John Unguris

Electron Vortex Beams with High Quanta of Orbital Angular Momentum

Diffraction holograms are used to create electron vortex beams that should enable higher-resolution imaging. Electron beams with helical wavefronts carrying orbital angular momentum are expected to provide new capabilities for electron microscopy and other applications. We used nanofabricated diffraction holograms in an electron microscope to produce multiple electron vortex beams with well-defined topological charge. Beams carrying quantized amounts of orbital angular momentum (up to 100ℏ) per electron were observed. We describe how the electrons can exhibit such orbital motion in free space in the absence of any confining potential or external field, and discuss how these beams can be applied to improved electron microscopy of magnetic and biological specimens.

Scanning electron microscopy with polarization analysis (SEMPA)

The high spatial resolution imaging of magnetic microstructure has important ramifications for both fundamental studies of magnetism and the technology surrounding the magnetic recording industry. One technique for imaging surface magnetic microstructure on the 10‐nm‐length scale is scanning electron microscopy with polarization analysis (SEMPA). This technique employs a scanning electron microscope (SEM) electron optical column to form a medium energy (10–50 keV), small probe ( 1 nA) on a ferromagnetic specimen. Secondary electrons excited in the ferromagnet by the high spatial resolution probe retain their spin‐polarization orientation as they leave the sample surface. The spin polarization of the emitted secondary electrons can be related directly to the local magnetization orientation. A surface magnetization map is generated when the spin polarization of the secondary electrons is analyzed as the electron beam is rastered point‐by‐point across the ferromagnet’s surface. In th...

Low‐energy diffuse scattering electron‐spin polarization analyzer

A new, compact (approximately fist sized), efficient electron‐spin analyzer is described. It is based on low‐energy (150 eV) diffuse scattering from a high‐Z target, for example, an evaporated polycrystalline Au film opaque to the incident electron beam. By collecting a large solid angle of scattered electrons, a figure of merit S2I/I0=10−4 is achieved with an analyzing power S=0.11. The figure of merit degrades only marginally (<10%) for beams with an energy width of 40 eV or after one month of operation at 10−8 Torr. The electron optical acceptance is of order 100 mm2 sr eV. The details of the design and construction are discussed and its performance is compared to six other spin analyzers. Illustrative results are presented from an application to scanning electron microscopy with polarization analysis (SEMPA) to image magnetic microstructure.

Oscillatory exchange coupling in Fe/Au/Fe(100)

Scanning electron microscopy with polarization analysis was used to investigate the interlayer exchange coupling in Fe/Au/Fe(100) sandwich structures. The films were epitaxially grown on single‐crystal Fe(100) substrates. Electron diffraction measurements revealed that the Au spacer film grew with a surface reconstruction consistent with that observed for bulk Au crystals. The exchange coupling oscillates between primarily ferromagnetic and antiferromagnetic coupling for Au spacer layers up to 65 layers (13 nm) thick, but a significant biquadratic coupling component was also observed. The oscillatory coupling exhibited two components with periods of 2.48±0.05 layers (0.506±0.010 nm) and 8.6±0.3 layers (1.75±0.06 nm). The measured periods are in excellent agreement with those calculated from spanning vectors of the Au Fermi surface.

Realization of Ground-State Artificial Skyrmion Lattices at Room Temperature

Magnetic Skyrmions exhibit topologically protected quantum states, not only offering exciting new mechanisms for ultrahigh density and low dissipation information storage, but also providing an ideal platform for explorations of unique topological phenomena. Prerequisite are systems exhibiting skyrmion lattices at ambient conditions. Here, we demonstrate the realization of artificial Bloch skyrmion lattices over extended areas in their ground state at room temperature by patterning asymmetric magnetic nanodots with controlled circularity on an underlayer film with perpendicular magnetic anisotropy (PMA) [1], shown in Fig. 1.

Probing electric field control of magnetism using ferromagnetic resonance

Exchange coupled CoFe/BiFeO3 thin-film heterostructures show great promise for power-efficient electric field-induced 180° magnetization switching. However, the coupling mechanism and precise qualification of the exchange coupling in CoFe/BiFeO3 heterostructures have been elusive. Here we show direct evidence for electric field control of the magnetic state in exchange coupled CoFe/BiFeO3 through electric field-dependent ferromagnetic resonance spectroscopy and nanoscale spatially resolved magnetic imaging. Scanning electron microscopy with polarization analysis images reveal the coupling of the magnetization in the CoFe layer to the canted moment in the BiFeO3 layer. Electric field-dependent ferromagnetic resonance measurements quantify the exchange coupling strength and reveal that the CoFe magnetization is directly and reversibly modulated by the applied electric field through a ~180° switching of the canted moment in BiFeO3. This constitutes an important step towards robust repeatable and non-volatile voltage-induced 180° magnetization switching in thin-film multiferroic heterostructures and tunable RF/microwave devices.

