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.

No abstract

In the past year, several groups have observed evidence for long-range spin-triplet supercurrent in Josephson junctions containing ferromagnetic (F) materials. In our work, the spin-triplet pair correlations are created by non-collinear magnetizations between a central Co/Ru/Co "synthetic antiferromagnet" (SAF) and two outer thin F layers. Here we present data showing that the spin-triplet supercurrent is enhanced up to 20 times after our samples are subject to a large in-plane magnetizing field. This surprising result can be explained if the Co/Ru/Co SAF undergoes a "spin-flop" transition, whereby the two Co layer magnetizations end up perpendicular to the magnetizations of the two thin F layers. Direct experimental evidence for the spin-flop transition comes from scanning electron microscopy with polarization analysis and from spin-polarized neutron reflectometry.

The topological nature of magnetic skyrmions leads to extraordinary properties that provide new insights into fundamental problems of magnetism and exciting potentials for novel magnetic technologies. Prerequisite are systems exhibiting skyrmion lattices at ambient conditions, which have been elusive so far. 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 with perpendicular magnetic anisotropy (PMA). Polarity is controlled by a tailored magnetic field sequence and demonstrated in magnetometry measurements. The vortex structure is imprinted from the dots into the interfacial region of the underlayer via suppression of the PMA by a critical ion-irradiation step. The imprinted skyrmion lattices are identified directly with polarized neutron reflectometry and confirmed by magnetoresistance measurements. Our results demonstrate an exciting platform to explore room-temperature ground-state skyrmion lattices.

No abstract

Scanning Electron Microscopy with Polarization Analysis (SEMPA) is a technique for the high spatial resolution imaging of magnetic microstructure. It employs a Scanning Electron Microscope (SEM) to create a finely focused electron beam on the surface of a ferromagnet; secondary electrons excited by the incident beam retain their spin-polarization when exiting the surface. A two dimensional map of the electron spinpolarization of these secondary electrons reveals the surface magnetization distribution for ferromagnetic (or ferrimagnetic) systems. This chapter describes salient features of the electron probe forming optics, the electron spin-polarization analyzers with associated transport optics, and the signal processing electronics. We also give examples illustrating how SEMPA provides high resolution magnetization images of various classes of micromagnetic structure.

No abstract

We report local probe investigations of the magnetic interaction between BiFeO 3 films and a ferromagnetic Co 0.9 Fe 0.1 layer. Within the constraints of intralayer exchange coupling in the Co 0.9 Fe 0.1 , the multiferroic imprint in the ferromagnet results in a collinear arrangement of the local magnetization and the in-plane BiFeO 3 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 BiFeO 3 films. We ascribe this breakdown in the exchange coupling to a weakening of the antiferromagnetic order in the ultrathin BiFeO 3 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 BiFeO 3 .