1,720,993 research outputs found
Magneto-mechanical dynamics at the nanoscale
No full textGyromagnetic experiments, the Barnett effect (magnetization by rotation) and the Einstein-de Haas effect (rotation by magnetization), were initially devised to test the molecular current hypothesis and determine the electron g-factor at the dawn of quantum mechanics. Advances in fabrication techniques and an ever increasing control of materials has led to continued miniaturization of not only integrated circuits but also of mechanical elements, such as cantilevers, giving rise to the field of micro-electro-mechanical devices (MEMS) and eventually nano-electro-mechanical devices (NEMS). In this thesis, we focused on the Barnett effect and the interplay of this effect with its close relative, the Einstein-de Haas effect in mesoscopic or nanoscopic systems. We perform a feasibility study concerning the Barnett effect in magnetic nanostructures and thin films. Furthermore, we study the the interplay of the Barnett and Einstein-de Haas effects in a suspended quasi one-dimensional magnetic wire containing a tail-to-tail domain wall and in a freely rotating grain containing a magnetic moment.Kavli Institute of Nanoscience DelftApplied Science
Coupled spin, elastic and charge dynamics in magnetic nanostructures
No full textIn this Thesis, I address the interaction of magnetic degrees of freedom with charge current and elastic dynamics in hybrid systems composed of magnetic and non-magnetic materials. The objective, invariably, is to control and study spin dynamics using charge and elastic degrees of freedom. In certain cases, an understanding of this coupling can be exploited reciprocally to employ magnetic fields in controlling the charge and/or elastic dynamics. In chapter 2, I develop the generic theoretical description of a characterization technique called torque differential magnetometry (TDM). The TDM method entails attaching a magnetic specimen to a mechanical resonator and recording the mechanical resonance frequency (and quality factor) as a function of the applied magnetic field. A magnet experiences a torque under the influence of an external magnetic field, which adds to the elastic torque acting to restore the mechanical resonator to its equilibrium position. Since the mechanical resonance frequency depends on the stiffness (rate of change with deviation from equilibrium) of this restoration torque, the former depends on the magnetic torque, and hence, on the applied magnetic field. Considering the magnet to be single-domain, it can be characterized by a free energy that depends on the applied magnetic field and direction of the magnetic moment. I calculate the magnetic torque, and hence magnetic stiffness, experienced by the specimen for an arbitrary free energy density, and relative orientation of the applied magnetic field, mechanical resonator, and magnetic moment. This allows to calculate the observed mechanical resonance frequency for any applied magnetic field in terms of the parameters characterizing the magnetic free energy. Comparing experimental data with the calculations allows for quantitative extraction of the magnetic parameters such as anisotropy field and saturation magnetization. Chapter 3 continues to discuss the advantages of TDM method as compared to other magnetometry techniques. I present a new TDM setup that operates over a broad temperature and pressure range, including ambient conditions, with high sensitivity. So far, all TDM experiments have been carried out using micro-fabricated cantilevers, at low temperatures and pressures, using optical detection techniques. This renders the TDM method expensive, sensitive to disturbances, and limited in operation range. On the other hand, my setup employs low-cost commercially available piezoelectric tuning forks to offer an all electrical operation. I demonstrate measurement of anisotropy constant and saturation magnetization of an iron wire with a high signal to noise ratio under ambient conditions. The work presented in chapters 2 and 3 makes TDM highly attractive as a magnetometry method, and is expected to invigorate research and applications in this direction. In chapters 4 and 5, I discuss interaction between spin and elastic waves in a cubic crystalline magnet. Magneto-elastic coupling (MEC) in a magnetic solid results from the dependence of the magnetic anisotropy, dipolar energies and exchange interaction on the crystal lattice configuration. For wave propagation along a symmetry axis, only the transverse polarized elastic waves couple to spin waves up to the linear order in the field variables. MEC leads to hybridization of the spin and elastic waves into magneto-elastic waves (MEWs). The effect of hybridization is significant only around the (anti) crossing point between the uncoupled spin and elastic dispersion relations, where a fully mixed excitation magneto-phonon polaron (MPP) is formed. I formulate magneto-elastic boundary conditions at a ferromagnet|non-magnet interface, and investigate energy transport across it in terms of a scattering matrix approach. It is found that MPPs can be excited via elastic waves injection using a non-magnetic transducer with a high efficiency. I also theoretically demonstrate excitation of spin waves in a thin magnetic film using elastic waves injection. This allows injection of pure spin current into an adjacent normal metal via spin pumping using coherent elastic waves. The chapter 6 discusses heterostructures formed by depositing a normal metal layer (N) on a ferromagnetic insulator (FI). In