Synthesis, structure and magnetic properties of Fe@Pt core-shell nanoparticles

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.