Brillouin Light Scattering Study on Magnetization Dynamics in Superparamagnetic Co–Al–O Granular Films Down to 15 K

Superparamagnetic (SPM) excitations of Co–Al–O nano-granular films, especially of Co50.3Al20.4O29.3 film, were intensively investigated by means of Brillouin light scattering in a temperature range between 300 and 15 K under external magnetic fields of up to 4.5 kOe. We successfully observed a pair of broad peaks with asymmetric intensity in our spectrum through the above temperature range. A numerical analysis of the spectrum by employing the theory developed by Camley and Mills reveals that the SPM excitations consist of the bulk-type and Damon–Eshbach (DE)-type modes and exhibits a singlet–doublet peak structure. We determined the temperature and magnetic-field developments of the gyromagnetic ratio, the field-induced magnetization, the peak frequencies and width, the effective magnetic anisotropy field, and the peak intensities. We found that the peak intensities at 15 K are only reduced about 50% of the intensities at 300 K. Furthermore, the SPM peak intensities can be well fitted by a linear function of temperature T given by A + BT below 150 K in contrast with the T-proportional intensities expected from the high-temperature approximation for the Bose–Einstein factor in the spin-wave response function. While the bulk and DE-type peak frequencies are almost independent of temperature in a wide temperature range above 50 K, the peak width shows a broad peak and the peak-maximum temperature shifts to higher temperature side for higher magnetic field. These temperature behaviors can be qualitatively reproduced by the Néel–Brown relaxation model and the Debye relaxation function. We obtained τ0 = 2.2 × 10−12 s for the attempt relaxation time and KV/kB = 213 K for the activation energy. Finally the statistical ferrimagnetic structure model qualitatively reproduces the temperature and magnetic field developments of the gyromagnetic ratio γ/2π.