The nanoscale spin structure of head-to-head domain walls in mesoscopic ferromagnetic rings has been studied by high-resolution nonintrusive photoemission electron microscopy as a function of both ring width (100-730 nm) and film thickness (2-38 nm). Depending on the geometry, two types of head-to-head domain walls are found (vortex and transverse walls). The experimental phase diagram, which identifies the transition between the wall types, is compared to analytical calculations of the energy and micromagnetic simulations, which are found to agree well with the experimental results.

Polar ionic surfaces with bulk termination are inherently unstable because of their diverging electrostatic surface energy. Nevertheless, they are frequently observed in nature, mainly because of charge neutralization by adsorbates, but occur also under atomically clean conditions. Several mechanisms have been invoked to explain the stability of atomically clean polar surfaces, but the frequently observed periodic nanoscale pattern formation has not yet been explained. Here we propose that long-range interactions between alternating electropositive and electronegative regions of different surface terminations minimize the electrostatic energy of the surface and thus stabilize the nanoscale pattern. This is illustrated using the example of polar Fe oxide surfaces by combining scanning tunneling microscopy and spectroscopy results with results from density functional theory-based calculations and dipole–dipole interaction models

MnAs films were deposited by molecular-beam epitaxy on GaAs͑001͒ and GaAs͑111͒B surfaces. Imaging of the temperature-dependent magnetic structure by x-ray magnetic circular dichroism photoemission electron microscopy, and the comparison with magnetization measurements by superconducting quantum interference device ͑SQUID͒ magnetometry, is used to study the impact of the different strain state of MnAs/GaAs͑001͒ and of MnAs/GaAs͑111͒B films on the phase transition between ferromagnetic ␣-MnAs and paramagnetic ␤-MnAs, the spatial distribution of the two structural and magnetic phases, and the transition temperature. For the isotropically strained MnAs/GaAs͑111͒B films, the phase coexistence range is much wider than for the anisotropically strained MnAs/GaAs͑001͒ films. The characteristic change of the saturation magnetization with film thickness is found to be a universal property of films with different epitaxial orientation, if at least one MnAs͗1120͘ direction is in the film plane. For MnAs/GaAs͑001͒ films this variation is related to the striped coexistence of ␣ and ␤ MnAs and the changing intra-and inter-stripe magnetic interaction with film thickness and temperature. The magnetic structure of MnAs/GaAs͑111͒B is more complex due to the existence of three symmetry-equivalent ␣-phase domains superimposed by a honeycomb-like network of the coexisting ␤ phase. The magnetic properties ͑saturation magnetization, domain size͒ of thin MnAs/GaAs͑001͒ films can be improved by postgrowth annealing. Above a certain film thickness this is inhibited by the complex magnetic structure of the film.

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The magnetization states of 20-nm-thick rectangular Co thin-film elements are studied with micromagnetic modeling and x-ray magnetic circular dichroism photoemission electron microscopy. The energies of ten domain configurations obtained in the modeling are compared with the frequency of occurrence of the corresponding virgin domain structures as a function of aspect ratio from 1:1 to 1:3 and of width from 200 to 600 nm. The results show that the abundance of the virgin states is largely determined by the magnetic energy densities of the elements.

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Low energy electron diffraction and low energy electron microscopy microspot I ∕ V analysis of the ( 4 × 4 ) O structure on Ag(111): Surface oxide or reconstruction?The structural phase transitions in the multiphase system indium ͑In͒ on Si͑111͒ are studied as a function of coverage at different temperatures between Ϫ100 and 550°C by low energy electron microscopy ͑LEEM͒ and low energy electron diffraction ͑LEED͒. All phase transitions observed with increasing In coverage are first order. Nucleation of a new phase starts only after the previous phase is completed. At high temperatures only a disordered monolayer with high atomic density forms. When In is deposited at room temperature on a (ͱ3ϫͱ3)-R30°-In surface four new phases are observed: a ͑2ϫ1͒, a near coincidence (1.5ͱ3ϫ1.5ͱ3), a ''(1ϫ1)-R30°'' and a ͑6ϫ6͒ phase. At room temperature and up at least to 400°C a double layer forms. Below 120°C this layer is compressed and ordered in two coexisting structures, (ͱ7ϫͱ3) and ''͑1ϫ1͒-R30°''. Above 120°C it shows a ͑1ϫ1͒ LEED pattern, which is attributed to a disordered layer on the Si͑111͒-͑1ϫ1͒ surface. A third monolayer with the packing density of a slightly compressed In͑111͒ plane is unstable at room temperature against formation of three-dimensional crystals but becomes stable during growth at temperatures below about Ϫ80°C. Three-dimensional crystals grow at room temperature on the double layer in the Stranski-Krastanov mode. These crystals are primarily ͑100͒-oriented and are bounded by the equilibrium planes known from bulk crystals, including the reconstructed ͑100͒ surface, which is no longer reconstructed above 120°C.