Magneto-striction in iron crystals

The phenomenon of magneto-striction, or change in dimensions of a body when magnetised, has been known for a long time. It was discovered by Joule in 1847, and has since been the subject of many careful investigations. The phenomenon has been measured in many kinds of iron, nickel, and cobalt. The results have shown that the exact nature of the phenomenon depends to a great extent on the physical state of the specimen examined, and may be considerably altered by physical treatment, such as annealing. This result follows from the known effect of crystalline structure on the properties of ferro-magnetic substances. It is to be expected that the results, and their interpretation, would be considerably simplified if found in single crystals. In this case, instead of there being indeterminate heterogeneous crystalline mass, the specimen would have a definite crystal lattice with reference to which the axes of the specimen could be determined. This has been pointed out by various writers, and some work has been done in this direction. Heaps (‘Phvs. Rev.,’ vol. 22, p. 486, 1923; vol. 23, p. 60, 1924) has investigated the magneto-striction of single crystals of iron and magnetite. The experiments on iron were carried out on a disc cut from a single crystal. The elongation parallel to the magnetizing field was measured for various directions in the disc, and it was found that the elongation varied considerably with the direction. The direction of the crystal axes were not determined, and the interpretation of the results is, therefore, difficult. Moreover, the crystal contained about 3½ per cent, of silicon. These experiments are also open to the objection that, with specimens of the shape used, it is difficult to obtain uniform magnetization except with very strong magnetic fields. The experiments on magnetite were more complete. A single crystal approximately spherical in form was used, and the position of the crystal axes known. In this case, the elongation was found to vary in a regular way with reference to the crystal axes. In neither of these experiments, however, was the intensity of magnetization measured. It may be mentioned that the intensity of magnetization is probably an important factor in the phenomenon of magnetostriction, as this phenomenon seems to be intimately connected with the orientation of the atoms in the magnetic field.