Buch, Englisch, 206 Seiten, Format (B × H): 145 mm x 210 mm
Buch, Englisch, 206 Seiten, Format (B × H): 145 mm x 210 mm
ISBN: 978-3-89722-707-1
Verlag: Logos
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 generatedby 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 modulationsare considered, as described below.
The comprehensive experimental study of "magnetic barriers" generated inthis way is relevant for a number of areas, including lateral spintransistors in semiconductor magneto-electronics, micromagnetometry, andfundamental semiconductor research (e.g. quantum states in inhomogeneousmagnetic fields, electron-electron scattering in 2DEGs). The three mainresults chapters are descibed in more detail in the following. Each ofthem 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 peakgenerated by a cobalt rectangle on the surface that has only one edgein the active area of the device. The additional resistance causedby this type of barier was first demonstrated by V. Kubrak et al.[Physica E 6, 755 (2000)] and subsequently extended to barriers oflarger amplitude, e.g. in Proceedings ICPS 25 and Proceedings of8th MMM-Intermag. In addition to the results already published, my thesisalso presents Monte Carlo simulations. The simulations clearly show thatscattering-assisted transmission and electrons skipping along the deviceedge (some kind of "edge states") both contribute to the conductionacross large-amplitude barriers. Some experimental results at temperatureswell below 1K are also shown, and the observed conductance steps oroscillations may be the first evidence for the special quantum stateswithin the barrier.
2) Transport Through A Sign-Alternating Magnetic Barrier (36 pages)In contrast to the first system, a bipolar magnetic field modulationis obtained by a thin cobalt stripe running across the Hall bar. Thisbarrier type was first demonstrated by V. Kubrak et al. in[Physica B 256-258, 380 (1998)]. It was subsequently demonstratedthat the dependence of the resistance on the magnetization state of thecobalt stripe can be used to infer hard-axis magnetization curves of thestripe [Appl. Phys. Lett. 74, 2507 (1999)]. In my thesis, these resultsare complemented by results from further samples and a discussion oftwo theoretical models. Although the Monte Carlo simulation and asemiclassical snake-orbit model both show that scattering into and outof the snake-orbits (trajectories trapped at the zero-field contourin the centre of the barrier) influences the overall conduction throughthe barrier, neither is able to explain the experimental resultsquantitatively.
3) Transport Through Arrays Of Sign-Alternating Barriers (26 pages)The final chapter discusses the magnetoresistance due to ferromagneticgratings when the external field is applied in the plane of the 2DEG.In these systems, a resistance component proportional to T^2 was recentlyobserved by Overend et al. [Physica B 249-251, 326 (1998)] and by Katoet al. e.g. [Phys. Rev. B 58, 4876 (1998)]. This T^2 component was ascribedto electron-electron umklapp scattering from the periodic magnetic fieldmodulation. My thesis presents new results (not yet published elsewhere)showing that the size of the T^2 component is the same for periodic andaperiodic gratings, ruling out umklapp scattering. The semiclassical modelfor the magnetoresistance (predicting a MR proportional to the resistivityand thus approximatly linear in T at low T) is also discussed in more detailthan elsewhere.
Kubrak
Transport of Two-Dimensional Electrons Through Magnetic Barriers jetzt bestellen!
The comprehensive experimental study of "magnetic barriers" generated inthis way is relevant for a number of areas, including lateral spintransistors in semiconductor magneto-electronics, micromagnetometry, andfundamental semiconductor research (e.g. quantum states in inhomogeneousmagnetic fields, electron-electron scattering in 2DEGs). The three mainresults chapters are descibed in more detail in the following. Each ofthem 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 peakgenerated by a cobalt rectangle on the surface that has only one edgein the active area of the device. The additional resistance causedby this type of barier was first demonstrated by V. Kubrak et al.[Physica E 6, 755 (2000)] and subsequently extended to barriers oflarger amplitude, e.g. in Proceedings ICPS 25 and Proceedings of8th MMM-Intermag. In addition to the results already published, my thesisalso presents Monte Carlo simulations. The simulations clearly show thatscattering-assisted transmission and electrons skipping along the deviceedge (some kind of "edge states") both contribute to the conductionacross large-amplitude barriers. Some experimental results at temperatureswell below 1K are also shown, and the observed conductance steps oroscillations may be the first evidence for the special quantum stateswithin the barrier.
2) Transport Through A Sign-Alternating Magnetic Barrier (36 pages)In contrast to the first system, a bipolar magnetic field modulationis obtained by a thin cobalt stripe running across the Hall bar. Thisbarrier type was first demonstrated by V. Kubrak et al. in[Physica B 256-258, 380 (1998)]. It was subsequently demonstratedthat the dependence of the resistance on the magnetization state of thecobalt stripe can be used to infer hard-axis magnetization curves of thestripe [Appl. Phys. Lett. 74, 2507 (1999)]. In my thesis, these resultsare complemented by results from further samples and a discussion oftwo theoretical models. Although the Monte Carlo simulation and asemiclassical snake-orbit model both show that scattering into and outof the snake-orbits (trajectories trapped at the zero-field contourin the centre of the barrier) influences the overall conduction throughthe barrier, neither is able to explain the experimental resultsquantitatively.
3) Transport Through Arrays Of Sign-Alternating Barriers (26 pages)The final chapter discusses the magnetoresistance due to ferromagneticgratings when the external field is applied in the plane of the 2DEG.In these systems, a resistance component proportional to T^2 was recentlyobserved by Overend et al. [Physica B 249-251, 326 (1998)] and by Katoet al. e.g. [Phys. Rev. B 58, 4876 (1998)]. This T^2 component was ascribedto electron-electron umklapp scattering from the periodic magnetic fieldmodulation. My thesis presents new results (not yet published elsewhere)showing that the size of the T^2 component is the same for periodic andaperiodic gratings, ruling out umklapp scattering. The semiclassical modelfor the magnetoresistance (predicting a MR proportional to the resistivityand thus approximatly linear in T at low T) is also discussed in more detailthan elsewhere.
Autoren/Hrsg.
Fachgebiete
- Technische Wissenschaften Elektronik | Nachrichtentechnik Elektronik Bauelemente, Schaltkreise
- Technische Wissenschaften Elektronik | Nachrichtentechnik Elektronik Mikroprozessoren
- Naturwissenschaften Physik Elektromagnetismus Elektrizität, Elektrodynamik
- Naturwissenschaften Physik Elektromagnetismus Magnetismus
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