Dynamics of Magnetic Nanoparticles Studied by Neutron Scattering

The fluctuations of the magnetization direction in nanoparticles are currently subject to much attention. Studies of these phenomena are motivated not only by interest in the fundamental properties of ultrafine particles but also by the applications of such particles in, for example, magnetic recording media, ferrofluids, catalysts, etc. [1,2]. Most experimental studies of magnetic fluctuations in nanoparticles are carried out by use of ac and dc susceptibility measurements and by Mossbauer spectroscopy. Combining these techniques, it is possible to cover relaxation times in the range from more than 10 4 to 10 29 s. The superparamagnetic relaxation times have been found to vary with temperature in accordance with the Neel-Brown theory [3,4], apart from some observations at very low temperatures which suggest that macroscopic quantum tunneling may be the most important mechanism under these conditions [5]. At temperatures where the thermal energy is comparable to or larger than the magnetic anisotropy energy, the superparamagnetic relaxation time may be in the picosecond range. Apart from the superparamagnetic relaxation (fluctuations of the magnetization vector among the easy directions of magnetization), the magnetization vector also performs fast oscillations in the vicinity of the easy direction of magnetization (collective magnetic excitations) [6,7], with frequencies up to the THz range. The time scales of magnetization measurements and Mossbauer spectroscopy are too long to be used to measure these frequencies. In order to understand the full dynamics of the magnetization in nanoparticles, new