Browsing by Subject "Spin transport"

Now showing 1 - 4 of 4
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    Spin polarized charge carrier injection, transport, and detection in organic semiconductors.
    (2011-05) Yunus, Mohammad
    In this thesis we explore spin polarized charge carrier injection, transport, and detection in organic semiconductors. Device structures considered have one or more ferromagnetic contacts to the organic semiconductor, and the condition for which charge carrier injection from ferromagnetic contacts is strongly spin polarized is discussed. Spin injection into semiconductors can be greatly enhanced if the injection mechanism is spin selective, such as is the case for tunnelling from ferromagnetic contacts. By contrast, if the carrier injection is by thermionic emission or another process that does not depend on spin, the injection is only weakly spin polarized. To discuss spin transport and spin detection, we consider a unipolar organic spin valve consisting of an organic semiconductor layer sandwiched between two ferromagnetic contacts. The polarizations of the magnetic contacts can be parallel or anti-parallel. Spin and charge carrier transport in the organic semiconductor is described by spin dependent transport equations in drift-diffusion approximation and the spin detection process is through magneto-resistance. We discuss the impact of various degrees of spin relaxation in organic semiconductors on the spatial variation of the spin current and its effect on magneto-resistance. The spatial profile of the spin current inside the organic semiconductor depends not only on the spin diffusion length but also on the alignment of the contact polarizations. However, the magneto-resistance decreases strongly with decreasing spin diffusion length. Electron tunnelling from a ferromagnetic contact can have significant spin dependence because the spatial part of the electron wave function is different for the majority and minority spin states of the ferromagnetic contacts. The tunnelling process occurs from the ferromagnetic contact through an insulating layer into the organic semiconductor. The insulating layer is modeled first as an ohmic layer with spin dependent contact resistances. The effectiveness of spin dependent contact resistances on spin polarized injection and magneto-resistance is examined on the basis of a simple analytical model. We then model the insulating layer as a tunnel barrier with spin dependent rate equations. Both majority and minority spin electrons of the ferromagnetic contact tunnel through the insulating layer into the localized molecular states of the organic semiconductor at the semiconductor/insulator interface. Tunnelling matrix elements and transition rates of the two spin types are calculated using a Transfer Hamiltonian approach. The transition rates are thus spin dependent and used in rate equations to calculate the injected (extracted) current for carriers of either spin direction. We explore the various aspects of the ferromagnetic contacts, the thickness and barrier height of the insulating layer, and the energy of the localized molecular states on spin injection and magneto-resistance. Consistent with the experimental data, the spin injection from ferromagnetic contacts can be either positive or negative, and the magneto-resistance decreases strongly with the applied bias across the device.
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    Spin transport and current induced magnetization dynamics in magnetic nanostructures.
    (2010-12) Chen, Xi
    The study of the interaction between conducting electrons and magnetization in a ferromagnet has stimulated much interest following the discovery of the giant magnetoresistive effect two decades ago. With the advance of fabrication techniques at the nanometer length scale, a variety of new magnetic nanostructures have emerged. These structures are interesting from both a scientific and technological perspective. Some of them have successfully led to applications in information storage industry. This thesis theoretically studies some of these structures and focuses on two aspects: (1) the effect of surface roughness in magnetoresistive devices, (2) spin transfer torque induced magnetization dynamics. Surface roughness is known to be an important source of scattering in small structures. We employ Landauer's formalism to study spin dependent electron transport in structures like spin valve, magnetic tunnel junction and nanowires. An efficient algorithm is developed to solve the scattering problem numerically. It is found that the resistivity and magnetoresistance are strongly influenced by the surface roughness scattering. The coupling between spin polarized current and local magnetic moment results in a torque on the magnetization. This induces dynamic effects such as magnetization reversal and switching. We propose an exchange coupled composite structure to study current induced reversal and show that this structure can significantly reduce the critical current. The spin torque can cancel the damping torque and induce steady precession. This type of spin torque oscillator is attractive as a microwave device at the nanoscale. Several of these oscillators can couple together and oscillate in a phase coherent manner. The mechanism for the coupling is studied analytically and using micromagnetic simulation. It is found that the coupling exhibits an oscillatory behavior through a spin wave mediated interaction.
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    Spin-dependent transport phenomena in ferromagnet/semiconductor heterostructures
    (2014-06) Geppert, Chad Christopher
    This dissertation examines several aspects of spin-dependent transport phenomena in epitaxially grown ferromagnet/n-GaAs heterostructures. Further maturation of the field of semiconductor-based spintronics is hindered by difficulties in evaluating device performance across materials systems. Using Fe/n-GaAs and Co2MnSi/n-GaAs heterostructures as a test case, the main goal of this work is to demonstrate how such difficulties may be overcome by (1) specifying a more quantitative framework for evaluating transport parameters and (2) the introduction of a new spin-to-charge conversion phenomenon which may be parameterized by bulk semiconductor parameters. In the introductory chapter, this work is placed in the broader context of developing improved methods for the generation, modulation, and detection of spins. The lateral spin-valve geometry is presented as a concrete example of the typical measurement procedures employed. Chapter 2 presents the charge-based transport properties of these samples and establishes the notation and calculation techniques to be employed in subsequent chapters. In particular, we examine in detail the calculation of the electrochemical potential for a given carrier concentration. Chapter 3 provides a full derivation of the equations governing spin-dependent transport in the large polarization regime. This is applied to the case of extracting spin lifetimes and diffusion rates, demonstrating how quantitative agreement with theoretical predictions may be obtained upon properly accounting for both device geometry and material parameters. Further examination of the boundary conditions applicable to the heterojunctions of these samples demonstrates to what extent device performance may be parameterized across materials systems. Chapter 4 presents experimental observations of a new spin-to-charge conversion phenomenon using a non-magnetic probe. In the presence of a large non-equilibrium spin accumulation, the combination of a non-constant density of states and energy-dependent conductivity generates an electromotive force (EMF). It is shown that this signal dephases in the presence of applied and hyperfine fields, scales quadratically with the polarization, and is comparable in magnitude to the spin-splitting. Since this spin-generated EMF depends only on experimentally accessible parameters of the bulk material, its magnitude is used to quantify the injected spin polarization in absolute terms, independent of any assumptions regarding the spin-resistance of the interface. Chapter 5 examines spin-dependent contributions to signals measured in the Hall geometry. In particular, a large scattering asymmetry develops in the presence of hyperfine interactions with dynamically polarized nuclei. A pulsed measurement technique is introduced which allows the polarization of the electron spin system and nuclear spin system to be manipulated independently. Based on these results, a possible mechanism is presented based on inhomogeneities in the nuclear polarization. This motivates a phenomenological model which is compared against experimental data using the modeling techniques of the previous chapters.
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