Coherent magnetic imaging by TEM

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...

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