The picosecond and subpicosecond dynamics of spins in intrinsic and n-doped GaAs is investigated at room temperature using the time-resolved, pump and probe, photoinduced, near-resonant magneto-optical Kerr effect between 1.44 and 1.63 eV. Three components with different temporal and spectral behavior are distinguished in both Kerr rotation and ellipticity, and their origin is discussed in relation with theoretical predictions and simulations. Two contributions are attributed to the splitting of the spin sublevels. The first one, present only as long as pump and probe pulses coincide in time, is a coherent response accounted for in terms of the optical Stark effect, whereas the longer decay of the second one, up to 35 ps, is attributed to electron spin relaxation. The third component is assumed to arise from the difference in population of the photoexcited states and decays within a time smaller than the pulse duration, which is attributed to the energy relaxation of electrons towards the bottom of the conduction band through carrier-carrier and carrier-phonon scattering.
Optical and magneto-optical properties of maghemite superparamagnetic particles dispersed (f=0.28 vol % γ-Fe2O3) in a silica gel matrix have been studied at room temperature in the 1.5–3.7 eV spectral range. A permanent optical anisotropy can be induced when applying a magnetic field during the sol-gel freezing process. The permanent linear dichroism and birefringence in such in-plane magnetically textured samples are attentively studied. The spectral dispersion of Faraday effects in our textured samples is analyzed taking into account the in-plane optical anisotropy. Such a composite material is a promising candidate for future magneto-optical applications.
This paper is devoted to magnetic and structural properties of anisotropic ␥-Fe 2 O 3 superparamagnetic particles dispersed in a transparent xerogel matrix. The effect of frozen anisotropy axes and magnetic texture, induced by a magnetic field applied during the solidification of the matrix on the in-field magnetization process, is studied by alternating gradient force magnetometry and first and second order magneto-optical effects. The changes of magnetization curves with respect to the ferrofluid solution at the same particle concentration are interpreted on the basis of an existing statistical approach extended to systems with particle size distribution, which has to be taken into account for real samples. A very good agreement between the experiment and theory was achieved for a log-normal distribution of diameters which well resembles that deduced from electron microscopy observations in different imaging modes. This structural analysis states the parameter values used in calculations and confirms the relevance of basic assumptions of the model for the specimens studied. The experimental results and the related theoretical discussion should be of use to understand magnetic properties of other magnetically textured superparamagnetic systems.
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