Optimized synthesis and magnetic properties of intermetallic Au ₃Fe_1-x, Au₃Co_1-x, and Au ₃Ni_1-x nanoparticles

Au and the 3d transition metals are immiscible under equilibrium conditions, but nonequilibrium alloys and intermetallic compounds of these elements are of interest for their potential multifunctional optical, catalytic, and magnetic properties. Here we report an optimized synthesis of intermetallic compounds with nominal compositions of Au₃Fe_1-x, Au ₃Co_1-x, and Au₃Ni_1-x as nanoparticles. Identification and optimization of the key synthetic variables (solvent, order of reagent addition, stabilizer, heating rate) led to the generation of nanoparticles with high phase purity and sample sizes of > 30 mg, which is an order of magnitude larger than what was previously achievable. These intermetallic nanoparticles, which have diffraction patterns consistent with the L1₂ structure type, were characterized by powder XRD, TEM, EDS, electron diffraction, UV-visible spectroscopy, and SQUID magnetometry. Aliquot studies showed that Au₃Fe_1-x formed through the initial nucleation of Au nanoparticles, followed by subsequent incorporation of Fe. Magnetic studies of powdered samples identified Au₃Fe _1-x and Au₃Co_1-x as superparamagnetic with T_B=7.9 and 2.4 K, respectively. Au₃Ni_1-x is paramagnetic down to 1.8 K.