No abstract

Abstrcrr+--A novel transmission electron microscope optimised for the shdy of magnetic materials is described Usiag this instmment, a new method for revealing magnetic structures, coherent Foucault imaging, can be milised Images appear directly in the form of magnetic interferograms and provide immediate access to a quantitative description of the induction distribution across the specimen. A simple analytical approach is given to the underlying theory and the main fatUtes are confinned by computer modelling. Experimental images of small regular permaky elements illustrate the power of the technique for studying domain structures. I. INTRODWCTIONTo understand the properties of modern magnetic materials, it is frecluently necessary to have a detailed knowledge of the spatial distribution of magnetisation within the specimen with as high a qmtd resolution as possible. If the specimen is in thin film form, or if the idormation sought can be extracted from a thinned section, transmission electron microscopy 0 is an attractive option. However, the standard high performance microscope is ill-equipped for examining magnetic materials as the specimen is immersed in a high magnetic field which cbtxoys, or substantially modifies, the domain structure of interest. Simply turning off the objective lens and using only existing lenses more distant from the specimen allows magnetic stiuctures to be imaged but with, at best, modest resolution. In the case of an instrument equipped with a thermionic electron source, resolution in fixedkxn (CTEM) mode is typically >IOW and in scanning (STEM) mode >50nm [ 11. Access to both scanning and fixed-beam modes is desirable as different modes of Lorentz microscopy are advantageous for different studies. Thus, the Fresnel and Foucault modes, shown schematically in Fig. 1, are most diciently practised in a CTEM. They provide rapid information on the domain geometry and the approximate direction of magnetsation in each domain and their simplicity of implementation makes them very well suited for use in in-situ dynanuc experiments. However, under normal operational Oondltions, they are unsuitable for deriving quantitative information on the detailed spatial distribution of induction throughout the specimen. For this purpose the differential phase contrast imaging mode [2], which can be realised only in a STEM, Manuscript received Apnl I , I994and electron holography [3], whch requires the use of an electron biprism, are more suitable.In the following section of this paper we describe a recently developed electron microscope, equipped with a field emission gun (FEG), whose performance has been optimised for the study of magnetic materials. Thereafter, in section 111, we concentrate on the imaging performance in fixed beam mode and show how the small size of the electron source allows the effectiveness of the Foucault mode to be increased very sigmficantly so that quantitative information can be extracted. Indeed, if so-called coherent Foucault (CF) imaging is undertaken using a phase-shifting t...

No abstract

Energy dispersive X-ray detectors are frequently attached to electron microscopes to enable microanalysis to be performed, but because such detectors accept X-rays generated within an appreciable solid angle, the recorded spectra usually include some spurious contributions from the instrument. This paper describes instrumental modifications firstly to reduce and secondly to permit the subtraction of the residual extraneous contributions. The probable accuracy of this subtraction procedure is examined.Results are presented showing the effects of various instrumental modifications on spect.ra from thin specimens and demonstrate that by careful attention to experimental details it is possible to separate the spectrum due to the thin specimen alone from all other extraneous signals.Two test specimens and a number of test procedures for investigating the analytical performance of (scanning) transmission electron microscopes are presented.Sources of error and the method of their correction when the thin specimen Bremsstrahlung is used for quantitative analysis are discussed. I N T R O D U C T I O NIn transmission electron microscopes (TEMs) equipped with energy dispersive X-ray detectors even the best designed systems show some spurious contributions to the X-ray spectra. The recorded spectra include along with the X-rays from the thin specimens, Bremsstrahlung and characteristic X-rays from the instrument and, to a lesser extent, directly recorded electrons. To suppress the instrumental characteristic lines, a solution which has been frequently adopted is to construct part of the specimen holder of carbon or beryllium (Russ, 1977; Saubermann et al., 1981; Panessa et al., 1978); however, this will leave an unknown Bremsstrahlung background which may be high. In materials science where the intensity of the characteristic lines is of prime importance the aim of most workers has been to try to eliminate instrumental peaks, and X-rays from regions of the specimen not under analysis, by careful attention to collimation of the electron beam above the specimen (e.g. Goldstein & Williams, 1978; Nicholson et al., 19771. In the literature little attention has been given to the design of the parts of the instrument beneath the specimen, and to extraneous signals whose intensities are dependent upon the electron scattering introduced by the specimen itself.There are two areas of interest where knowledge of the Bremsstrahlung from the thin specimen alone is important. For analysis of specimens with organic matrices such as biological specimens, in which the matrix produces some characteristic lines which cannot be detected with an energy dispersive detector, the Hall method (1971) of quantitation is frequently used. The method uses an integrated region of the Bremsstrahlung to normalize characteristic line intensities far variations in specimen mass thickness and incident beam current. Thus it is

The electron microprobe microanalyser has been used to measure the concentrations of Ca, P and S in the predentine of young rat incisors. The specimens were prepared as alcohol embedded ultrathin sections, unfixed vacuum embedded dry cut ultrathin sections and as thin cryostat sections. The results show the influence of preparation on the measured compositions and indicate that Ca is tightly bound to the matrix, whereas P can be easily washed out. Measurements along the dentine-predentine border demonstrated zones of Ca enrichment, the average size of which suggests that the zones could be the prestages of calcospherites. A mineralisation mechanism is discussed in which the high Ca concentration activates pyrophosphosphatase or ATPase before the onset of nucleation.