Synthesis and magnetism of Co_xRh_1-x and Co_xRu_1-x nanoparticles

The simultaneous decomposition of Co(cycloctenyl)(COD) (COD: 1,5-cyclooctadiene) and Rh(Acac)(COD) (Acac: acetylacetonate) or Ru(COD)(COT) (COT: 1,3,5-cyclooctatriene) in mild conditions of temperature and pressure leads, in the presence of a polymer (poly(vinylpyrrolidone)), to spherical nanoparticles (NPs), well dispersed and isolated in the organic matrix. All samples display a mean diameter of about 2 nm and a narrow size distribution. Structural investigations by high-resolution electron microscopy (HRTEM) and wide-angle X-ray scattering (WAXS) as well as magnetization measurements by SQUID magnetometry demonstrate the formation of bimetallic alloyed nanoparticles of defined compositions. Cobalt-rich nanocrystals adopt an unusual non periodic polytetrahedral structure while the 4d metal-rich samples display the main features of the bulk structure. More interestingly, the magnetic data evidence a strong enhancement of the magnetic moment per atom compared to bulk values. The coercive field compared to Co monometallic NPs increases strongly with the 4d metal ratio. The alloying with 4d metals is then presented as a precise tool to tune the magnetic properties of nanomagnets and to get softer or harder magnetic materials with high magnetization. The origin of these unusual properties will be discussed in relation with the synthesis conditions and the structure of the NPs.