  Structural and magnetic properties of Fe@Pt core-shell nanostructure prepared
by a sequential reduction process are reported. Transmission electron
microscopy (TEM) shows nearly spherical particles fitting a lognormal size
distribution with $D$$_o$= 3.0 nm and distribution width $\lambda$$_D$= 0.31.
In x-ray diffraction, Bragg lines due to Pt shell only are clearly identified
with line-widths yielding crystallite size =3.1 nm. Measurements of
magnetization $M$ vs. $T$ (2 K - 350 K) in magnetic fields up to 90 kOe show a
blocking temperature $T$$_B$ = 13 K below which hysteresis loops are observed
with coercivity $H$$_C$ increasing with decreasing $T$ reaching $H$$_C$ = 750
Oe at 2 K. Temperature dependence of the ac susceptibilities at frequencies
$f$$_m$ = 10 Hz to 5 kHz is measured to determine the change in $T$$_B$ with
$f$$_m$ using Vogel-Fulcher law. This analysis shows the presence of
significant interparticle interaction, the N\'{e}el-Brown relaxation frequency
$f$$_o$ = 5.3 x 10$^{10}$ Hz and anisotropy constant $K$$_a$ =3.6 x10$^6$
ergs/cm$^3$. A fit of the $M$ vs. $H$ data up to $H$ = 90 kOe for $T$ $>$
$T$$_B$ to the modified Langevin function taking particle size distribution
into account yields magnetic moment per particle consistent with the proposed
core-shell structure; Fe core of 2.2 nm diameter and Pt shell of 0.4 nm
thickness.
