Temperature Effects in Domain Patterns from Single Crystals of Haematite ( -Fe2O3)

Electron shadow patterns have been used to study the configuration of domains present in single crystals of haematite (α-Fe 2 O 3 ) in the temperature range 0 to 700 °C. The observations were made using a furnace designed to fit into the specimen chamber of the electron diffraction camera. It has been found that once a crystal has been heated to above its Curie temperature the configuration of domains present in the specimen at room temperature is partially stable. Repeated heat treatment of the crystal to above its Curie point leads to a partial `memory effect' similar to that described by Blackman, Haigh and Lisgarten in 1957 for the low temperature transition in haematite. A study of the variation with temperature of the sizes of features in the shadow patterns provides a method of determining the Curie temperature of haematite crystals with an accuracy to 2 to 3 degrees. In this way Curie temperatures between 643 and 688 °C have been observed for crystals from different origins. Finally an attempt has been made to correlate the Curie temperature of various crystals with their lattice parameters as determined by x-ray powder photographs.