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We present inelastic neutron scattering studies of 4 nm maghemite nanoparticles. In applied magnetic fields (1-4 T) at 300 K, we observe a clear signal from collective magnetic excitations (spin precession of the whole particle moment). The precession is coherent since it is present at the antiferromagnetic wave vector transfer, 1.31 Å−1 . At high fields, the precession frequency varies linearly with field, corresponding to a Landé factor of g = 2.03(2). The intensity of the precession signal varies with temperature (in the range 6-300 K) and applied magnetic field in quantitative agreement with a simple thermodynamic model. The effective anisotropy field is estimated to 0.3(1) T.

For pt.I. see ibid, vol.6, p.3081-90, (1994). The magnetic structure of ultrafine maghemite particles was studied by Mossbauer spectroscopy. We have compared the degree of spin alignment obtained in applied magnetic fields in particulate samples with different frozen-in orientational distributions of the easy directions of the magnetization. Full alignment of the spins was not obtained even in large applied fields (4.35 T). The degree of alignment with the applied field was found to be independent of the orientation of the easy directions in the sample when the applied field was larger than 0.75 T. This result shows that the incomplete alignment of the spins in ultrafine maghemite particles subjected to large applied fields is not due to incomplete alignment of the net particle magnetization due to large magnetic anisotropy, but rather stems from a canting of individual spins.

A sol-gel synthesis was used to produce iron oxide aerogels. These nanocrystalline aerogels have a pore-solid structure similar to silica aerogels but are composed entirely of iron oxides. Mössbauer experiments and x-ray diffraction showed that the as-prepared aerogel is an amorphous or poorly crystalline iron oxide, which crystallized as a partially oxidized magnetite during heating in argon. After further heat treatment in air, the nanocrystallites are fully converted to maghemite. The particles are superparamagnetic at high temperatures, but the magnetic properties are strongly influenced by magnetic interactions between the particles at lower temperatures.

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