Hyperfine Interactions and Spin Transport in Ferromagnet-Semiconductor Heterostructures

Measurements and modeling of electron spin transport and dynamics are used to characterize hyperfine interactions in Fe/GaAs devices with $n$-GaAs channels. Ga and As nuclei are polarized by electrically injected electron spins, and the nuclear polarization is detected indirectly through the depolarization of electron spins in the hyperfine field. The dependence of the electron spin signal on injector bias and applied field direction is modeled by a coupled drift-diffusion equation, including effective fields from both the electronic and nuclear polarizations. This approach is used to determine the electron spin polarization independently of the assumptions made in standard transport measurements. The extreme sensitivity of the electron spin dynamics to the nuclear spin polarization also facilitates the electrical detection of nuclear magnetic resonance.Comment: Submitted to Phys. Rev.

Rashba spin-orbit coupling and spin relaxation in silicon quantum wells

Silicon is a leading candidate material for spin-based devices, and two-dimensional electron gases (2DEGs) formed in silicon heterostructures have been proposed for both spin transport and quantum dot quantum computing applications. The key parameter for these applications is the spin relaxation time. Here we apply the theory of D'yakonov and Perel' (DP) to calculate the electron spin resonance linewidth of a silicon 2DEG due to structural inversion asymmetry for arbitrary static magnetic field direction at low temperatures. We estimate the Rashba spin-orbit coupling coefficient in silicon quantum wells and find the $T_{1}$ and $T_{2}$ times of the spins from this mechanism as a function of momentum scattering time, magnetic field, and device-specific parameters. We obtain agreement with existing data for the angular dependence of the relaxation times and show that the magnitudes are consistent with the DP mechanism. We suggest how to increase the relaxation times by appropriate device design.Comment: Extended derivations and info, fixed typos and refs, updated figs and data. Worth a re-downloa

Low-temperature spin relaxation in n-type GaAs

Low-temperature electron spin relaxation is studied by the optical orientation method in bulk n-GaAs with donor concentrations from 10^14 cm^{-3} to 5x10^17 cm^{-3}. A peculiarity related to the metal-to-insulator transition (MIT) is observed in the dependence of the spin lifetime on doping near n_D = 2x10^16 cm^{-3}. In the metallic phase, spin relaxation is governed by the Dyakonov-Perel mechanism, while in the insulator phase it is due to anisotropic exchange interaction and hyperfine interactio

Structural and transport properties of GaAs/delta<Mn>/GaAs/InxGa1-xAs/GaAs quantum wells

We report results of investigations of structural and transport properties of GaAs/Ga(1-x)In(x)As/GaAs quantum wells (QWs) having a 0.5-1.8 ML thick Mn layer, separated from the QW by a 3 nm thick spacer. The structure has hole mobility of about 2000 cm2/(V*s) being by several orders of magnitude higher than in known ferromagnetic two-dimensional structures. The analysis of the electro-physical properties of these systems is based on detailed study of their structure by means of high-resolution X-ray diffractometry and glancing-incidence reflection, which allow us to restore the depth profiles of structural characteristics of the QWs and thin Mn containing layers. These investigations show absence of Mn atoms inside the QWs. The quality of the structures was also characterized by photoluminescence spectra from the QWs. Transport properties reveal features inherent to ferromagnetic systems: a specific maximum in the temperature dependence of the resistance and the anomalous Hall effect (AHE) observed in samples with both "metallic" and activated types of conductivity up to ~100 K. AHE is most pronounced in the temperature range where the resistance maximum is observed, and decreases with decreasing temperature. The results are discussed in terms of interaction of 2D-holes and magnetic Mn ions in presence of large-scale potential fluctuations related to random distribution of Mn atoms. The AHE values are compared with calculations taking into account its "intrinsic" mechanism in ferromagnetic systems.Comment: 15 pages, 9 figure

Spin relaxation in low-dimensional systems

We review some of the newest findings on the spin dynamics of carriers and excitons in GaAs/GaAlAs quantum wells. In intrinsic wells, where the optical properties are dominated by excitonic effects, we show that exciton-exciton interaction produces a breaking of the spin degeneracy in two-dimensional semiconductors. In doped wells, the two spin components of an optically created two-dimensional electron gas are well described by Fermi-Dirac distributions with a common temperature but different chemical potentials. The rate of the spin depolarization of the electron gas is found to be independent of the mean electron kinetic energy but accelerated by thermal spreading of the carriers.Comment: 1 PDF file, 13 eps figures, Proceedings of the 1998 International Workshop on Nanophysics and Electronics (NPE-98)- Lecce (Italy

Linear polarization of the photoluminescence of quantum wells

The degree and orientation of the magnetic-field induced linear polarization of the photoluminescence from a wide range of heterostructures containing (Cd,Mn)Te quantum wells between (Cd,Mn,Mg)Te barriers has been studied as a function of detection photon energy, applied magnetic field strength and orientation in the quantum well plane. A theoretical description of this effect in terms of an in-plane deformation acting on the valence band states is presented and is verified by comparison with the experimental data. We attempted to identify clues to the microscopic origin of the valence band spin anisotropy and to the mechanisms which actually determine the linear polarization of the PL in the quantum wells subject to the in-plane magnetic field. The conclusions of the present paper apply in full measure to non-magnetic QWs as well as ensembles of disk-like QDs with shape and/or strain anisotropy.Comment: 21 pages, 10 figure

Manipulation of the Spin Memory of Electrons in n-GaAs

We report on the optical manipulation of the electron spin relaxation time in a GaAs based heterostructure. Experimental and theoretical study shows that the average electron spin relaxes through hyperfine interaction with the lattice nuclei, and that the rate can be controlled by the electron-electron interactions. This time has been changed from 300 ns down to 5 ns by variation of the laser frequency. This modification originates in the optically induced depletion of n-GaAs layer

Optical Orientation in Ferromagnet/Semiconductor Hybrids

The physics of optical pumping of semiconductor electrons in the ferromagnet/semiconductor hybrids is discussed. Optically oriented semiconductor electrons detect the magnetic state of the ferromagnetic film. In turn, the ferromagnetism of the hybrid can be controlled optically with the help of the semiconductor. Spin-spin interactions near the interface ferromagnet/semiconductor play crucial role in the optical readout and the manipulation of ferromagnetism.Comment: The review is written for a special issue "Spin Orientation" of Semiconductor Science and Technology journal dedicated to the memory of B.P. Zakharcheny