Crystallographic Orientation and Oxidation of Submicron Whiskers of Iron, Iron—Cobalt, and Cobalt

Abstract

Individual single-crystal iron, iron—cobalt, and cobalt whiskers with diameters from 250 to 7000 Å have been studied by selected area electron diffraction. All of the iron and iron—cobalt diffraction patterns showed a normal bcc structure with the 〈100〉 direction along the long axis of the whisker. All but one of the cobalt whiskers were hexagonal with the 〈11.0〉 along the whisker axis. This one was a smaller whisker which was fcc. Most of the diffraction patterns also showed a second pattern due to the oxide film on the whisker surfaces coherent with the base metal, but many of the patterns from whiskers smaller than about 700 Å contained no oxide spots. The absence of oxide reflections in the diffraction pattern can be attributed to an increasing degree of crystal perfection with decreasing whisker size. In the case of iron, the oxide was identified as Fe3O4 (or γ-Fe2O3) with its 〈110〉 direction coincident with the 〈100〉 of the iron. The (100) plane of iron was parallel to the (100) of the oxide. The iron—cobalt alloy whiskers had cobalt ferrite spots with the same orientation relationship. The cobalt whiskers had their 〈11.0〉 direction and (00.1) plane coincident with the 〈110〉 and (111) of the CoO. Oxidation at temperatures up to 120°C was followed by the changes in magnetic moment and coercive force. Electron diffraction indicated the formation of α-Fe2O3 accompanied by the loss of the single-crystal structure.