We present in-situ x-ray scattering measurements performed during the growth of two rare-earth metals, gadolinium and samarium, onto molybdenum (110) single crystals. The results have been interpreted using a diffusive growth model to determine the degree of interlayer mass transport in the initial stages of deposition. Both elements are shown to grow generally in a layerwise manner but with significant roughness after the initial layer is complete. A raised substrate temperature modifies the growth; the best Gd single layer is produced at a temperature of 140°C when deposited at a rate of 0.067 monolayers/min while for Sm the growth becomes increasingly islanded at higher temperatures. The presence of oxygen at the surface encourages layer-by-layer growth for both Gd and Sm, although a significant proportion of the atoms are in upper layers before the lower ones are complete. The mechanism for improved layerwise growth is oxide formation at the interface, producing a large amount of small islands that encourage interlayer mass transport. The growth of Sm on Mo (110) is generally more rough than Gd on Mo(110) due to the dynamic size change associated with the coordination induced valence transition for the Sm atoms.
We have used soft x-ray photoemission electron microscopy (XPEEM) combined with x-ray
magnetic circular dichroism (XMCD) and DC SQUID (superconducting quantum
interference device) magnetometry to probe the magnetic ground state in Fe thin films
produced by depositing size-selected gas-phase Fe nanoparticles with a diameter of 1.7 nm
(∼200 atoms) onto Si substrates. The depositions were carried out in ultrahigh vacuum conditions and
thicknesses of the deposited film in the range 5–50 nm were studied. The magnetometry data are
consistent with the film forming a correlated super-spin glass with a magnetic correlation length
∼5 nm. The XPEEM magnetic maps from the cluster-assembled films were compared to those
for a conventional thin Fe film with a thickness of 20 nm produced by a molecular beam
epitaxy (MBE) source. Whereas a normal magnetic domain structure is observed
in the conventional MBE thin film, no domain structure could be observed in
any of the nanoparticle films down to the resolution limit of the XMCD based
XPEEM (100 nm) confirming the ground state indicated by the magnetometry
measurements. This observation is consistent with the theoretical prediction that an
arbitrarily weak random anisotropy field will destroy long-range magnetic order.
Previous aging and cueing studies suggest that automatic orienting driven by peripheral cues is preserved with aging; however, inconsistencies can be found. One issue might be the use of response times (RT) to assess cueing effects (invalid RT--valid RT), which, in many cases, may not have clear quantitative predictions. We propose an ideal observer (IO) analysis of accuracy estimating participants' internal value of cue validity, or weight, which should equal the actual cue validity. The weight measures the use of information provided by the cue and is insensitive to variations in set size and difficulty, thus potentially providing advantages to RT. Older (n = 54) and younger (n = 58) participants performed a yes/no detection task of a two-dimensional (2-D) Gaussian (60 ms). Square peripheral precues (150 ms) indicated likely target locations (70% valid) across two or six locations (set sizes). For cueing effects, (valid--invalid hit rates), younger participants had set-size effects (larger cueing effects for set size 6), while older participants did not. The opposite pattern was found for weights (younger: no set-size effects, older: set-size effects) due to the IO predicting larger cueing effects for larger set sizes. Comparisons to the ideal weight (cue validity) suggested that older participants used the cue information effectively with set size 2 (as or more so than younger participants), but not with set size 6. These results suggest that attentional deficits from aging in peripheral cueing tasks may only arise as difficulty increases, such as larger set sizes.
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