  The scaling form of the normalized ZFC and FC susceptibility of
superparamagnets (SPM's) is presented as a function of the normalized
temperature $y$ ($=k_{B}T/K_{u}< V>$), normalized magnetic field $h$
($=H/H_{K}$), and the width $\sigma$ of the log-normal distribution of the
volumes of nanoparticles, based on the superparamagnetic blocking model with no
interaction between the nanoparticles. Here $<V>$ is the average volume,
$K_{u}$ is the anisotropy energy, and $H_{K}$ is the anisotropy field. Main
features of the experimental results reported in many SPM's can be well
explained in terms of the present model. The normalized FC susceptibility
increases monotonically increases as the normalized temperature $y$ decreases.
The normalized ZFC susceptibility exhibits a peak at the normalized blocking
temperature $y_{b}$ ($=k_{B}T_{b}/K_{u}< V>$), forming the $y_{b}$ vs $h$
diagram. For large $\sigma$ ($\sigma >0.4$), $y_{b}$ starts to increase with
increasing $h$, showing a peak at $h=h_{b}$, and decreases with further
increasing $h$. The maximum of $y_{b}$ at $h=h_{b}$ is due to the nonlinearity
of the Langevin function. For small $\sigma$, $y_{b}$ monotonically decreases
with increasing $h$. The derivative of the normalized FC magnetization with
respect to $h$ shows a peak at $h$ = 0 for small $y$. This is closely related
to the pinched form of $M_{FC}$ vs $H$ curve around $H$ = 0 observed in SPM's.
