Electron Holography of Magnetic Materials

Abstract

Transmission electron microscopy (TEM) involves the use of high-energy (60-3000 keV) electrons that have passed through a thin specimen to record images, diffraction patterns or spectroscopic information from a region of interest. Many different TEM techniques have been developed over the years into highly sophisticated methodologies that have found widespread application across scientific disciplines. Because the TEM has an unparalleled ability to provide structural and chemical information over a range of length scales down to atomic dimensions, it has developed into an indispensable tool for scientists who are interested in understanding the properties of nanostructured materials and in manipulating their behavior (Smith, 2007). State-of-the-art TEMs are now equipped with spherical and chromatic aberration correctors and can provide interpretable image resolutions of 0.05 nm (Erni et al., 2009). However, in addition to conventional TEM techniques that can be used to provide structural and compositional information about materials, the TEM also allows magnetic and electrostatic fields in specimens to be imaged with nanometer spatial resolution. One of the most powerful techniques for providing this information is electron holography, which was originally proposed as a means to compensate for lens aberrations and to improve electron microscope resolution (Gabor, 1949). Electron holography is still the only technique that provides direct access to the phase shift of the electron wave that has passed through a thin specimen, in contrast to more conventional TEM techniques that record only spatial distributions of image intensity. Electron holography has only recently become widely available on commercial electron microscopes. The earliest studies using electron holography were restricted by the limited brightness and coherence of the tungsten filaments that were used as electron sources (Haine & Mulvey, 1952). The availability of high brightness, stable, coherent field emission electron guns now allows electron holography to be applied to a wide variety of materials such as quantum well structures, magnetic thin films, semiconductor devices, natural rocks and biominerals.