Browsing by Subject "Spintronics"
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Item Current driven magnetization dynamics in ferromagnets and antiferromagnets(2015-05) Wang, Cheng, doctor of physics; Tsoi, Maxim; Li, Xiaoqin; MacDonald, Allan H; Niu, Qian; Banerjee, Sanjay KThe development of spintronics and potential applications demands a thorough understanding of various novel phenomena in ferromagnets and antiferromagnets. Magnetotransport measurements, which have been implemented in current data storage and magnetoresistive sensing technology, provide convenient and powerful approach to the characterizations of magnetism. We conduct point-contact magnetotransport investigations in metallic magnetic multilayers and antiferromagnetic insulators, aiming at probing the electron transports associated with local magnetic properties in those different materials. For metallic exchange biased spin valves, both radiofrequency (rf) and dc currents are injected through point contacts and we detect the rectified electrical signals. Point contacts with contact sizes of the order of 10-100 nm allow to probe the spins in very local scale. It is found that both linear ferromagnetic resonance and nonlinear parametric resonance can be observed driven by oscillating currents. Particularly, the parametric excitation driven by ac spin transfer torque (STT) is a promising candidate of techniques for realizing fast magnetic switching in spin torque based devices. As for investigating the single crystals of antiferromagnetic Mott insulator Sr₂IrO₄ (SIO), a large anisotropic magnetoresistance (AMR) signal originated from the entanglement of orbital physics and magnetic moments was revealed, shedding lights into the unexplored physics in heavy transition metal oxides in presence of comparable magnitudes of electron correlations and spin-orbit coupling. The crystalline AMR found in SIO may point out a practical path to the sensing of antiferromagnetic order in future AFM-based devices. Furthermore, detailed point-contact study of the electron transport in SIO under high electric biases discovers an electrically tunable transport band gap in this iridate, suggesting a very interesting playground for developing functional devices based on transition metal oxides.Item Current-driven non-linear magnetodynamics in magnetic nano-devices(2016-05) Seinige, Heidi; Tsoi, Maxim; Li, Xiaoqin E.; Lai, Keji; MacDonald, Allan H.; Banerjee, Sanjay K.Spintronis is an emerging electronic technology that is built on interconnections between the electron’s electric charge and its quantum-mechanical spin. The interconnections allow altering the electrical transport properties of magnetic nano-devices by changing the magnetic configuration, and vice versa. It opens up a possibility of denser and faster magnetic memory and logic devices. In this work, we conducted electronic transport studies using nanco-scale point-contacts in CoSiBFeNb, exchange-biased spin valves, and the antiferromagnetic Mott insulator Sr2IrO4 and Sr3Ir2O7. Magnetic domain switching and evidence of spin-transfer torque in CoSiBFeNb were observed. Furthermore, by simultaneously measuring the rectification signal and microwave absorption, we were able to directly compare electrical detection of ferromagnetic resonance and conventional absorption measurements. We found a good agreement between the methods and showed that here the point-contact acts as a nano-scale bolometer, monitoring the absorption of microwave current. Measurements in exchange-biased spin valves showed that parametric resonance can be excited next to ferromagnetic resonance. These non-linear excitations are driven by spin-transfer torque and due to the field-like component shift with applied dc bias. Parametric resonance can potentially be used as a new and faster method to switch the magnetization in magnetic memory and logic devices. Last, we studied electrical transport in Sr2IrO4 and Sr3Ir2O7. Both compounds revealed a decrease in activation energy with increasing dc bias, which was well fitted by a field-effect model and explained by small lattice distortions. Moreover, a small resistive switching due to the transition between meta-stable states at a critical current was observed. High-frequency measurements in Sr3Ir2O7 showed a resonance-like peak structure in the rectification signal as a function of dc bias at sufficiently high microwave power. We attribute these features to magnonics that can be excited in Sr3Ir2O7 when the lattice is distorted in an ac electric field. Our results show that transition metal oxides such as Sr2IrO4 and Sr3Ir2O7 are a new class of materials that allow for modifying band structures via dc and ac currents in spintronic applications.Item Electronic and spintronic transport in germanium nanostructures(2014-05) Liu, En-Shao; Tutuc, Emanuel, 1974-The digital information processing system has benefited tremendously from the invention and development of complementary metal-oxide-semiconductor (CMOS) integrated circuits. The relentless scaling of the physical dimensions of transistors has been consistently delivering improved overall circuit density and performance every technology generation. However, the continuation of this trend is in question for silicon-based transistors when quantum mechanical tunneling becomes more relevant; further scaling in feature sizes can lead to increased leakage current and power dissipation. Numerous research efforts have been implemented to address these scaling challenges, either by aiming to increase the performance at the transistor level or to introduce new functionalities at the circuit level. In the first approach, novel materials and device structures are explored to improve the performance of CMOS transistors, including the use of high-mobility materials (e.g. III-V compounds and germanium) as the channel, and multi-gate structures. On the other hand, the overall circuit