The Magnetic Structure of Cobalt

The magnetic powder method has been used to investigate the magnetic structure of cobalt crystals at room temperature. A stable colloid of gamma-ferric-oxide was deposited on suitable cuts of cobalt crystals by a magnetic field applied normal to the surface under observation. Smooth surfaces were prepared by metallurgical polishing followed by additional electrolytic polishing. (The latter method for polishing cobalt is described.) The surfaces chiefly studied approximated basal or prism planes. On all cuts three related patterns were found corresponding to no applied field or a normal field applied outward or inward. The two patterns with field were reciprocal in the sense that their spaces and deposits were interchanged. On basal planes the pattern appeared lace-like possessing considerable detail which was not easily modified by increasing the applied field. On prism planes the no-field pattern consisted of straight lines running parallel to the hexagonal axis. A small applied normal field spread the colloid in these lines into the alternate spaces between them. Further increase in field widened the resulting stripes in a somewhat erratic manner. Other interesting details of the patterns are described. In the discussion are listed the various energies involved in the demagnetization of an ideal crystal, i.e., one in which all energies concerned are reversible. The attempts of other authors to use these energies for devising magnetic structures are examined for their relation to magnetic powder patterns. It is concluded that the cobalt patterns do not accord with the structure demanded by simple theory which neglects irreversible energy changes accompanying demagnetization. The structure which seems best suited to explain the various patterns of cobalt is then proposed and discussed. To make this structure appear reasonable, the process by which a crystal goes from the fully magnetized to the demagnetized state (as suggested by various pattern observations) is described in terms of the dendritic growth of regions of reversed magnetization.