Powder Patterns on Ferromagnetic Crystals

The regular patterns which, as Bitter has found, form on ferromagnetic single crystals from a suspension of magnetic iron oxide have been studied on large crystals of 3.5 percent silicon-iron. Three different types of patterns were observed at low, medium, and high fields, respectively. The direction of Type I and Type II patterns is determined by the crystal orientation; that of Type III is always nearly normal to the applied field. Type I patterns were found only after polishing or otherwise straining an annealed sample, and the line spacing stayed constant (except at H=0) as the field was increased. We conclude that surface strains are essential in producing this type of pattern. Type II patterns were little affected by stress, and the lines multiplied when the longitudinal or normal field component was increased. Certain dissymmetries of the lines and their behavior near a hole in the sample were studied. It is concluded that the Type II lines are formed at the intersections of magnetic "sheets," lying in dodecahedral (110) planes, with the surface. Type III patterns form near surface inhomogeneities where the magnetic flux is forced to cross the surface and where the magnetization accordingly has a normal component.