Cells of magnetotactic bacteria are used as model systems for studying the magnetic properties of ferrimagnetic nanocrystals. Each individual bacterial strain produces magnetosomes (membrane-bounded magnetic crystals) that have distinct sizes, shapes, crystallographic orientations and spatial arrangements, thereby providing nanoparticle systems whose unique magnetic properties are unmatched by synthetic chemically-produced crystals. Here, we use off-axis electron holography in the transmission electron microscope to study the magnetic properties of isolated and closely-spaced bullet-shaped magnetite (Fe3O4) magnetosomes biomineralized by the following magnetotactic bacterial strains: the cultured Desulfovibrio magneticus RS-1 and the uncultured strains LO-1 and HSMV-1. These bacteria biomineralize magnetite crystals whose crystallographic axes of elongation are parallel to <100> (RS-1 and LO-1) or <110> (HSMV-1). We show that the individual magnetosome crystals are single magnetic domains and measure their projected in-plane magnetization distributions and magnetic dipole moments. We use analytical modelling to assess the interplay between shape anisotropy and the magnetically preferred <111> magneto-crystalline easy axis of magnetite.

The successful correction of spherical aberration is an exciting and revolutionary development for the whole field of electron microscopy. Image interpretability can be extended out to sub-Ångstrom levels, thereby creating many novel opportunities for materials characterization. Correction of lens aberrations involves either direct (online) hardware attachments in fixed-beam or scanning TEM or indirect (off-line) software processing using either off-axis electron holography or focal-series reconstruction. This review traces some of the important steps along the path to realizing aberration correction, including early attempts with hardware correctors, the development of online microscope control, and methods for accurate measurement of aberrations. Recent developments and some initial applications of aberration-corrected electron microscopy using these different approaches are surveyed. Finally, future prospects and problems are briefly discussed.

Off-axis electron holography is used to measure electrostatic potential profiles across a silicon p-n junction, which has been prepared for examination in the transmission electron microscope (TEM) in two different specimen geometries using focused ion beam (FIB) milling. Results are obtained both from a conventional unbiased FIB-milled sample and using a novel sample geometry that allows a reverse bias to be applied to an FIB-milled sample in situ in the TEM. Computer simulations are fitted to the results to assess the effect of TEM specimen preparation on the charge density and the electrostatic potential in the thin sample.

Ultrahigh-resolution imaging may be achieved using modifications of the off-axis holography scheme in a scanning transmission electron microscopy (STEM) instrument equipped with one or more electrostatic biprisms in the illuminating system. The resolution is governed by the diameter of a reference beam, reduced by channeling through a line of atoms in an atomic-focuser crystal. Alternatively, the off-axis holography may be combined with the Rodenburg method in which a four-dimensional data set is obtained by recording a nanodiffraction pattern from each point of the specimen as the incident beams are scanned. An ultrahigh-resolution image is derived by computer processing to give a particular two-dimensional section of this data set. The large amount of data recording and data processing involved with this method may be avoided if the two-dimensional section is derived by recording the hologram while the four beams produced by two perpendicular biprisms are scanned in opposing directions across the specimen by varying the voltages on the biprisms. An equivalent scheme for conventional TEM is also possible. In each case, the complex transmission function of the specimen may be derived and resolutions of about 0.05 nm may be expected.

Through recent years, off-axis electron holography has helped us to understand and to overcome some experimental restrictions in transmission electron microscopy. With development of powerful electron microscopes, slow-scan CCD cameras, and computers, holography is not an academic technique anymore used by specialized laboratories. Holography has proven its wide range of applications in solving real-world problems in materials science and biology. At medium resolution, that is, on nanometer scale, holography allows access to large area phase contrast produced by magnetic fields and electric potentials. In the high-resolution domain, holography unveils its power by unscrambling amplitude and phase of the electron wave, resulting in an improved lateral resolution up to the information limit. Holography is a thoroughly quantitative method, and, in combination with the perfect zero-loss filtering inherent to this method, the interpretation of the reconstructed data is strongly simplified. After outlining the basics of holography, in this tutorial we focus on development of a step-by-step procedure for recording and reconstruction of holograms. At the end, some recent applications are discussed.

Through recent years, off-axis electron holography has helped us to understand and to overcome some experimental restrictions in transmission electron microscopy. With development of powerful electron microscopes, slow-scan CCD cameras, and computers, holography is not an academic technique anymore used by specialized laboratories. Holography has proven its wide range of applications in solving real-world problems in materials science and biology. At medium resolution, that is, on nanometer scale, holography allows access to large area phase contrast produced by magnetic fields and electric potentials. In the high-resolution domain, holography unveils its power by unscrambling amplitude and phase of the electron wave, resulting in an improved lateral resolution up to the information limit. Holography is a thoroughly quantitative method, and, in combination with the perfect zero-loss filtering inherent to this method, the interpretation of the reconstructed data is strongly simplified. After outlining the basics of holography, in this tutorial we focus on development of a step-by-step procedure for recording and reconstruction of holograms. At the end, some recent applications are discussed.