Magnetic properties of ferromagnetic ultrafine particles prepared by vacuum evaporation on running oil substrate

Ultrafine particles of ferromagnetic metals (Fe, Co and Ni) were prepared by the vacuum evaporation onto a running oil substrate. Particles thus obtained were suspended in the oil and their average diameter was about 25 Å. An electron diffraction analysis indicated that the particles were oxidized and the main component was Fe₃O₄, CoO and NiO for Fe, Co and Ni fine particles, respectively. From magnetic measurements in the temperature range between 4.2 and 300 K, the main magnetic behavior of the fine particles can be explained in terms of the superparamagnetism or superantiferromagnetism. Furthermore, it was concluded that the Fe₃O₄ particles may be covered with several atomic layers of α-Fe₂O₃. For CoO and NiO particles, existence of an imperfect oxide structure was required besides the main components in order to explain the magnetic behavior.