Chien-Cheng Kuo

Patterning Co nanoclusters on thin-film Al2O3/NiAl(100)

Self-organized patterning of supported nanoclusters by virtue of low cost and readiness for mass production is considered as one of the most promising methods; however, this approach is challenging, since the capability of controlling the patterns relies on a suitable combination of clusters and templates. In this paper we demonstrate that Co nanoclusters grown from vapour deposition over Al2O3 thin films on NiAl(100) substrate make a perfect combination for self-organized patterning. Uniform and sizeable Co nanoclusters are formed only on crystalline Al2O3 films and they are highly aligned by protrusion structures of the crystalline Al2O3. Through simple thermal treatments we can pattern the crystalline Al2O3 films and consequently the grown Co nanoclusters. The patterns are robust as they are sustained even when the Co nanoclusters are flashed to 750 K, exposed to atmosphere or the coverage is increased to coalescence. Moreover, the patterns can be further refined by using STM tips. The results imply potential applications in both fundamental and applied researches for electronic and magnetic nanodevices as well as catalysis.

The nontrivial electronic structure of Bi/Sb honeycombs on SiC(0001)

We discuss two-dimensional (2D) topological insulators (TIs) based on planar Bi/Sb honeycombs on a SiC(0001) substrate using first-principles computations. The Bi/Sb planar honeycombs on SiC(0001) are shown to support a nontrivial band gap as large as 0.56 eV, which harbors a Dirac cone lying within the band gap. Effects of hydrogen atoms placed on either just one side or on both sides of the planar honeycombs are examined. The hydrogenated honeycombs are found to exhibit topologically protected edge states for zigzag as well as armchair edges, with a wide band gap of 1.03 and 0.41 eV in bismuth and antimony films, respectively. Our findings pave the way for using planar bismuth and antimony honeycombs as potential new 2D-TI platforms for room-temperature applications.

Self-aligned Co nanoparticle chains supported by single-crystalline Al2O3∕NiAl(100) template

We present Co nanoparticle chains grown by vapor deposition over a single-crystalline Al2O3 layers on NiAl(100) with such features as self-limiting size distribution with the average size of ∼2.7nm, well-ordered alignment, and high thermal stability. We attribute these features to peculiar one-dimensional long stripes with ∼4nm interdistance on the surface of the ultrathin Al2O3 template. This nanostructure may open the door to numerous applications, such as catalysis and nanostorage, where large area well-ordered nanodots are desired.

Growth of Co clusters on thin films Al2O3∕NiAl(100)

We present a scanning tunnel microscopy study of Co clusters grown through vapor deposition on Al(2)O(3) thin films over NiAl(100) at different coverages and temperatures. Formation of Co clusters was observed at 90, 300, 450, and 570 K. At the three lower temperatures, we find narrow cluster size distributions and the mean sizes (with a diameter of 2.6 nm and a height of 0.7 nm) do not change significantly with the coverage and temperature, until the clusters start to coalesce. Even on 3-4-nm-wide crystalline Al(2)O(3) strips where the deposited Co atoms are confined, the same features sustain. Only at 570 K the normal growth mode where the cluster size increases with the deposition coverage is observed, although the data are less conclusive. A simple modeling of kinetic surface processes on a strip confirms the normal growth mode, but fails to show a favored size unless additional energetic constraints are applied on the cluster sizes. Increasing Co coverages to cluster coalescence, a larger preferable size (mean diameter of 3.5 nm and height of 1.4 nm) appears for growth at 450 K. These two sizes are corroborated by morphology evolution of high Co coverages deposited at 300 K and annealed to 750 K, in which the coalescence is eliminated and the two preferable geometries appear and coexist.

Unusual high-temperature ferromagnetism of PbPd0.81Co0.19O2 nanograin film

Single-phase PbPd0.81Co0.19O2 film with a body-centered orthorhombic structure was prepared using the sol-gel spin-coating technique and an oxidation treatment. Film resistivity has a power dependence on temperature. The insulator-metal transition temperature was 358 K, markedly higher than the reported values of similar material systems. Ferromagnetism and superparamagnetism coexisted in the film and the ferromagnetism persisted up to 380 K. As temperature increased, the notable increasing tendencies were found for the film’s saturation magnetization and for the magnetic field where saturation magnetization decreases abruptly. The special spin gapless band structure and the film’s nanograin microstructure are likely responsible for these interesting properties.