capability could be increased if other state variables are exploited in the electronic devices, such as the electron spin degree of freedom (e.g. spintronics). Here we explore the potential of germanium nanowires in both CMOS and beyond-CMOS applications, studying the electronic and spintronic transport in this material system. Germanium is an attractive replacement to silicon as the channel material in CMOS technology, thanks to its lighter effective electron and hole mass. The nanowire structures, directly synthesized using chemical vapor deposition, provide a natural platform for multi-gate structures in which the electrostatic control of the gate is enhanced. We present the realization and scaling properties of germanium-silicon-germanium core-shell nanowire n-type, [omega]-gate field-effect transistors (FETs). By studying the channel length dependence of NW FET characteristics, we conclude that the intrinsic channel resistance is the main limiting factor of the drive current of Ge NW n-FETs. Utilizing the electron spins in semiconductor devices can in principle enhance overall circuit performance and functionalities. Electrical injection of spin-polarized electrons into a semiconductor, large spin diffusion length, and an integration friendly platform are desirable ingredients for spin based-devices. Here we demonstrate lateral spin injection and detection in Ge NWs, by using ferromagnetic metal contacts and tunnel barriers for contact resistance engineering. We map out the contact resistance window for which spin transport is observed, manifestly showing the conductivity matching required for spin injection.Item Ferromagnetic resonance in magnetic tunnel junctions under high dc biases(2016-08) Williamson, Morgan Cole; Tsoi, Maxim; Lai, KejiFerromagnetic resonance (FMR) is a standard spectroscopic technique which is used to probe the magnetodynamics of ferromagnetic materials in order to understand and improve performance of spintronics applications such as magnetic random-access memory (MRAM). In our experiments, we use rf microwave currents to excite FMR in magnetic tunnel junctions (MTJs) via spin-transfer torque (STT-FMR) that allows us to electrically detect magnetodynamics by means of a small rectified voltage which develops across the MTJ at resonance. The MTJ pillars used in this work have diameters on the order of 100 nm and consist of free and pinned CoFeB layers separated by a MgO barrier with typical tunneling magnetoresistances (TMRs) of about 100% at room temperature. As expected, the frequency-field relationship of the observed resonances can be well fitted by Kittel’s equation. However, as a function of the dc bias applied to the MTJ we observe an unexpected shift of the resonance field. This shift is symmetric about zero bias and may be a result of the out-of-plane voltage controlled magnetic anisotropy (VCMA) in the otherwise in-plane magnetized MTJ. In addition to the effective field due to VCMA, an out-of-plane field was produced by canting the applied field. A generalized angular dependent version of Kittel's equation revealed little influence of the out-of-plane applied field with respect to the effective VCMA field. Also, two measurement techniques for detecting FMR, rf amplitude modulation and applied magnetic field modulation are reviewed and compared.Item Imaging and control of magnetization dynamics for spintronic devices(2013-05) Birt, Daniel; Li, Xiaoqin; Tsoi, MaximAs features on integrated circuits continue to shrink, currently at 22 nm and predicted to approach 10 nm by 2020, the semiconductor industry is rapidly brushing up against the fundamental limits of electric charge and current based devices. These limits are due to the fact that charges are being pushed around in tiny areas and they repel one another with significant force. Fortunately, there are many other degrees of freedom in solids that do not suffer from these limitations and are just waiting to be harnessed in useful devices. This idea is behind all of the fields that have lately been proliferating ending in -onics, photonics, plasmonics, phononics, and of most relevance to this dissertation spintronics. Spintronics refers to a field of research wherein ways are sought to utilize the spin property of the electron in devices. One of the most attractive aspects of electron spin is that it can be used to store (transiently or permanently), process, and transmit information. The main challenge in spintronics is accessing the spin degree of freedom. Until the discovery of the giant magnetoresistance effect in the late 1980's, the only way to manipulate the electron spin was through a magnetic field. Recent developments have shown that electron spins can be controlled with direct currents of both heat and electrons, which have the benefit of being easy to generate and direct without interfering over a large area. The purpose of this dissertation is to study methods of controlling the dynamics of magnetization in thin films for spintronic applications by imaging the spin wave intensity in devices. To this end we have constructed a micro-focus Brillouin Light Scattering system to map the intensity of spin waves propagating in thin ferromagnetic films on the sub-micron scale. We have studied issues relating to fundamental issues of spin wave propagation in thin films. We have investigated the possibility of spin wave amplification with direct charge currents and spin currents generated by the spin Hall effect. Furthermore, we have demonstrated the ability to measure the magnon and phonon temperatures, which is important for studies of thermal transport.Item Interaction between collective coordinates and quasiparticles in spintronic devices(2006) Núñez, Álvaro Sebastián; MacDonald, Allan H.In this dissertation several aspects of the interaction of collective and quasi-particle degrees of freedom are studied. This is done in the context of spin dependent transport