Synthesis and Magnetic Properties of Nanostructured Maghemite
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Synthesis and magnetic properties of nanostructured maghemite D. Vollath and D. V. Szab´o Institut f¨ur Materialforschung III, Forschungszentrum Karlsruhe, P.O. Box 3640, D-76021 Karlsruhe, Germany
R. D. Taylor and J. O. Willis Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 9 August 1996; accepted 19 March 1997)
Nanocrystalline maghemite, g –Fe2 O3 , can be synthesized in a microwave plasma using FeCl3 or Fe3 (CO)12 as the precursor. Electron microscopy revealed particle sizes in the range of 5 to 10 nm. In general, this material is superparamagnetic. The magnetic properties are strongly dependent on the precursor. In both cases the production process leads to a highly disordered material with the consequence of a low magnetization. The assumption of a disordered structure is also supported by electron energy loss (EEL) and M¨ossbauer spectroscopy. The structure of this material shows a nearly identical number of cations on tetrahedral and octahedral lattice sites.
I. INTRODUCTION
Nanocrystalline oxides of iron are known to be superparamagnetic, provided that the particle size is sufficiently small. Maghemite (g –Fe2 O3 ) with grain sizes in the range of micrometers is ferrimagnetic.1 Superparamagnetism of solids is caused by thermal fluctuations of the direction of the magnetic moment (N´eel rotation). The transition temperature between the ferrimagnetic and the superparamagnetic state is called the “blocking temperature”. Superparamagnetic materials exhibit a magnetization curve with diminishing hysteresis in the range between the blocking temperature and the Curie temperature. At temperatures below the blocking temperature a remanence is observed. Additionally, measurements of the M¨ossbauer effect show a pure quadrupole splitting above and a magnetic hyperfine structure below the blocking temperature. This is the only possibility to distinguish superparamagnetism caused by spin rotation from a similar phenomenon due to the rotation of the whole particle (Brownian rotation).2,3 In most cases, superparamagnetic materials are synthesized by wet chemical methods. It is the aim of this paper to demonstrate the possibilities of the microwave plasma process4–6 to synthesize superparamagnetic maghemite. Additionally, it will be demonstrated that the magnetic properties are extremely sensitive to the synthesis path. As described earlier,4–6 a microwave plasma enhances the kinetics of chemical reactions. The reason for this is the interaction of charged particles such as electrons, ions, or radicals in the plasma with the J. Mater. Res., Vol. 12, No. 8, Aug 1997
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uncharged species. Because of this interaction, some chemical reactions can be performed at significantly lower temperatures, where they are thwarted kinetically under normal conditions. The use of low reaction temperatures is essential, because this reduces the probability of the formation of
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