particular, I report the measurement of low frequency spin current noise in Platinum (Pt) deposited on yttrium iron garnet (YIG). This is accomplished via conversion of spin to charge current noise in Pt via the inverse spin Hall effect, and measuring the resulting charge current (voltage) noise across the Pt. The measured voltage noise power is found to be consistent with the fluctuation-dissipation (FD) theorem and the recently discovered spin Hall magnetoresistance (SMR) effect. Since spin Hall currents underlie the SMR effect, I call the measured noise - spin Hall noise. I also discuss a stochastic linear response theory to elucidate the spin nature of the observed dependence of voltage noise power on YIG magnetization. Although the spin Hall noise originates in Pt, the YIG allows to isolate it from the purely charge based Johnson-Nyquist noise via the magnetization dependent spin current scattering from the Pt|YIG interface. The stochastic theoretical analysis presented, in conjunction with the FD theorem, may be considered an alternate derivation of the SMR effect. Understanding interaction between different subsystems in a magnetic hybrid structure is the key to achieving an efficient control of and access to the parameters of interest. The present thesis has been an attempt at studying interaction between two subsystems at a time, thereby enriching our knowledge and application oriented toolbox. To exploit the full potential of magnetic hybrids, it may be desirable to study all the subsystems together. Multiferroics constitute such an example and have been the subject of considerable research activity in the recent years. It is natural to ask the questions, that the present thesis addresses for ferromagnetic system, for advanced materials like multiferroics and antiferromagnets. It is hoped that the present thesis will motivate and serve as a good foundation for such an investigation.Applied PhysicsApplied Science
Waves in inhomogeneous media
No full textIn this thesis we study wave propagation in inhomogeneous media. Examples of the classical (massless) waves we consider are acoustic waves (sound) and electromagnetic waves (light, for example). Interaction with inhomogeneities embedded in a reference medium alter the propagation direction, velocity and amplitude of waves. We describe the properties of these (multiple) scattering processes to answer some questions in the field of waves in complex media, both on a fundamental level and from the point of view of applications. In the introductory chapter we motivate our research by discussing two applications for which studying wave propagation in inhomogeneous media is essential, namely seismic exploration to image the earth's subsurface, and diffuse optical tomography (DOT) with near infrared (NIR) light for medical imaging purposes. Chapter 2 discusses propagation of monochromatic waves in one-, two- and three-dimensional (3D) inhomogeneous media. Here, the complex medium is an acoustic reference medium with many embedded, similar, spherical scatterers with a finite radius smaller than the wavelength. In chapter 3 we study boundary conditions between large scattering objects when wave energy propagation is primarily diffuse. These boundary conditions are applied to a diffuse imaging problem in chapter 4. The fifth and final chapter of this thesis deals with a numerical method, the recursive Green function technique, to solve the classical wave equation in disordered 2D media. We use this method to study the arrival time and amplitudes of head waves from rough interfaces.Applied Science
Entanglement in Solid-State Nanostructures
No full textThe goal of this thesis is to investigate theoretically the generation and behaviour of multipartite entanglement for solid-state nanosystems, in particular electron spin quantum bits (so-called 'qubits') in quantum dots. A quantum dot is a tiny potential well where a single electron can be trapped. A quantum bit can be implemented in this system by applying a magnetic field, and thereby lifting the degeneracy of the spin states of the electron. These spins can then be used as single qubits, and engineering many of these quantum dots next to each other gives as a register of qubits. In this scheme, the so-called Loss-DiVincenzo quantum computer, the single spins can be rotated e.g. by applying a time dependent magnetic field, and two spins can interact through controlling the potential barrier between them. A qubit cannot only be in a superposition of the two computational states 0 and 1 at the same time, but an even stranger characteristic arises for multiple qubits: this phenomenon is called entanglement and refers to a strong correlation between two or more qubits, which can not be achieved within the framework of classical physics, and exponentially enlarges the possible states for a N-qubit system as compared to a classical N-bit system. In this thesis we devise algorithms how to generate multipartite entangled states in electron spin qubits in quantum dots. We compare which classes of entangled states can be generated efficiently in this system. Once the states are created, they decay due to a process called decoherence. We compare how entangled states can be generated and detected in a realistic experiment, and which classes of states are the most suitable. Furthermore, we compare which classes conserve the entanglement, and quantify the robustness of various classes of entangled states. In the last chapter, we devise a scheme of how to execute a simple quantum algorithm, the Deutsch-Jozsa algorithm, in a system containing another type of solid-state qubit, the so-called flux qubit.Kavli Institute of Nanoscience DelftApplied Science
