Coupling between magnetic and optical properties of stable Au–Fe solid solution nanoparticles

Abstract

Au-Fe nanoparticles constitute one of the simplest prototypes of a multifunctional nanomaterial that can exhibit both magnetic and optical (plasmonic) properties. This solid solution, not feasible in the bulk phase diagram in thermal equilibrium, can be formed as a nanostructure by out-of-equilibrium processes. Here, the novel magnetic, optical and magneto-optical properties of ion-implanted Au-Fe solid solution nanoparticles dispersed in a SiO(2) matrix are investigated and correlated. The surface plasmon resonance of the Au-Fe nanoparticles with almost equicomposition is strongly damped when compared to pure Au and to Au-rich Au-Fe nanoparticles. In all cases, the Au atoms are magnetically polarized, as measured by x-ray magnetic circular dichroism, and ferromagnetically coupled with Fe atoms. Although the chemical stability of Au-Fe nanoparticles is larger than that of Fe nanoparticles, both the magnetic moment per Fe atom and the order temperature are smaller. These results suggest that electronic and magnetic properties are more influenced by the hybridization of the electronic bands in the Au-Fe solid solution than by size effects. On the other hand, the magneto-optical transitions allowed in the vis-nIR spectral regions are very similar. In addition, we also observe, after studying the properties of thermally treated samples, that the Au-Fe alloy is stabilized, not by surface effects, but by the combination of the out-of-equilibrium nature of the ion implantation technique and by changes in the properties due to size effects.

Superparamagnetic Nanoparticles as High Efficiency Magnetic Resonance Imaging T2 Contrast Agent

“…A number of different methods have been previously reported for the preparation of bimetallic Au/Fe alloy nanoparticles, 13−15,20−26 core−shell or dumbbell nanoparticles, 27,28 with protocols including thermal decomposition, 15,23 pulsed laser deposition, 29,30 microemulsion techniques, 16,31 thermal vaporization, 32,33 laser-assisted synthesis in solution, 13,20 and aqueous reduction by borohydride derivates. 14 Chemical reduction of metal ions by sodium borohydrides has previously been used to prepare nanocrystalline magnetic materials, nanoalloys, and amorphous metals.…”

X-ray absorption spectroscopy for metal-implanted silica

“…The possibility of synthesizing binary alloy metal cluster is interesting, since the cluster composition is one additional parameter that can be used to tailor the composite optical or magnetic response (1,20). Ion implantation of metals in silica is an effective route to obtain binary metal nanoclusters, either in the form of alloy or core-shell nanostructures (14,20).…”

“…Ion implantation of metals in silica is an effective route to obtain binary metal nanoclusters, either in the form of alloy or core-shell nanostructures (14,20). In this respect, it is worth noting that the nanometric size of the aggregates coupled with the non-equilibrium character of the ion implantation process can allow the formation of binary structures not stable in the bulk phase (20). Moreover, proper annealing in selected atmosphere and at defined temperature, as well as ion irradiation, allow to further modify the cluster composition.…”

“…XAS also allowed a detailed structural analysis on the alloy formation in sequentially implanted silica and on the structural modifications induced by annealing in selected atmospheres (9,(16)(17)(18)(19): in this particular case the alloy cluster formation was not only detected by measuring the interatomic distance, but was also detected by the presence of both atomic species in the first shell surrounding each dopant species. More recently, the use of X-ray magnetic circular dichroism on metastable Au−Fe nanoalloys produced by Au + Fe ion implantation in silica showed how X-ray spectroscopy can complement standard magnetic characterizations (20).…”

Gold-based nucleation in implanted silica studied by x-ray absorption spectroscopy

“…The coupling of ion implantation with proper annealing at defined temperature and atmosphere and/or irradiation by laser or ion beams widens the palette of possible morphologies and compositions. Moreover, alloy nanoclusters whose corresponding bulk phase is unstable or metastable at room temperature can be easily obtained [7,8].…”