Coercivity enhancement near blocking temperature in exchange biased Fe/FexMn1−x films on Cu(001)

Exchange bias is found in the Fe/FexMn1−x/Cu(001) bilayer films. The coercivity Hc is enhanced at blocking temperature Tb for the films with composition x between 0.25 and 0.35, but not for those between 0.1 and 0.25. A simple model based on the discrepancy of the Neel temperature TN and Tb is proposed, which may reveal the physical origins of these two temperature points.

Dramatic depression of Curie temperature for magnetic Co/Cu(1 0 0) ultrathin films upon deposition at elevated temperature

The correlation between the crystalline structures and magnetic properties, such as magnetization and Curie temperature (TC) were investigated systematically by varying the deposition temperature of 2 ML fcc cobalt films on Cu(1 0 0) substrate. At variation of the deposition temperature a dramatic change in Curie temperature and coercivity of the films was observed. The drastic raising of Curie temperature were found for the films with the deposition temperatures from 340 K (TC=170 K) down to 275 K (TC=325 K). A simple theoretical estimation was proposed to evaluate the Curie temperature of the films in terms of magnetic moment as well as anisotropy with quantitative success. The enhanced Curie temperature can be attributed to the increase of the magnetic moments due to the alteration of film morphology with various deposition temperatures.

Hysteretic behavior of magnetic particles with dipole interaction

Energy barrier height and coercivity variation of a pair of interacting particles has been studied as a function of the bonding angle β for 0°⩽β⩽π/2 and applied field using the fixed step Monte Carlo simulation approach supplemented by the perturbation method. Coercivity Hc(β) of the system has local maxima at β=0 and π/2, and a global minimum at β≃65°, and remains depressed compared to the noninteracting case for β⩽30° at any temperature. The barrier height behaves similarly. Due to the random nature of distribution dipole interaction, on average, leads naturally to a reduction in coercivity and energy barrier height in comparison with the noninteracting case.

Exchange bias in Co/Fe/FexMn1−x/Cu(100) ultrathin films

Stable and well-grown face-centered-cubic Fe films were prepared on buffer layers with varying lattice constants by depositing FexMn1−x alloy film on Cu(100) single crystal. No ferromagnetic ordering was observed at the stage of 30 ML Fe on the FexMn1−x/Cu(100) systems in the temperature range from 100 to 350 K. Furthermore, capping of Co on Fe/FexMn1−x/Cu(100) was employed as the probe of antiferromagnetic ordering by study of exchange bias coupling in these films. The exchange bias of the hysteresis loops can be observed after field cooling of the films. Further analyses by varying the measurement temperature and Fe coverage of the films were also carried out to clarify the origin of the exchange bias coupling observed. The exchange bias field found here is attributed to the interlayer coupling between the Co and Fe–Mn films through the spacing layer Fe.

Giant enhancement of magneto-optical response and increase in perpendicular magnetic anisotropy of ultrathin Co/Pt(111) films upon thermal annealing

The annealing effects on the magnetic properties, crystallographic structure, and alloy formation, were studied for Co/Pt (111) ultrathin films at coverages up to 6.6 ML, using in situ magneto-optical Kerr effect, low energy electron diffraction, Auger electron spectroscopy, and ultraviolet photoelectron spectroscopy. After the postdeposition annealing in the temperature range of 500–800 K, a significant increase in perpendicular magnetic anisotropy at high coverages and a large enhancement of magneto-optical response with the value about 200%–300% of that before annealing for all coverages investigated are observed. Both findings are shown to be correlated to the formation of a kind of Co–Pt interface (surface) alloy. This is expected to be mainly attributed to the transfer of strong spin-orbit coupling of the Pt into the Co magnetic orbital due to the hybridization during interface alloy formation.