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January 15, 2003 Structure of Manganese Zinc Ferrite NanomagnetsS. Calvin1, E.E. Carpenter1, V.G. Harris1, and S.A. Morrison2 Scientists at the Naval Research Laboratory have synthesized nanoscale manganese zinc ferrite (MZFO) particles using colloidal aggregates of amphiphilic molecules (having a water-soluble and a water-insoluble component). By using a technique called extended x-ray absorption fine structure (EXAFS) spectroscopy at beamline X11A, the scientists have determined the distribution of the manganese, zinc, and iron ions between the lattice sites of MZFO to a precision of eight percentage points.
We created monodisperse nanoparticles made of manganese zinc ferrite (MZFO) by using micelles (figure 1), which are colloidal aggregates of hundreds of amphiphilic molecules (having a water-soluble and a water-insoluble component). The micelles were formed by adding a small amount of the surfactants nonylphenol poly(oxyethylene)5 and nonylphenol poly(oxyethylene)9 and water to cyclohexane. Micelle self-assembly sequesters the water into tiny droplets of a uniform size determined by the water-to-surfactant ratio, which limits the size of particles formed via aqueous reactions. In our case, the particles were approximately 11 nanometers in diameter.
In addition to the problem of monodispersion, the atomic-level structure of nanomagnets in their early development is not well known, so that the characteristics of devices made of nanomagnets are not well defined either. In the case of MZFOs, the manganese, zinc, and iron ions can each potentially reside in two different environments: tetrahedral sites surrounded by four oxygens, or octahedral sites surrounded by six oxygens. By knowing the distribution of the ions between these sites, scientists and engineers can better predict the performance of the materials made of MZFOs. We studied the structure of MZFO by using extended x-ray absorption fine structure (EXAFS) spectroscopy at beam line X11A of the National Synchrotron Light Source at Brookhaven National Laboratory. To determine the distributions of the manganese, zinc, and iron ions, we first measured the ratio of absorbed photons versus the total incident photons. Then, we compared the experimental spectra to theoretical spectra, and developed a model of mixed ferrite materials to extract information from the absorption spectra.
In this study, we fit the models to all three metal edges simultaneously, so that the result was constrained to agree with the stoichiometry of the material and lead to a precision of about eight percentage points for the distribution of manganese, zinc, and iron ions (figure 2). The technique outlined here promises to be an important tool in determining the structure of magnetic nanoparticle materials.
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The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
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Nanoscale magnets hold great promise for a wide range of future
technologies: Aside from improving the performance of microwave
filters and power supplies, they could be used in targeted delivery
of pharmaceuticals and as biological sensors. Although scientists
have been synthesizing nanoscale magnets for many years, the
magnetic particles have generally been produced with a wide range of
sizes, whereas most applications require particles with a narrow
size distribution, called monodisperse particles. 
Previous EXAFS studies of magnetic nanoparticles have looked at
each metal absorption spectrum separately, allowing scientists to
identify, for example, that the zinc ions were in a tetrahedral
environment and that the iron and manganese were distributed between
tetrahedral and octahedral environments. But, in these studies,
scientists could not easily untangle the tetrahedral and octahedral
contributions, and determine precise quantitative distributions.