On the Magnetic Transformation and the Domain Structure of Magnetite

The behaviour of magnetite has been examined in the region of its low-temperature transformation (similar 120 °K) using the electron shadow technique. A marked change in the pattern was found, and the variation in the size of the features in the pattern was used to determine the transformation temperature and the breadth of the transformation. Synthetic crystals had markedly smaller transformation breadths (similar 0.6 °K) than the natural crystals (9° to 11 °K). In one case (a Zermatt magnetite) it was established that the material in the central region of the crystal had an appreciably lower transformation temperature than that found for the original crystal as a whole; thus the inhomogeneous nature of the crystal was demonstrated. The shadow patterns also allowed an estimate to be made of the domain size. Below the transformation this was about 60 μm for natural crystals and about 30 μm for the synthetic crystals. Above the transformation the surface magnetic fields were much weaker and the domain sizes smaller by a factor of 2.