Browsing by Subject "magnetoresistance"

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    Non-equilibrium Quantum Transport in Two Dimensional Electron Gases in Modulation Doped Heterostruvtures
    (2017-08) Ebner, Quentin
    This thesis focuses on experimental investigations of transport in two dimensional electron gases (2DEG) driven out of equilibrium by microwaves and dc current. Part I presents the background information in four sections that describe the basics of magneto transport, Shubnikov-de Haas oscillations (SdHO), microwave-induced resistance oscillations (MIRO), and Hall field-induced resistance oscillations (HIRO). The second part of this thesis is broken into three sections that each discuss experimental findings of MIRO and HIRO in various 2DEGs. The fifth section discusses the observation of MIRO and HIRO in p-type Germanium quantum wells. This is significant because previous studies into a variety of different materials never showed any MIRO response. P-type Germanium is the second material for which MIRO and HIRO have been observed, making MIRO and HIRO not unique to GaAs quantum wells. The sixth section looks into the role of introducing alloy disorder into the quantum well on MIRO and HIRO. The quantum scattering rate was found to increase with the amount of alloy disorder and the MIRO prefactor that is decided by scattering times was not found to have a measurable change with increasing alloy disorder concentration. It was found the amplitude of HIRO is controlled by sharp disorder scattering contribution in mobility. The seventh section provides a systematic study into the effects of density on MIRO, which is not well understood. The analysis focuses on when one energy sub-band is populated, but also gives some observations of when two sub-bands are populated. When one sub-band is populated, increasing density was found to suppress the effective mass extracted from MIRO. The change in MIRO amplitude was found to be described by a combination of the zero-field resistance and the increase of quantum lifetime as density increased. When the second sub-band was populated, MIRO was found to be highly suppressed and this suppression cannot be explained by the MIRO mechanisms, quantum lifetime, or MIRO’s power factor.
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    Transition Shifts in Heat-Assisted Magnetic Recording and Magnetoresistance Enhancement by Wave-Vector Filtering
    (2017-12) Wang, Ziran
    In the first part of this thesis, micromagnetic simulations are employed to study transition shifts induced by nonequilibrium spin dynamics in HAMR. These dynamical effects, including superparamagnetic (SP) switching, spin temperature lag, and switching time, cannot be captured by static analytical models. It is found that changes in head velocity and damping both introduce transition shifts due to their effects on the spin cooling rate. The transition positions obtained from simulations are compared to the predictions of an analytical model. It is shown that the analytical model overestimates the write temperature when the thermal profile is wide because SP switching is neglected; whereas for a narrow thermal profile, the analytical model underestimates the write temperature because the switching time is assumed to be zero. By investigating the transition shifts due to nonzero head field risetime (~0.1 ns), the effect of SP switching is found to be the same order of magnitude as the risetime and is dominant. Approaches based on spontaneous magnetization, damping constant of a block of spins, and spin energy are proposed to determine spin temperature and its lag relative to lattice temperature. The lag is determined to be of the order of 0.01 ns with typical HAMR parameters. Finally, the switching speed of a HAMR process is extracted from the head velocity effect, and is found to be ~0.05 ns. This high switching speed is confirmed by atomistic simulations under decreasing temperature. In the second part, the magnetoresistance (MR) of Fe/Ag/Fe/InAs/Ag(100) in the current-perpendicular-to-plane geometry is studied based on a tight-banding model with full spd bands, the Landauer-Büttiker formalism, and the recursive Green’s function technique. Results show that the system's MR can reach values above 1000%. This MR enhancement mainly is a result of the wave-vector filtering effect imposed by the InAs layer, restricting conductance within a small region around the Gamma point in the 2D Brillouin zone. Calculations also reveal that when the Fermi level sits in the InAs band gap, the MR gradually saturates as a function of the InAs thickness; whereas when the Fermi level is in the InAs conduction band and close to the band bottom, the MR exhibits an oscillatory behavior with a large period. MR oscillations are also observed with respect to the Ag thickness, with amplitudes determined by the Fermi level position relative to the InAs conduction band edge. The oscillation periods in both cases can be well explained by the concept of quantum-well states, and are determined by the spanning vector of the Fermi surface belly of the corresponding material. These MR oscillations are due to the quantum interference of conduction electrons near the Gamma point. The MR and area-resistance product (RA) profiles at a wide range of Fermi energies are compared. Near the MR peak (with MR above 1000%) in the InAs conduction band, the RA can be as low as 8.8 Ωμm^2. This feature of large MR but small RA results from the wave-vector filtering effect of doped InAs, and it makes the structure under study distinct from conventional giant-magnetoresistance systems (small MR, small RA) and magnetic tunnel junctions (large MR, large RA).