Absolute Magnetometry Using Electron Holography: Magnetic Superlattices and Small Particles
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Magnetometry Using Electron Holography: Magnetic Superlattices and Small Particles
Marian Mankos, J.M. Cowley, and M.R. Scheinfein Introduction Synthesized magnetic structures are of interest due to their unique and unusual properties, which are governed by their micromagnetic structure. For example, giant-magnetoresistance (GMR) multilayer structures composed of magnetic layers separated by nonmagnetic spacers,1 and granular GMR films composed of magnetic and nonmagnetic metals2 exhibit phenomena whose interpretation requires knowledge of both the physical and micromagnetic structure at nanometer-length scales. Techniques for magnetic-microstructure imaging are based on the interaction between a probe and either the magnetic microstructure 3 4 itself (magnetization ' ) or a physical quantity related to the magnetization distribution (e.g., magnetostriction, mag57 netic induction). Transmission methods are sensitive to bulk magnetic microstructure averaged along the direction of the incident probe; surface structure is 3 8 lost. Reflection techniques ' interact with the near-surface region and no information is obtained about the bulk structure aside from those properties that can be inferred from appropriate
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BULLETIN/OCTOBER 1995
boundary conditions.9 Electron-optical methods represent the widest class of high-spatial-resolution, magnetic-domain imaging techniques.10 The most advanced techniques provide the highest contrast, sensitivity, and point resolution (1 nm). Electron holography offers quantitative micromagnetic information11"16 at high spatial resolution, a feature missing in most magneticimaging techniques. Quantitative information can be extracted from the absolutely calibrated electron wavelength and a knowledge of electron phase shifts in electromagnetic fields. High sensitivity, nanometer spatial resolution, and absolute calibration make electron holography a powerful tool for examining magnetic microstructure. In electron holography, both the amplitude and phase of the transmitted electron waves can be recovered in contrast to conventional electron microscopy where only the intensity is available. The phase, containing information about the local distribution of electromagnetic fields, can be retrieved from an electron 1116 hologram.
Electron Holography In off-axis scanning transmission electron microscopy (STEM) holography,13"16 an electron biprism 17 —a conductive wire about 0.5 μ^n in diameter held at a constant potential—is placed in the electron microscope's illuminating system. The wave emitted from the electron source is split by the biprism into two wave packets resulting in the formation of two identical coherent electron probes, which are subsequently imaged onto the specimen. Typically, one of the two sources passes in vacuum around the sample. This is the reference wave. The second wave packet that interacts with the sample is the object wave. The reference and object waves recombine in the detector plane and form an interference pattern—a hologram, which is simply a fringe-modul
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