Enhanced phase stabilization of CoPt in the presence of Ag

We report the synthesis of CoPt and CoPt-Ag nanoparticles by the simultaneous reduction of platinum(II) acetylacetonate (Pt(acac)(2)), cobalt(II) acetylacetonate (Co(acac)(2)), and silver nitrate (AgNO(3)) (for CoPt-Ag) in the presence of poly(N-vinyl-2-pyrrolidone) (PVP). Here the changes in the structural and magnetic properties of these nanoparticles subjected to heat treatment and the effect of adding silver to CoPt in comparison to pure CoPt have been investigated. TEM observations reveal a narrow size distribution with particle diameter of about 2-3 nm. The XRD studies reveal a disordered fcc phase for the as synthesized nanoparticles which after annealing at higher temperatures show peaks corresponding to an ordered L1(0) phase. The structural phase transition is also followed by enhanced magnetic properties. The magnetic measurements show the transition of a magnetically soft alloy to a magnetically hard material with a maximum coercivity of similar to 1 T after annealing at 700 degrees C. The addition of silver (Ag) to CoPt alloy induces a reduction in the ordering temperature by 100 degrees C compared to pure CoPt, with an increased coercivity of 1.2 T. Such thermally stable and structurally ordered hard magnetic nanoparticles are promising candidates for magnetic storage applications. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3615939]