Smallest Bimetallic CoPt3 Superparamagnetic Nanoparticles

Abstract

We present for the first time a nonaqueous sol gel route to produce ultrasmall (<2 nm) magnetic bimetallic CoPt3 nanoparticles (NPs). The one-pot procedure is carried out at low temperature (180 degrees C) using benzyl alcohol, acting as both reducing agent and solvent. The highly monodisperse CoPt3 NPs were investigated with innovative advanced X-ray methods (whole powder pattern modeling), HR-STEM, XPS, and SQUID magnetometry. XPS showed Co was mostly in metallic form, but with a very small amount of CoO on the NP surface. The spherical NPs had an ultrasmall diameter of 1.6 nm and could self-assemble in aligned linear chains, or nanobelts, of single NPs. They are superparamagnetic, with blocking temperature of similar to 20 K and coercivity at 10 K of 27.9 kA m(-1) (similar to 350 Oe). However, there is evidence of a secondmagnetic phase (probably CoO) in the ZFC magnetization curve, which enhances their magnetization values, without significantly affecting their superparamagnetism.