Rafal E. Dunin-Borkowski

Electron Holography of Magnetic Materials

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.

Lorentz microscopy and off-axis electron holography of magnetic skyrmions in FeGe

Magnetic skyrmions are vortex-like spin structures that are of great interest scientifically and for applications in low-power magnetic memories. The nanometer size and complex spin structure require high-resolution and quantitative experimental methods to study the physical properties of skyrmions. Here, we illustrate how Lorentz TEM and off-axis electron holography can be used to study the spin textures of magnetic skyrmions in the noncentrosymmetric B20-type helimagnet FeGe as a function of temperature and applied magnetic field. By reversing the magnetic field inside the microscope, the switching mechanism of the skyrmion lattice at 240 K is followed, showing a transition of the skyrmion lattice to the helical structure before the anti-skyrmion lattice is formed.

Research data supporting "Dislocations in AlGaN: core structure, atom segregation and optical properties"

Figure 1. Plan-view aberration-corrected HAADF-STEM image of the AlGaN sample, showing the core structure of an edge-type dislocation (5/7-atom ring)(a,d,g), an undissociated mixed-type dislocation (double 5/6-atom ring)(b,e,h) and a dissociated mixed-type dislocation (7/4/8/4/9-atom ring)(c,f,i). Raw unfiltered images (a-c), and ABSF-filtered (average background subtraction filter) (d-f) with atomic columns identified to guide the eye (g-i). Figure 2. (a) Unfiltered HAADF-STEM image of an undissociated mixed-type dislocation. (b) ABSF-filtered image of (a) with geometric phase analysis overlay showing the x-x strain component (x-axis parallel to [11-20]). (c) EDX line scan showing the composition of Al, Ga, and N along the line depicted in (b) (with a ca. 1 nm analysis width). Figure 3. (a) AFM, (b) CL integrated intensity, and (c) CL peak emission energy of the same region in the AlGaN sample. Figure 4. (a) AFM, (b) CL integrated intensity, and (c) CL peak emission energy of the same region in the InGaN sample. Figure 5. Simulation of the emission energy shift in the vicinity of an edge-type dislocation.

Research data supporting "Carrier Localization in the Vicinity of Dislocations in InGaN"

FIG. 1. AFM (a), SEM (b), panchromatic CL (c), and ADF-STEM (d) performed on the same micrometre-scale area. To guide the eye, a few dislocations are indicated by arrows in each picture. (e) High-resolution (HR) STEM image of the dislocation indicated by a square in (a)-(d), enabling the identification of the core structure (here dissociated 7/4/8/5-atom ring), and (f) geometric phase analysis (GPA) showing the e_xx strain component of the dislocation core region. n nFIG. 2. Schematic showing the electron probe in the SEM-CL scanning across a V-pit. The scale of the schematic, although indicative, is representative of the experimental conditions in which the experiment was conducted. Distance to nearest neighbor dependence of the intensity ratio (a)(c) and energy shift (b)(d) measured at the center (a)(b) and facet (c)(d) of the V-pits. n nFIG. 3. (a) Histogram of the number of In-N chains as a function of the number of indium atoms in the chains, located within a 10 A radius centered on the dislocation, in the case of a random distribution of indium (i.e. initial configuration of the simulation) or segregation of indium (i.e. equilibrium configuration of the simulation). Abstract representation of the data in (a), in the case of a random distribution (b) or segregation (c) of indium atoms. n nFIG. 4. ADF-STEM image of the clustered dislocations 26 (a) and 87 (b). The white strain-related contrast between the neighboring dislocations is indicated by an arrow. Aberration-corrected HAADF-STEM image of the core of dislocation 26 (dissociated 7/4/8/4/9-atom ring)(c) and 87 (undissociated double 5/6-atom ring)(d). An ABSF-filter (Average Background Subtraction Filter) has been applied to (c) and (d) in order to remove noise from the images. n nFIG. 5. 16K CL integrated intensity (a)(c) and peak emission energy (b)(d) maps of isolated n n(a)(b) and clustered (c)(d) dislocations. To guide the eye, the position of the bright spots, directly observable in (a) and (c), is indicated by circles in all the maps. To emphasize the relative variations in intensity and energy between isolated and clustered configurations, a common color scale is used in (a) and (c) and in (b) and (d).