Mixed Iron-Manganese Oxide Nanoparticles Jriuan Lai, ²,‡ Kurikka V. P. M. Shafi, ²,‡ Abraham Ulman,* ,²,‡ Katja Loos, ²,‡ Nan-Loh Yang, §,‡ Min-Hui Cui, §,‡ Thomas Vogt, | Claude Estourne` s, ⊥ and Dave C. Locke # Department of Chemical Engineering, Chemistry & Material Science, Polytechnic UniVersity, 6 Metrotech Center, Brooklyn, New York 11201, Department of Chemistry, CUNY at Staten Island, 2800 Victory BouleVard, Staten Island, New York, Physics Department and Center for Functional Nanomaterials, BrookhaVen National Laboratory, P.O. Box 5000, Upton, New York 11973-5000, UMR7504 CNRS-ULP, Institut de Physique et Chimie des Mate´ riaux de Strasbourg, Cedex, France, The NSF MRSEC for Polymers at Engineered Interfaces, and Department of Chemistry and Biochemistry, CUNY Queens College, 60-30 Kissena BouleVard, Remsen 206, Flushing, New York 11367 ReceiVed: January 7, 2004; In Final Form: April 14, 2004 Designing nanoparticles for practical applications requires knowledge and control of how their desired properties relate to their composition and structure. Here, we present a detailed systematic study of mixed iron-manganese oxide nanoparticles, showing that ultrasonication provides the high-energy reaction conditions required for complete atomic level mixing of Fe(III) and Mn(III) when amorphous Fe 2 O 3 nanoparticles are irradiated in the presence of Mn 2 (CO) 10 in ambient atmosphere. X-ray diffraction (XRD) results reveal that the crystal structure of manganese iron mixed oxide nanoparticles changes from spinel to bixbyite with increasing of Mn(III) content. The results of room-temperature magnetization curves are consistent with the XRD patterns and spin density from electron paramagnetic resonance measurements, showing samples converting from superparamagnetic to antiferromagnetic, when the crystal structures of these samples transform from spinel to bixbyite. Introduction Confinement and quantum size effects in nanoparticles induce properties that are significantly different from those of the bulk material as a result of their reduced dimensions. 1 Because of these unique properties, nanomaterials have become a focus of research in modern technology. For example, semiconductor nanoparticles (quantum dots) exhibit discrete energy bands and size-dependent band gap energies; conducting nanoparticles exhibit large optical polarizabilities and nonlinear electrical conductance; and ferromagnetic nanoparticles become super- paramagnetic, with size-dependent magnetic susceptibilities. 2-7 However, studies on magnetic nanoparticles are scant compared to, say, those on semiconductor nanoparticles. Magnetic nanoparticles have applications in information storage, 8 color imaging, 9 magnetic refrigeration, 10 bioprocess- ing, 11 medical diagnosis, 12,13 and controlled drug delivery 14 and as ferrofluids. 15 Thus, developing new synthetic routes for magnetic nanoparticles and the investigation of their properties are of great importance. 16 Ferrites, the transition metal oxides having a spinel structure, are used in magnetic inks 17 and magnetic fluids 18 and for the fabrication of magnetic cores of read/write heads for high-speed digital tapes or for disk recording. 19 One important example is the electrically conductive manganese oxide, which stores electrical charge by a double insertion of electrons and cations into the solid state. 20 Maghemite [γ-Fe 2 O 3 , the ferrimagnetic cubic form of iron(III) oxide] is being used widely for the production of magnetic materials and in catalysis. Because of the small coercivity of Fe 2 O 3 nanoparticles, they can be used as magnetooptical devices. In this paper, we discuss the magnetic properties of sonochemically synthesized mixed iron manganese oxide nanoparticles. Various methods have been reported for the synthesis of metal oxide nanoparticles, such as wet chemical, 20,21 electrochemical, 22 and pyrolysis 23,24 techniques, sol-gel reactions, 25,26 and chemi- cal oxidation in micellar media 27 or in polymer 28,29 or mineral matrixes. 30,31 The wet chemical methods include coprecipita- tion, 32-34 spray drying, 35 and hydrothermal processes. 36 The conventional high-temperature ceramic method for the prepara- tion of ferrites can result in the loss of their fine particle nature. The fine ferrite particles are also produced by grinding coarse powders of high-purity bulk material in the presence of kerosene and oleic acid (an organic surfactant). 37 Surfactant nanostructures (reverse micelles) have also been used to synthesize nanosized ferrite particles. 38-40 Chemists are accustomed to thinking in terms of activation energy. However, in acoustic cavitation, where temperatures in the excess of 5000 K and pressures of 800 atm are produced, 41-43 activation energy becomes meaningless, and chemical reactions can take place that otherwise are impossible. As a result, using ultrasonication, one can achieve mixing at the atomic level of the constituent ions in the amorphous phase so that the crystalline phase can be obtained by annealing at relatively low temperatures. 44 Sonochemistry has been used to prepare various kinds of nanostructured amorphous magnetic materials. 45-52 * To whom correspondence should be addressed. E-mail: aulman@ duke.poly.edu. ² Polytechnic University. ‡ The NSF MRSEC for Polymers at Engineered Interfaces. § CUNY at Staten Island. | Brookhaven National Laboratory. ⊥ Institut de Physique et Chimie des Mate´riaux de Strasbourg. # CUNY Queens College. 14876 J. Phys. Chem. B 2004, 108, 14876-14883 10.1021/jp049913w CCC: $27.50 © 2004 American Chemical Society Published on Web 08/27/2004