the effective magnetic anisotropy and the effective volume from the field dependence of T(B). From the obtained values, we show the presence of interparticle interaction for the Fe grown at 323 K, and discuss the effective magnetic anisotropy of randomly oriented particles grown above 573 K.