We report on a systematic study of optical properties of (Ga,Mn)As epilayers spanning the wide range of accessible substitutional MnGa dopings. The growth and post-growth annealing procedures were optimized for each nominal Mn doping in order to obtain films which are as close as possible to uniform uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the midinfrared absorption spectra whose position exhibits a prevailing blue-shift for increasing Mn-doping. In the visible range, a peak in the magnetic circular dichroism blue shifts with increasing Mndoping. These observed trends confirm that disorder-broadened valence band states provide a better one-particle representation for the electronic structure of high-doped (Ga,Mn)As with metallic conduction than an energy spectrum assuming the Fermi level pinned in a narrow impurity band.PACS numbers: 74.20. Mn, 74.25.Nf, 74.72.Bk, 74.76.Bz The discovery of ferromagnetism in (Ga,Mn)As above 100 K [1] opened an attractive prospect for exploring the physics of magnetic phenomena in doped semiconductors and for developing advanced concepts for spintronics. Assessment of a wide range of magnetic and transport properties of the material [2][3][4] showed that in ferromagnetic (Ga,Mn)As with Mn dopings x > 1%, disorderbroadened and shifted host Bloch bands represent a useful one-particle basis for describing this mixed-crystal degenerate semiconductor. The common kinetic-exchange model implementation of this valence band theory and the more microscopic tight-binding Anderson model or ab-initio density functional theory can all be shown [5] to be mutually consistent on the level of atomic and orbital resolved band structure. The main utility of valence band theories have been in providing a qualitative and often semi-quantitative description of phenomena originating from the exchange split and spin-orbit coupled electronic structure and in assisting the development of prototype spintronic devices [4]. Other basic physical properties of (Ga,Mn)As, namely those reflecting the vicinity of the metal-insulator transition and localization and electronelectron interaction effects, remain to be fully understood and require to go beyond the commonly employed perturbative or disorder averaged Bloch-band theories.In the insulator non-magnetic regime (x 1%), the system is readily described by localized Fermi level states residing inside a narrow impurity band separated from the valence band by an energy gap of magnitude close to the isolated Mn Ga impurity binding energy. Recently, a debate has been stirred by proposals, based in particular on optical spectroscopy measurements [6], that the narrow impurity band persists in high-doped (Ga,Mn)As with metallic conduction. Several phenomenological variants of the impurity band model have been proposed for the high-doped regime [6][7][8][9][10] which are mutually inconsistent from the perspective of the assumed atomic orbital nature of the impurity band states [5]. Further theoretical inconsistencies arise when recreating ...

We report on a quantitative experimental determination of the three-dimensional magnetization vector trajectory in GaMnAs by means of the static and time-resolved pumpand-probe magneto-optical measurements. The experiments are performed in a normal incidence geometry and the time evolution of the magnetization vector is obtained without any numerical modeling of magnetization dynamics. Our experimental method utilizes different polarization dependences of the polar Kerr effect and magnetic linear dichroism to disentangle the pump-induced out-of-plane and in-plane motions of magnetization, respectively. We demonstrate that the method is sensitive enough to allow for the determination of small angle excitations of the magnetization in GaMnAs. The method is readily applicable to other magnetic materials with sufficiently strong circular and linear magneto-optical effects.

Magnetic linear dichroism and birefringence in (Ga,Mn)As epitaxial layers is investigated by measuring the polarization plane rotation of reflected linearly polarized light when magnetization lies in the plane of the sample. We report on the spectral dependence of the rotation and ellipticity angles in a broad energy range of 0.12-2.7 eV for a series of optimized samples covering a wide range on Mn dopings and Curie temperatures and find a clear blueshift of the dominant peak at energy exceeding the host material band gap. These results are discussed within the framework of the k · p and mean-field kinetic-exchange model of the (Ga,Mn)As band structure. We infer that disorder-induced nondirect transitions significantly influence magneto-optical properties of (Ga,Mn)As.

Investigation of magnetic materials using the first-order magneto-optical Kerr effects (MOKEs) is well established and is frequently used. On the other hand, the utilization of the second-order (or quadratic) magneto-optical (MO) effects for the material research is rather rare. This is due to the small magnitude of quadratic MO signals and the fact that the signals are even in magnetization (i.e., they do not change a sign when the magnetization orientation is reversed), which makes it difficult to separate second-order MO signals from various experimental artifacts. In 2005 a giant quadratic MO effect-magnetic linear dichroism (MLD)-was observed in the ferromagnetic semiconductor (Ga,Mn)As. This discovery not only provided a new experimental tool for the investigation of in-plane magnetization dynamics in (Ga,Mn)As using light at normal incidence, but it also motivated the development of experimental techniques for the measurement of second-order MO effects in general. In this paper we compare four different experimental techniques that can be used to measure MLD and to separate it from experimental artifacts. We show that the most reliable results are obtained when we monitor the polarization of reflected light while the magnetization of the sample is rotated by applying an external magnetic field. Using this technique we measure the MLD spectra of (Ga,Mn)As in a broad spectral range from 0.1 eV to 2.7 eV and we observe that MLD has a magnitude comparable to the polar MOKE signals in this material.

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