Synthesis, structural characterization and magnetic properties of Fe/Pt coreshell nanoparticles.

Structural and magnetic properties of Fe/Pt core-shell nanostructure prepared by a sequential reduction process are reported. Transmission electron microscopy shows nearly spherical particles fitting a lognormal size distribution with D_o=3.0 nm and distribution width λ_D=0.31. In x-ray diffraction, Bragg lines only from the Pt shell are clearly identified with line-widths yielding crystallite size=3.1 nm. Measurements of magnetization M vs. T (2K-350K) in magnetic fields up to 90 kOe show a blocking temperature T_B=13K below which hysteresis loops are observed with coercivity H_C increasing with decreasing T reaching H_C=750Oe at 2 K. Temperature dependence of the ac susceptibilities at frequencies f_m=10 Hz-5 kHz is measured to determine the change in T_B with f_m using the Vogel-Fulcher law. This analysis shows the presence of significant interparticle interaction, the Neel-Brown relaxation frequency f_o=5.3×10¹⁰Hz and anisotropy constant K_a=3.6×10⁶ ergs/ cm³. 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. [ABSTRACT FROM AUTHOR]