Nanostructured materials have been intensively studied in recent years particularly because the physical properties of these materials are quite different from those of the bulk. 1-3 A variety of chemical and physical preparative methods have been developed to produce materials with nanometer domain size, including metal evaporation, 4, 5 reduction of metal salts by borohydride derivatives, 6, 7 laser pyrolysis, 8 and thermal decomposition. 9 Colloids of ferromagnetic materials are of special interest due to their many important technological applications as ferrofluids. 10, 11 Such magnetic fluids find uses in information storage media, magnetic refrigeration, audio reproduction, and magnetic sealing. 12, 13 Commercial magnetic fluids are generally produced by exhaustive grinding of magnetite (Fe3O4) in ball or vibratory mills for several weeks in the presence of surfactants, which produces a very broad particle size distribution. 10 Chemical methods such as thermolysis of organometallic compounds and metal evaporation have also been applied to produce colloids of ferromagnetic materials. 14, 15 We present here a new method for the preparation of stable ferromagnetic colloids of iron using high-intensity ultrasound to sonochemically decompose volatile organometallic compounds. These colloids have narrow size distributions centered at a few nanometers and are found to be superparamagnetic. Sonochemistry arises from acoustic cavitation, the formation, growth, and implosive collapse of bubbles in a liquid. 16 The collapse of bubbles generates localized hot spots with transient temperatures of∼ 5000 K and cooling rates in excess of 1010 K/s. 17, 18 Volatile organometallic compounds inside the cavitating bubble are decomposed to yield individual metal atoms as shown by the observed sonoluminescence from electronic exited states of metal atoms, including Fe. 19 In alkanes, in the absence of any trapping agent, these atoms agglomerate to produce various highly porous nanostructured materials including amorphous metals, alloys, and carbides. 19-23 We have now found it possible to stabilize the nanometer-sized cluster produced during cavitation, preventing their agglomeration and permitting the isolation of stable nanocolloids. In this work, a volatile organometallic precursor, Fe (CO) 5, was sonochemically decomposed in the presence of a stabilizer to create a nanosized iron colloid. Ultrasonic irradiation of 0.2 mL of Fe (CO) 5 in 20 mL of octanol with 1 g of polyvinylpyrrolidone (PVP, average molecular weight of 40 000, Sigma Chemicals) at 20 C under a rigorously oxygen free argon atmosphere produced a black colloidal solution. The TEM (transmission electron microscopy) image in Figure 1 shows that iron particles in the polymer matrix range in size from 3 to 8 nm. Electron microdiffraction shows that these iron clusters are amorphous as initially formed (Figure 1, bottom left). By irradiating the particles with a high-intensity electron beam in the TEM chamber, crystallization was induced presumably from local heating in the electron beam. Electron microdiffraction following the in situ crystallization revealed dense ring patterns with d spacings of 2.0, 1.4, and 1.2 Å, which match the standard body centered cubic iron lines (Figure 1, bottom right). Electron microdiffraction also shows a very weak ring pattern with d spacing of 2.5 Å, which corresponds to a slight contamination from the (111) line of cubic iron oxide FeO. Oleic acid (octadec-9-ene-1-carboxylic acid) can also serve as a colloid stabilizer. 10 The tail of oleic acid is kinked at the double bond, which is critical to its effectiveness: stearic acid, for example, does not stabilize the colloid …