Interfacial coupling in multiferroic/ferromagnet heterostructures

We report local probe investigations of the magnetic interaction between BiFeO3 films and a ferromagnetic Co0.9Fe0.1 layer. Within the constraints of intralayer exchange coupling in the Co0.9Fe0.1, the multiferroic imprint in the ferromagnet results in a collinear arrangement of the local magnetization and the in-plane BiFeO3 ferroelectric polarization. The magnetic anisotropy is uniaxial, and an in-plane effective coupling field of order 10 mT is derived. Measurements as a function of multiferroic layer thickness show that the influence of the multiferroic layer on the magnetic layer becomes negligible for 3 nm thick BiFeO3 films. We ascribe this breakdown in the exchange coupling to a weakening of the antiferromagnetic order in the ultrathin BiFeO3 film based on our x-ray linear dichroism measurements. These observations are consistent with an interfacial exchange coupling between the CoFe moments and a canted antiferromagnetic moment in the BiFeO3.

Oscillatory magnetic coupling in Fe/Ag/Fe(100) sandwich structures

Abstract The magnetic coupling in Fe/Ag/Fe sandwich structures has been studied using scanning electron microscopy with polarization analysis (SEMPA). when atomically well-ordered Ag spacer layers are grown epitaxially on an Fe whisker substrate, the coupling between the Fe substrate and the top Fe film is found to oscillate between ferromagnetic and antiferromagnetic as a periodic function of the Ag thickness. The magnetic coupling is composed of two oscillatory components with periods of 2.37±0.07 and 5.73±0.05 Ag layers. The oscillation persist for Ag spacer films that are up to at least 50 layers thick. These periods are consistent with theoretical models that predict that the oscillatory coupling length scale is determined by spanning vectors of the spacer materials Fermi surface. Biquadratic coupling was also observed in these structures. The relative strength of biquadratic to bilinear coupling was found to depend on the thickness of the top Fe film.

High spatial resolution quantitative micromagnetics (invited)

Magnetization profiles at surfaces are observed with scanning electron microscopy with polarization analysis (SEMPA). This technique allows for quantitative analysis of the vector magnetization profile with 70 nm spatial resolution. Magnetization profiles in surface Neel walls which terminate bulk 180° Bloch walls at surfaces have been calculated by solving the micromagnetic equations using energy minimization. The micromagnetic calculations show that the surface Neel wall penetrates a distance from the surface comparable to a Bloch wall width and that the surface Neel wall width is at least twice the bulk Bloch wall width. The dependence of the domain wall magnetization on sample thickness is calculated for Fe, and model predictions of the wall widths that would be determined by transmission Lorentz microscopy are compared with the experimental results. The magnetic field outside of the sample, which gives rise to contrast with the Bitter technique and magnetic force microscopy (MFM), is a complicated superposition of contributions from both bulk and surface walls. Moreover, a strong mutual interaction between the sample and the MFM tip may alter the sample magnetization.Magnetization profiles at surfaces are observed with scanning electron microscopy with polarization analysis (SEMPA). This technique allows for quantitative analysis of the vector magnetization profile with 70 nm spatial resolution. Magnetization profiles in surface Neel walls which terminate bulk 180° Bloch walls at surfaces have been calculated by solving the micromagnetic equations using energy minimization. The micromagnetic calculations show that the surface Neel wall penetrates a distance from the surface comparable to a Bloch wall width and that the surface Neel wall width is at least twice the bulk Bloch wall width. The dependence of the domain wall magnetization on sample thickness is calculated for Fe, and model predictions of the wall widths that would be determined by transmission Lorentz microscopy are compared with the experimental results. The magnetic field outside of the sample, which gives rise to contrast with the Bitter technique and magnetic force microscopy (MFM), is a complicated su...

Improved low‐energy diffuse scattering electron‐spin polarization analyzer

An improved low‐energy diffuse scattering electron‐spin polarization analyzer is described. It is based on the low‐energy (150 eV) diffuse scattering of polarized electrons from polycrystalline evaporated Au targets. By collecting large solid angles and efficiently energy filtering the scattered electrons, a maximum figure of merit, FOM=S2I/I0=2.3×10−4 is achieved. Maximum measured values of the Sherman function were S=0.15. Further, the instrumental (false) asymmetry due to changes in the trajectory of the incident electron beam has been minimized by balancing the angular and displacement asymmetries. A total residual scan asymmetry as low as 0.0035/mm has been measured over 4‐mm scan fields at the Au target in the detector. This instrumental asymmetry would produce a maximum error in the polarization in a SEMPA experiment of less than 0.3% for a 100‐μm full‐field scan. Details of the design and performance of the new detector are given.