Spin-to-Orbital Angular Momentum Conversion and Spin-Polarization Filtering in Electron Beams

We propose the design of a space-variant Wien filter for electron beams that induces a spin half-turn and converts the corresponding spin angular momentum variation into orbital angular momentum of the beam itself by exploiting a geometrical phase arising in the spin manipulation. When applied to a spatially coherent input spin-polarized electron beam, such a device can generate an electron vortex beam, carrying orbital angular momentum. When applied to an unpolarized input beam, the proposed device, in combination with a suitable diffraction element, can act as a very effective spin-polarization filter. The same approach can also be applied to neutron or atom beams.Comment: 9 pages, 5 figure

Magnetoresistance, Micromagnetism, and Domain Wall Scattering in Epitaxial hcp Co Films

Large negative magnetoresistance (MR) observed in transport measurements of hcp Co films with stripe domains were recently reported and interpreted in terms of a novel domain wall (DW) scattering mechanism. Here detailed MR measurements, magnetic force microscopy, and micromagnetic calculations are combined to elucidate the origin of MR in this material. The large negative room temperature MR reported previously is shown to be due to ferromagnetic resistivity anisotropy. Measurements of the resistivity for currents parallel (CIW) and perpendicular to DWs (CPW) have been conducted as a function of temperature. Low temperature results show that any intrinsic effect of DWs scattering on MR of this material is very small compared to the anisotropic MR.Comment: 5 pages, 5 Figures, submitted to PR

Surface Plasmons: a Sensitive Diagnostic Tool for Characterizing Interfaces

There has been considerable interest in characterizing the electronic and chemical properties of interfaces and grain boundries at high spatial resolution. This abstract describes a technique which utilizes the energy dispersion of surface plasmons in the transmission electron energy loss spectrum to evaluate the local dielectric constant variation across interfaces. This technique is shown to yield extremely high spatial resolution.We have been conducting studies of interfaces in a VG HB-5 STEM located at NRRFSS which is equipped with a high resolution electron energy loss analyzer. In STEM, using small probes, a typical surface plasmon excited by 100 keV electrons (Al for example) reaches its asymptotic energy value at a scattering angle between.3 and.4 mr. Since we are convoluting the incident angular distribution with the surface plasmon intensities integrated over a collection aperture, the surface plasmon excitation energies are given by their asymptotic (in k-space) energy values. These asymptotic energy excitations are very sensitive functions of the thickness and dielectric constant [eg, 2-5] of the surrounding medium.</jats:p

Electron Energy Loss Spectroscopy at 5Å Spatial Resolution

One of the ultimate goals of electron energy loss spectroscopy within the electron microscope is to be able to obtain the electronic structure of interfaces at near atomic resolution. Though this goal has not yet been achieved, there has been considerable effort devoted to elemental composition at high spatial resolution using ELS (eg. References 1-3). In this paper we wish to present initial measurements made across different types of interfaces that show core and valence shell electron energy loss spectra changing within an 8Å spatial scale across the interface.All the measurements have been performed using a modified dedicated STEM (VG Scientific HB-5) equipped with beam blanking facilities, digital control and a wide-gap aberration connected energy loss spectrometer. The details of this instrument have been described elsewhere (4). The main point to be noted is that the incident illumination half angle was 7.5mrad for these experiments and the full width at half maximum of the probe was 4.6Å (measured). With these optical conditions, 90% of all the incident beam current is contained within a diameter of 9.0Å (5). For beam sensitive materials, the recording dose was kept to less than 1/3 the dose for observable sample degradation.</jats:p