Spin Caloritronic Phenomena Driven by Spin-orbit Coupling
No full textIn this thesis, we report several effects in spintronics and spin caloritronics related to relativistic spin-orbit coupling. In Chapter 2, we discuss the relativistic spin caloritronicHall effects in terms of a semiclassical theory for anomalous thermoelectric effects in ferromagnetic metals due to spin-orbit scattering at impurities, including the anomalous Nernst and Ettingshausen effect, the planar thermalHall effects, and thermolectric anisotropic magnetoresistance. The linear response relations between the currents and driving forces are derived for out-of-plane and in-plane magnetizations, respectively. In the out-of-plane configuration, there are anomalous thermoelectric Hall effects linear to the spin-orbit constant, while the thermoelectric anisotropic magnetoresistance and the planar Hall effect in the in-plane configuration are of second order in the spin-orbit coupling. The extrinsic theory systemizes the competing effects/mechanisms from a microscopic point of view and identifies the parameters needed to describe experiments. We developed a diffusion theory in Chapter 3 for the spin Hall magnetoresistance (SMR) in multilayers made from an insulating magnet F such as yttriumiron garnet (YIG), and a normal metal N with spin-orbit interactions, such as platinum (Pt). In an N|F bilayer system, the SMR requires spin-flip in N and spin-transfer at the N|F interface. Our results explain the SMR both qualitatively and quantitatively with transport parameters that are consistent with other experiments. The degrees of spin accumulation in N that can be controlled by the magnetization direction is found to be very significant. In the presence of an imaginary part of the spin-mixing conductance Gi we predicted an AHE-like signal (SHAHE), which has been observed experimentally and can be explained with values of Gi that agree with first principles calculations. We furthermore analyzed F|N|F spin valves for parallel and perpendicular magnetization configurations. The SMR torques under applied currents in N are expected to lead to magnetization dynamics of N|F and F|N|F structures. In Chapter 4,we generalized the SMR theory in Chapter 3 to a thin-film made of a metallic ferromagnet and take into account the out-of-plane spin currents generated by the spinHall effect, which were disregarded in Chapter 2. We predict a new contribution to the anisotropic magnetoresistance by the simultaneous action of the anomalous Hall effect and its inverse. By diffusion theory, we compare this contribution with the conventional anisotropic magnetoresistance, demonstrating that they can be distinguished experimentally by studying its dependence on the film thickness. The extra contribution to the magnetoresistance has a magnetization dependence different from that of the conventional AMR. While the conventional AMR is usually positive, the new contribution is always negative. In order to analyze the effect of interface and boundary roughness that was disregarded in Chapter 3, we reports in Chapter 5 a Boltzmann study to quantify how the surface/interface scattering affects the spin Hall physics. In a bilayer system made of N and FI, we observe an AHE-like transverse voltage induced by the spin dependent scattering at the FI|N interface, which is competing with the imaginary SMR predicted in Chapter 3. We further show that the spin diffusion equation on which the SMR in Chapter 3 is based, has to be corrected by the surface/interface roughness in the limit of thin-films. Our model provides an approach to analyze the role of roughness in recent measurements on layered systems. Even though the theories developed in Chapters 3–5 are not directly related to spin caloritronics, they can be easily generalized for their thermoelectric analogues by the formulation spelt out in Chapter 2, and can be useful for prospective research in spintronics and spin caloritronics.Quantum NanoscienceApplied Science
Quantum Pumping and Adiabatic Transport in Nanostructures
No full textThis thesis consists of a theoretical exploration of quantum transport phenomena and quantum dynamics in nanostructures. Specifically, we investigate adiabatic quantum pumping of charge in several novel types of nanostructures involving open quantum dots or graphene. For a bilayer of graphene we find that at the Dirac point and for a wide bilayer the pumped current scales linearly with the sample length when this length is much smaller than the interlayer coupling length, exhibits a maximum when both of these length scales are comparable, and crosses over to a logarithmic dependence if the sample length is much larger than the interlayer coupling length. This behavior is markedly different from the behavior of the conductance in a graphene bilayer. Futher we study possibilities for adiabatic evolution and computing in an extended version of the quantum Ising model, which includes beyond-nearest neighbour interactions and an additional site-dependent longitudinal magnetic field. We calculate the energy spectrum of this model, treating the interactions exactly and using perturbation theory in the longitudinal field and find that the presence of next-nearest-neighbour interactions enhances the minimal energy gap between the ground state and the first excited state.Kavli Institute of Nanoscience DelftApplied Science
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