Magnetic measurements with Lorentz microscopy

The electron microscope may be operated in a mode which permits the exploration of the magnetization configurations in thin magnetic films. This mode of operation, known as Lorentz microscopy, is a powerful technique for investigating thin NiFe films because it offers high resolution, because it provides an unequivocal identification of the local magnetization direction, and because it permits correlations to be made between the magnetic structure and the underlying physical (crystallographic) structure of the film. In the past, Lorentz microscopy has found fruitful employment in the analysis of the magnetization configurations of domain walls, in studies of various magnetization reversal processes, and in specialized investigations of unusual magnetic structure. Besides these primarily qualitative investigations, however, some quantitative measurements may be made with this instrument. Such measurements are useful not only because they permit direct evaluation of basic magnetic parameters of films being studied by Lorentz microscopy, but because they afford insight into the fundamental processes which occur in the standard macroscopic magnetic measurements of NiFe films deposited on glass substrates. The following measurements are discussed: 1) determination of the Curie temperature; 2) measurement of the anisotropy field H
k
by the standard hysteresigraph and the Feldtkeller techniques; 3) quantitative studies of wall motion by labyrinth propagation and by wall creep; 4) the investigation of anisotropy dispersion by the Crowther and Torok techniques. The accuracy of these measurements is, in general, lower than that of the analogous measurements made by macroscopic methods on films deposited on glass substrates. Nevertheless, macroscopic measurements performed on a film on a glass substrate showed good agreement with Lorentz measurements performed on a simultaneously-deposited film which was suitable for Lorentz microscopy.