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Transport of Two-Dimensional Electrons Through Magnetic Barriers

Volker Kubrak

ISBN 978-3-89722-707-1
206 pages, year of publication: 2001
price: 40.50 €
The thesis presents the results of more than three years of research into how one-dimensionally modulated magnetic fields influence the transport properties of a two-dimensional electron gas. The fields are generated by fabricating small cobalt stripes or rectangles on the surface of a GaAs wafer with a 2DEG about 50 nm below the surface. This results in magnetic field modulations with peak widths of a few hundred nanometres, and the amplitude of the modulation is varied between 0 and about 0.3 T (more than 0.5 T for certain geometries) by changing the magnetization state of the microstructured cobalt. Three different types of modulations are considered, as described below.

The comprehensive experimental study of "magnetic barriers" generated in this way is relevant for a number of areas, including lateral spin transistors in semiconductor magneto-electronics, micromagnetometry, and fundamental semiconductor research (e.g. quantum states in inhomogeneous magnetic fields, electron-electron scattering in 2DEGs). The three main results chapters are descibed in more detail in the following. Each of them contains results not yet published elsewhere.

1) Transport Through A Simple Magnetic Barrier (50 pages) A simple magnetic barrier is a single, unipolar magnetic field peak generated by a cobalt rectangle on the surface that has only one edge in the active area of the device. The additional resistance caused by this type of barier was first demonstrated by V. Kubrak et al. [Physica E 6, 755 (2000)] and subsequently extended to barriers of larger amplitude, e.g. in Proceedings ICPS 25 and Proceedings of 8th MMM-Intermag. In addition to the results already published, my thesis also presents Monte Carlo simulations. The simulations clearly show that scattering-assisted transmission and electrons skipping along the device edge (some kind of "edge states") both contribute to the conduction across large-amplitude barriers. Some experimental results at temperatures well below 1K are also shown, and the observed conductance steps or oscillations may be the first evidence for the special quantum states within the barrier.

2) Transport Through A Sign-Alternating Magnetic Barrier (36 pages) In contrast to the first system, a bipolar magnetic field modulation is obtained by a thin cobalt stripe running across the Hall bar. This barrier type was first demonstrated by V. Kubrak et al. in [Physica B 256-258, 380 (1998)]. It was subsequently demonstrated that the dependence of the resistance on the magnetization state of the cobalt stripe can be used to infer hard-axis magnetization curves of the stripe [Appl. Phys. Lett. 74, 2507 (1999)]. In my thesis, these results are complemented by results from further samples and a discussion of two theoretical models. Although the Monte Carlo simulation and a semiclassical snake-orbit model both show that scattering into and out of the snake-orbits (trajectories trapped at the zero-field contour in the centre of the barrier) influences the overall conduction through the barrier, neither is able to explain the experimental results quantitatively.

3) Transport Through Arrays Of Sign-Alternating Barriers (26 pages) The final chapter discusses the magnetoresistance due to ferromagnetic gratings when the external field is applied in the plane of the 2DEG. In these systems, a resistance component proportional to T^2 was recently observed by Overend et al. [Physica B 249-251, 326 (1998)] and by Kato et al. e.g. [Phys. Rev. B 58, 4876 (1998)]. This T^2 component was ascribed to electron-electron umklapp scattering from the periodic magnetic field modulation. My thesis presents new results (not yet published elsewhere) showing that the size of the T^2 component is the same for periodic and aperiodic gratings, ruling out umklapp scattering. The semiclassical model for the magnetoresistance (predicting a MR proportional to the resistivity and thus approximatly linear in T at low T) is also discussed in more detail than elsewhere.

Keywords:
  • Magnetoelectronics
  • Magnetotransport
  • Two-dimensional electron gases
  • Spatially in Lomogeneous magnetic fields
  • Micromagnetometry

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