The Brillouin Light Scattering experiment is a powerful and precise tool to investigate the magnetic properties of ultrathin films and nanostructures. The frequency shift of light inelastically scattered from magnons is measured and so the spin wave frequencies of a given system can be determined. By altering experimental parameters, such as the applied magnetic field or the angle between the field and the magnetic axes of the sample, many important magnetic variables (e.g. the saturation magnetisation, the g-factor, the anisotropy constants or the exchange stiffness) can be extracted with a high degree of accuracy. In our set up a 300 mW Ar+-laser (? = 514nm) is used to illuminate the sample. The frequency shift of the backscattered light is measured using a Tandem Fabry-Perot interferometer. Measurements can be performed ex situ, with an electromagnet capable of 1 T, or in situ, inside the Multiple Technique Chamber. The latter set up enables us to measure directly after the deposition of ferromagnetic material, on a sample surface that is virtually free of any contamination. As no capping layer is needed, the sample thickness can be increased gradually within a fraction of a monolayer before each measurement. Therefore in situ BLS represents an ideal tool to study the dynamic properties of ultrathin magnetic structures.
Recently we used in situ BLS to study the spin dynamics in ultrathin bcc Fe films grown on GaAs(001). This system exhibits at room temperature a thickness dependent superparamagnetic to ferromagnetic phase transition  and therefore offers an opportunity to investigate the influence of critical spin fluctuations at the transition thickness and to study the magnetism of nanoclusters in the superparamagnetic regime. We observed a sharp decrease in spin wave frequency as well as a significant broadening of the BLS peaks with decreasing film thickness in the ferromagnetic regime close to the critical thickness. Spin wave modes were also observed in the superparamagnetic regime. They were shown to be the collective modes of the system and to arise due to dipolar coupling between different Fe clusters.
 Y. B. Xu, E. T. M. Kernohan, D. J. Freeland, A. Ercole, M. Tselepi and J. A. C. Bland, Phys. Rev. B 58, 890 (1998)