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

Superparamagnetic nanoparticle ensembles

Magnetic single-domain nanoparticles constitute an important model system in magnetism. In particular ensembles of superparamagnetic nanoparticles can exhibit a rich variety of different behaviors depending on the inter-particle interactions. Starting from isolated single-domain ferro- or ferrimagnetic nanoparticles the magnetization behavior of both non-interacting and interacting particle-ensembles is reviewed. A particular focus is drawn onto the relaxation time of the system. In case of interacting nanoparticles the usual Neel-Brown relaxation law becomes modified. With increasing interactions modified superparamagnetism, spin glass behavior and superferromagnetism is encountered.Comment: Corrected formula: Eq. (1

Magnetic dot arrays modeling via the system of the radial basis function networks

Two dimensional square lattice general model of the magnetic dot array is introduced. In this model the intradot self-energy is predicted via the neural network and interdot magnetostatic coupling is approximated by the collection of several dipolar terms. The model has been applied to disk-shaped cluster involving 193 ultrathin dots and 772 interaction centers. In this case among the intradot magnetic structures retrieved by neural networks the important role play single-vortex magnetization modes. Several aspects of the model have been understood numerically by means of the simulated annealing method.Comment: 16 pages, 8 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

Characterization of magnetic microstructure at high spatial resolution

Advances in metal epitaxial growth techniques have produced renewed interest in magnetic materials through the unique properties of novel atomically engineered structures. Ultrathin layered and nanocrystalline, textured composites possess unique magnetic properties which are intimately connected to the physical microstructure on both the atomic and nanometer length scales. The length scales present in magnetic interactions span several orders of magnitude from the atomic length scales of the exchange interaction, to the mesoscopic length scales important in anti-ferromagnetic RKKY coupling between thin ferromagnetic films and small particles, to the macroscopic length scales determined by the long range magnetostatic interactions which are responsible for the formation of magnetic domains.Special characterization and computation techniques are required to analyze the micromagnetic and microstructural properties of these novel, low dimensional systems. In order to correlate the structure with the magnetic properties of any system, in-situ growth, structural, chemical and magnetic characterization is desirable.Most methods used for the observation of micromagnetic structure rely on a contrast mechanism derived from the magnetic fields present in ferromagnetic systems. We have implemented the conventional Fresnel, and less conventional Differential Phase Contrast modes of imaging magnetic microstructure in an HB5 STEM.</jats:p

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

Electronic and Chemical Analysis of a Metal-Insulator Interface Utilizing Transmission Electron Energy Loss Spectroscopy at 5Å Spatial Resolution

AbstractUsing a 0.5 nm diameter probe of 100 keV electrons, we have been able to detect significant changes in the transmission electron energy loss spectra in the region of valence shell and L23 shell excitation within a spatial extent of 0.4 nm of an Al-AlF3 interface. The spectra have been recorded with a dose significantly less than the critical dose for destruction of the AlF3.</jats:p