A comprehensive review is given on experimental studies of small particles with fivefold symmetry accompanied by an in-depth theoretical description of their characteristics and computer modeling. The cases of uniform and nonuniform deformations (disclination model), stability and relaxation of elastic stresses in pentagonal particles and needle-like crystals, models of their formation are discussed.

The magnetic circular dichroism ͑MCD͒ of core-level absorption ͑x-ray absorption spectroscopy, XAS͒ spectra in the soft x-ray region has been measured for the ferromagnetic Heusler alloy Co 2 Cr 0.6 Fe 0.4 Al at the Co, Fe, and Cr L II,III edges. The comparison of XAS spectra before and after in situ cleaning of polished surfaces revealed a pronounced selective oxidation of Cr in air. For clean surfaces we observed a MCD for all three elements with Fe showing the largest moment per atom. The MCD can be explained by the density of states of the 3d unoccupied states, predicted by linear muffin-tin orbital atomic sphere approximation. For Fe and Cr the orbital angular momentum component of the magnetic moment is considerably larger than the values reported for metallic alloys, whereas for Co a value close to its bulk value is observed. This observation is discussed in comparison with band structure calculations.

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Multiply twinned silver particles of rod-like shape and nanometer dimensions prepared by inert-gas aggregation technique have been studied by high-resolution electron microscopy. These pentagonal nanorods exhibit aspect ratio between 1.8 and 6 with the length of their fivefold axis ranging from 22 to 132 nm. Digital image processing and evaluation was utilised to characterise composition and lattice structure of the nanorods. Measuring the spacings of lattice plane fringes of nanorod subunits in various orientations revealed no deviation from the face centred cubic lattice type. There was also no indication of extended lattice defects found. Instead, a certain extent of non-regular lattice distortions recognised in the surface regions of the nanorods apparently is an effective means to achieve sufficient space filling. r

Heusler compounds are promising candidates for future spintronics device applications. The electronic and magnetic properties of Co 2 Cr 0.6 Fe 0.4 Al, an electron-doped derivative of Co 2 CrAl, are investigated using circularly polarized synchrotron radiation and photoemission electron microscopy (PEEM). Element specific imaging reveals needle shaped Cr rich phases in a homogeneous bulk of the Heusler compound. The ferromagnetic domain structure is investigated on an element-resolved basis using x-ray magnetic circular dichroism (XMCD) contrast in PEEM. The structure is characterized by micrometre-size domains with a superimposed fine ripple structure; the lateral resolution in these images is about 100 nm. The domains look identical for Co and Fe giving evidence of a ferromagnetic coupling of these elements. No ferromagnetic contrast is observed at the Cr line. Magnetic spectroscopy exploiting XMCD reveals that the lack of magnetic moment, detected in a SQUID magnetometer, is mainly due to the moment of the Cr atom.

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For Pd clusters on γ-Al2O3 epitaxially grown on NiAl(110) the lattice constant has been determined as a function of the cluster size by evaluating patterns induced by double diffraction involving the NiAl substrate and the cluster lattice. Similar to results reported previously for platinum and tantalum clusters on the same substrate (Surf. Sci. 1997, 391, 27; Surf. Sci. 1998, 413, 192), we observe a reduction of the distances within the lattice with decreasing cluster size. The highest observed reduction is 5% for clusters with a diameter of about 12 Å. Within the limits of the experimental errors the reduction of the lattice distances is isotropic. Although the samples have been exposed to air prior to investigation, the clusters are only weakly oxidized as concluded from XPS data.