Thermal evolution of the magnetization in nanocrystalline Fe particles investigated by electron holography

Micrometric, irregularly shaped Fe particles with a nanocrystalline structu re have been prepared by mechanical attrition through ball-milling. Electro n holography has been employed to visualize the stray field emerging from i solated Fe particles, both at 300 K and at selected temperatures T less tha n or equal to 1200 K, from which indirect information on the magnetic domai n configuration has been inferred. By complementary x-ray diffraction and t ransmission electron microscopy investigations a relationship has been esta blished between the changes of the leakage field and of the microstructure upon annealing: it indicates that the structural evolution is accompanied b y strong modifications in the interior magnetization pattern. This relation ship finds explanation in the framework of the random anisotropy model, inc luding temperature-induced reversible variations in the exchange correlatio n length and saturation magnetization. Moreover, the role played by the ove rall geometrical features of the particles in the determination of the actu al domain configuration has been investigated. (C) 2001 American Institute of Physics.