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 15K 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 15K 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 150K 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 = 213K for the activation energy. Finally the statistical ferrimagnetic structure model qualitatively reproduces the temperature and magnetic field developments of the gyromagnetic ratio γ/2π.
ASJC Scopus subject areas
- Physics and Astronomy(all)