Abstract
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.