Intrinsic magnetic properties of bimetallic nanoparticles elaborated by cluster beam deposition.

In this paper, we present some specific chemical and magnetic order obtained very recently on characteristic bimetallic nanoalloys prepared by mass-selected Low Energy Cluster Beam Deposition (LECBD). We study how the competition between d-atom hybridization, complex structure, morphology and chemical affinity affects their intrinsic magnetic properties at the nanoscale. The structural and magnetic properties of these nanoalloys were investigated using various experimental techniques that include High Resolution Transmission Electron Microscopy (HRTEM), Superconducting Quantum Interference Device (SQUID) magnetometry, as well as synchrotron techniques such as Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Magnetic Circular Dichroism (XMCD). Depending on the chemical nature of the nanoalloys we observe different magnetic responses compared to their bulk counterparts. In particular, we show how specific relaxation in nanoalloys impacts their magnetic anisotropy; and how finite size effects (size reduction) inversely enhance their magnetic moment.

Self-organisation of size-selected Co(x)Pt(1-x) clusters on graphite.

Sub-monolayer thin film morphologies obtained by deposition of size-selected CoxPt1-x clusters on graphite have been analyzed for different values of x. In all cases, the preformed clusters can easily diffuse on the surface and gather to form islands of clusters. By changing the cluster stoichiometry, very different morphologies can be obtained, going from large ramified islands to "bunches" of non-contacting incident clusters. We put into evidence that the introduction of platinum atoms in the incident particles drastically changes the interaction between clusters and offers the opportunity to control the coalescence process between them. In this way, by modifying the cluster reactivity, a local self-organization of size-selected magnetic nanoparticles can be achieved.

Low temperature ferromagnetism in chemically ordered FeRh nanocrystals.

In sharp contrast to previous studies on FeRh bulk, thin films, and nanoparticles, we report the persistence of ferromagnetic order down to 3 K for size-selected 3.3 nm diameter nanocrystals embedded into an amorphous carbon matrix. The annealed nanoparticles have a B2 structure with alternating atomic Fe and Rh layers. X-ray magnetic dichroism and superconducting quantum interference device measurements demonstrate ferromagnetic alignment of the Fe and Rh magnetic moments of 3 and 1µ(B), respectively. The ferromagnetic order is ascribed to the finite-size induced structural relaxation observed in extended x-ray absorption spectroscopy.

Multi-L1(0) domain CoPt and FePt nanoparticles revealed by electron microscopy.

The atomic structure of CoPt and FePt nanoparticles (with a diameter between 2 and 5 nm) has been studied by transmission electron microscopy. The particles have been produced by a laser vaporization cluster source and annealed under vacuum in order to promote chemical ordering. For both alloys, we observe a coexistence of crystalline and multiply twinned particles with decahedral or icosahedral shapes. In addition to particles corresponding to a single L1(0) ordered domain, we put into evidence that even small particles can display several L1(0) domains. In particular, the chemical order can be preserved across twin boundaries which can give rise to spectacular chemically ordered decahedral particles made of five L1(0) domains. The stability of such structures, which had been recently predicted from theoretical simulations, is thus unambiguously experimentally confirmed.

Playing with carbon and silicon at the nanoscale.

Because of its superior properties silicon carbide is one of the most promising materials for power electronics, hard- and biomaterials. In the solid phase, the electronic and optical properties are controlled by the stacking of double layers of Si and C atoms. In thin films, a change in the stacking order often requires stress, which can be achieved naturally in systems with nanometre length scale. For this reason, nanotubes, nanowires and clusters can be used as building blocks for the synthesis of novel materials. Furthermore, playing at the nanometre length scale enables the nature of the SiC bonding to be modified, which is of prime importance for atomic engineering of nanostructures. In this review, emphasis is placed on the theoretical principles associated with SiC cage-like clusters and experimental work resulting from them.