Magnetic Contrast in the SEM

Abstract

The magnetic microstructure of materials can be directly observed in the SEM by means of three different contrast mechanisms, which are designated by their chronological order of discovery. Type I magnetic contrast arises from the deflection of secondary electrons which have left the specimen by external magnetic fields and was first observed in recorded magnetic media by Dorsey (1969) and in natural magnetic crystals by Joy and Jakubovics (1968) and Banbury and Nixon (1967). Type II magnetic contrast originates from deflection of the beam electrons by internal magnetic fields; this contrast was first observed by Philibert and Tixier (1969) and the mechanism elucidated by Fathers et al. (1973a, 1974). Type III magnetic contrast arises from the polarization of the secondary electrons emitted from the surface of a magnetized material (Pierce and Celotta, 1981). The choice of the contrast mechanism to employ is determined by the type of magnetic microstructure which is to be imaged. In order to provide a basis for the discussion of the contrast mechanisms, the nature of magnetic microstructures will be discussed briefly. A more detailed description can be found in Craik and Tebble (1965) and Kittel (1966). This introduction will be followed by a description of the characteristics of the magnetic contrast mechanisms and the practical techniques required to obtain this contrast in images with the SEM.