We report ferromagnetic and spin wave resonance absorption measurements on high quality epitaxially grown Ga1−xMnxAs thin films. We find that these films exhibit robust ferromagnetic long-range order, based on the fact that up to seven resonances are detected at low temperatures, and the resonance structure survives to temperatures close to the ferromagnetic transition. On the other hand, we observe a spin wave dispersion which is linear in mode number, in qualitative contrast with the quadratic dispersion expected for homogeneous samples. We perform a detailed numerical analysis of the experimental data and provide analytical calculations to demonstrate that such a linear dispersion is incompatible with uniform magnetic parameters. Our theoretical analysis of the ferromagnetic resonance data, combined with the knowledge that strain-induced anisotropy is definitely present in these films, suggests that a spatially dependent magnetic anisotropy is the most likely reason behind the anomalous behavior observed.

We study the magnetic properties of metallic Ga 1−x Mn x As. We calculate the effective Ruderman-Kittel-Kasuya-Yoshida interaction between Mn spins using several realistic models for the valence band structure of GaAs. We also study the effect of positional disorder of the Mn on the magnetic properties. We find that the interaction between two Mn spins is anisotropic due to spin-orbit coupling both within the so-called spherical approximation and in the more realistic six band model. The spherical approximation strongly overestimates this anisotropy, especially for short distances between Mn ions. Using the obtained effective Hamiltonian we carry out Monte Carlo simulations of finite and zero temperature magnetization and find that, due to orientational frustration of the spins, noncollinear states appear in both valence band approximations for disordered, uncorrelated Mn impurities in the small concentration regime. Introducing correlations among the substitutional Mn positions or increasing the Mn concentration leads to an increase in the remanent magnetization at zero temperature and an almost fully polarized ferromagnetic state.

The electronic states of a free-standing widegap semiconductor quantum nanowire are investigated. We applied the variational technique using the effective mass approximation within the k · p model to a thin cylindrical GaAs nanowire. The energy dispersion, the distribution of the spinor components, and the optical absorption spectra are presented. The hole dispersion shows a typical "camel back" structure. The effect of the dielectric mismatch between the wire and the surrounding medium and the influence of the growth direction of the wire are examined. We find that the photoluminescence energy of the exciton is larger in the ͓001͔ configuration than the ͓111͔ configuration.

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We investigate the possibility of using local magnetic fields to produce one-dimensional traps in hybrid structures for any quasiparticle possessing spin degree of freedom. We consider a system composed of a diluted magnetic semiconductor quantum well buried below a micron-sized ferromagnetic island. Localized magnetic field is produced by a rectangular ferromagnet in close proximity of a single domain phase. We make quantitative predictions for the optical response of the system as a function of distance between the micromagnet and the quantum well, electronic g-factor, and thickness of the micromagnet.The use of the spin of quasi-particles, instead of their charge, as a basis for the operation of a new type of electronic devices has attracted the attention of a large interdisciplinary research community. The interest is not only in the phenomenology but also in the large scale production and applicability of such devices [1]. In this paper we show that the spin degree of freedom can be utilized for achieving spatial localization of charged quasiparticles such as electrons, holes or trions [2,3], as well as of neutral complexes such as excitons [4,5]. This is of interest for spintronic applications. In diluted magnetic semiconductors (DMS) like CdMnTe, due to the exchange interaction between delocalized band electrons and localized magnetic ions, the presence of a static magnetic field leads to a giant Zeeman splitting between band states for different spin components. As a consequence, the effective g-factor for the DMS is very high and temperature dependent [6,7].Here we consider a hybrid structure of a CdMnTe/CdMgTe quantum well (QW) at tens of nanometers below a rectangular ferromagnetic island. We find that due to the giant Zeeman interaction, the non-homogeneous magnetic field produced by the rectangular ferromagnetic island acts as an effective potential that can efficiently "trap" spin polarized quasi-particles in the QW. We present quantitative predictions for the optical response of the DMS where the localization of the quasi-particles is evident. We also discuss how these predictions are sensitive to the variation of certain parameters, such as the distance between the QW and the ferromagnetic island, the thickness of the micromagnet, and the electronic g-factors (g e -g-factor of the electron, g h -g-factor of the hole).We consider a typical rectangular micromagnet (iron) of dimensions D x =6 µm, D y =2 µm and D z =0.15 µm in a single domain state [8], with magnetization pointing in the x-direction [9, 10, 11], see Fig. 1. The single * Electronic address: Pawel.Redlinski.1@nd.edu domain state of the micromagnet was investigated with micro-magnetic simulation using the OOMMF package made available by NIST [12]. In the absence of an external magnetic field the micromagnet has a multi-domain structure. The simulation shows that after magnetizing the sample with a field of 1 T and then reducing the field to 0 T, a value of 0.2 T is enough to restore a state that is, for our purposes, sufficiently close ...