Synthesis of Crystalline Pyramidal ε-FeSi and Morphology- and Size-Dependent Ferromagnetism

Crystalline pyramidal epsilon-FeSi particles smaller than 1 mu m in size with {111} lateral facets are synthesized by a spontaneous chemical vapor deposition method. The nanocrystals initially nucleate from the amorphous film via self-clustering forming a rectangular epsilon-FeSi (001) terrace as a result of the cubic crystalline structure and subsequent anisotropic accumulation on the terrace produces the pyramidal morphology. Room-temperature ferromagnetism is observed from epsilon-FeSi particles larger than 250 nm and having the {111} facets. A model is postulated to explain the morphology- and size-dependent ferromagnetism based on the nonuniform Fe atomic arrangement that forms atomic-scale islands on the surface and dipole interaction among these islands in the large enough particles. The morphology- and size-dependent ferromagnetism allows control of the magnetic moments of mesostructures and is important to spintronics and other applications.