We report collective ferromagnetic behavior with high Curie temperatures (T(c)) in Fe dot assemblies supported by the Cu(111) surface. Our ability to tune the average size and spacing of the individual dots allows us to conclude that enhanced magnetic anisotropy cannot account for this high-T(c) ferromagnetic order. Because our Monte Carlo simulations have ruled out the dipolar interaction as the dominant factor in this system, we attribute the origin of the ferromagnetic order to indirect exchange coupling via the Cu(111) substrate.

The self-assembly of iron dots on the insulating surface of NaCl (001) is investigated experimentally and theoretically. Under proper growth conditions, nanometer-scale magnetic iron dots with remarkably narrow size distributions can be achieved in the absence of a wetting layer. Furthermore, both the vertical and lateral sizes of the dots can be tuned with the iron dosage without introducing apparent size broadening, even though the clustering is clearly in the strong coarsening regime. These observations are interpreted using a phenomenological mean-field theory, in which a coverage-dependent optimal dot size is selected by strain-mediated dot-dot interactions.PACS numbers: 81.07. Ta,61.46.+w,68.37.Ps,68.35.Md Clustering on surfaces by nucleation and growth during atom deposition has been an important subject in basic and applied science for decades [1,2]. Recent efforts have been focused on searching for methods to obtain nanometer-scale clusters with narrow size distributions. Such clusters or quantum dots are potentially valuable for optical, electronic, and magnetic device applications, but mass production of such structures by lithography or etching-based fabrication has proved to be exceptionally challenging [3,4]. Alternatively, it has been realized that the strain energy associated with the lattice mismatch between the dot and the substrate materials can be exploited to induce self-assembled formation of quantum dots with narrow size distributions. This has generated much excitement, particularly in the area of semiconductor quantum dots [3,4,5,6,7,8]. In such cases, the growth of the dots often proceeds in the StranskiKrastanow (SK) mode, which is characterized by the presence of a wetting layer prior to three-dimensional (3D) clustering. To date, the precise mechanism for size selection in semiconductor quantum dot systems remains a subject of active debate [4,5,6,9]: some attribute them to strain-induced thermodynamic equilibrium states, while others associate them with metastable configurations due to kinetic limitations.Although improved size uniformity can be achieved in quantum dot formation via the SK growth mode, the presence of a wetting layer is often undesirable, particularly for electronic and magnetic device applications of metallic/magnetic quantum dots. For this reason, it is preferred to fabricate quantum dots in the VolmerWeber (VW) growth mode, which is characterized by immediate 3D clustering on the substrate surface. Indeed, considerable recent efforts have been devoted to metallic/magnetic quantum dot formation on various substrates [10,11,12], but no significant size uniformity has been achieved in such studies.In this Letter we investigate the self-assembly of iron dots on NaCl(001), an insulating substrate, by thermal deposition and variable-temperature atomic force microscopy in ultrahigh vacuum. We show that, by properly choosing the growth conditions, nanometer-scale magnetic iron dots with remarkably narrow size distributions can be achieved in the absence of a wetting l...

In this article, we review recent progress in the exploration of the complex magnetic phases of the fcc Fe/Cu(111) system. In particular, we emphasize the magnetic properties realized by the synthesis of novel nanostructures of Fe on Cu(111). These include monolayer films, one-dimensional stripe arrays and nanodot arrays. The effects of spatial confinement, together with strong spin–lattice correlations, result in dramatically different magnetic behaviour for the various manifestations of the Fe/Cu(111) system. Multi-scale theoretical calculations have been used to provide an understanding of the magnetic behaviour in each case.

No abstract

We have prepared arrays of parallel Fe1−xCox alloy nanowires along the atomic step edges of a miscut W(110) surface. Their magnetic properties have been studied with the surface magneto-optical Kerr effect as a function of the relative concentration of the two materials. At low (<35%) cobalt concentrations, the wire arrays exhibit a ferromagnetic easy axis along the substrate [1 −1 0] direction, which is in the surface plane, but perpendicular to the wires. Unlike the bulk alloy, this system shows a decrease in its Curie temperature as cobalt is added to pure Fe. The Curie temperature drops sharply near x=0.35, indicating that cobalt frustrates magnetic ordering in the system.

We observe the spontaneous formation of parallel oxide rods upon exposing a clean NiAl͑110͒ surface to oxygen at elevated temperatures ͑850-1350 K͒. By following the self-assembly of individual nanorods in real time with low-energy electron microscopy ͑LEEM͒, we are able to investigate the processes by which the rods lengthen along their axes and thicken normal to the surface of the substrate. At a fixed temperature and O 2 pressure, the rods lengthen along their axes at a constant rate. The exponential temperature dependence of this rate yields an activation energy for growth of 1.2± 0.1 eV. The rod growth rates do not change as their ends pass in close proximity ͑Ͻ40 nm͒ to each other, which suggests that they do not compete for diffusing flux in order to elongate. Both LEEM and scanning tunneling microscopy ͑STM͒ studies show that the rods can grow vertically in layer-by-layer fashion. The heights of the rods are extremely bias dependent in STM images, but occur in integer multiples of approximately 2-Å-thick oxygen-cation layers. As the rods elongate from one substrate terrace to the next, we commonly see sharp changes in their rates of elongation that result from their tendency to gain ͑lose͒ atomic layers as they descend ͑climb͒ substrate steps. Diffraction analysis and darkfield imaging with LEEM indicate that the rods are crystalline, with a lattice constant that is well matched to that of the substrate along their length. We discuss the factors that lead to the formation of these highly anisotropic structures.

By observing with low-energy electron microscopy whether individual alumina islands grow or shrink for different substrate temperatures and O 2 pressures, we determine the stability of thin oxide layers on the NiAl͑110͒ surface. At each temperature, a well-defined O 2 pressure exists where islands do not change in size. Yet we conclude that the oxide cannot be in thermodynamic equilibrium with O 2 gas and NiAl bulk, because the O 2 pressures needed to attain this state are 20 orders of magnitude higher than expected. We discuss what kinetic processes can lead to the observed steady state, where the O 2 pressure needed for stability differs greatly from thermodynamic predictions.

We examine how translation-related domains form in thin alumina films synthesized by oxidizing a NiAl (110) surface. Low-energy electron microscopy observations reveal that translation-related domains (sometimes called antiphase domains in the literature) are created within isolated alumina islands as they grow or are annealed. Thus, the domains do not originate when islands with displaced lattices impinge, as frequently assumed in models of film growth. Even though the planar defects that bound the translation-related domains cost energy, the misfit dislocations that terminate the domain boundaries lower the film’s strain energy.