effects with applications for spintronics devices. In ferromagnetic metals the effects of quasi-particle currents on spin textures, either domain wall structures or spin waves, are discussed. In nano-magnetic heterostructures, the effects acquire the form of spin transfer torques. The microscopic origin of these effects, as discussed in this work, relies on the relation between exchange fields and spin densities. The presence of the current modifies the spin density. In consequence the exchange fields are also affected by the current. It is these modifi- cations on the exchange fields that are able to alter the dynamics of the collective fields. It is shown how this rather abstract picture of spin transfer reduces to the usual description, that can be found in the extensive literature on the subject, based on a bookkeeping argument and on spin conservation. The most important feature of this picture, as discussed in the text, is that it allows for generalizations of the spin transfer effects to systems were the spin conservation arguments fail or are of little use. We discuss applications of this view to spin transfer torques on systems with spin-orbit interaction and for systems with antiferromagnetic elements. In the latter case, a preliminary model study of spin dependent transport in antiferromagnets is reported, it has revealed that i) giant magnetoresistive effects are possible, and ii) nanostructures containing antiferromagnetic elements will exhibit current-induced magnetization dynamics. In particular it turns out that, contrary to the ferromagnetic case, the spin transfer torques act throughout the entire free antiferromagnet to cooperatively switch it, a result of the special symmetries of the antiferromagnetic state. This implies that the critical current for inducing collective magnetization dynamics is likely to be lower in antiferromagnetic metal nanostructures than in ferromagnetic spin valves.Item Nonequilibrium order parameter dynamics in spin and pseudospin ferromagnets(2009-08) Garate, Ion; MacDonald, Allan H.Research on spintronics has galvanized the design of new devices that exploit the electronic spin in order to augment the performance of current microelectronic technologies. The sucessful implementation of these devices is largely contingent on a quantitative understanding of nonequilibrium magnetism in conducting ferromagnets. This thesis is largely devoted to expanding the microscopic theory of magnetization relaxation and current-induced spin torques in transition metals ferromagnets as well as in (III,Mn)V dilute magnetic semiconductors. We start with two theoretical studies of the Gilbert damping in electric equilibrium, which treat disorder exactly and include atomic-scale spatial inhomogeneities of the exchange field. These studies enable us to critically review the accuracy of the conventional expressions used to evaluate the Gilbert damping in transition metals. We follow by generalizing the calculation of the Gilbert damping to current-carrying steady states. We find that the magnetization relaxation changes in presence of an electric current. We connect this change with the non-adiabatic spin transfer torque parameter, which is an elusive yet potentially important quantity of nonequilibrium magnetism. This connection culminates in a concise analytical expression that will lead to the first ab initio estimates of the non-adiabatic spin transfer torque in real materials. Subsequently we predict that in gyrotropic ferromagnets the magnetic anisotropy can be altered by a dc current. In these systems spin-orbit coupling, broken inversion symmetry and chirality conspire to yield current-induced spin torques even for uniform magnetic textures. We thus demonstrate that a transport current can switch the magnetization of strained (Ga,Mn)As. This thesis concludes with the transfer of some fundamental ideas from nonequilibrium magnetism into the realm of superconductors, which may be viewed as easy-plane ferromagnets in the particle-hole space. We emphasize on the analogies between nonequilibrium magnetism and superconductivity, which have thus far been studied as completely separate disciplines. Our approach foreshadows potentially new effects in superconductors.Item Semiclassical study of spin magnetic moment and spin orbit interaction(2010-05) Chuu, Chih-Piao; Niu, QianThis dissertation describes the theoretic studies of magnetic moment and spinorbit interaction in vacuum (Dirac wavepacket) and solid state systems, such as semiconductors. The semiclassical approach developed here provides a simple and intuitive picture for the origin of spin and spin-orbit coupling. In the Dirac model, the spin magnetic moment is originated from the self-rotating Dirac wavepacket with a correct g-value. The spin-orbit interaction is related to Berry connection (gauge potential) and the model is generalized to solid state systems. The Rashba effect caused by the spin-orbit coupling in a crystal with asymmetric potential in heterostructure quantum well is calculated by semiclassical spindependent scattering. The exact treatment of interface phase accumulation provides a justification of spin-dependent boundary condition at interface derived in previous treatment using Löwdin decomposition. Other spin-orbit coupling related phenomena in solid state system are also discussed in this thesis.Item Spin-Orbit Torque Driven Magnetization Dynamics in (Ga,Mn)As and (Ga,Mn)(As,P) Dilute Magnetic Semiconductors(2014-07-18) Vehstedt, Erin KathleenSpintronics-based technologies are poised to leapfrog the current limitations on the scaling, speed, and power consumption of electronic devices. Conventional devices rely on complex structures and magnetic-field-based switching to manipulate data. In order to overcome these limits, new methods must be developed to reliably transmit and store data more efficiently. The understanding and manipulation of magnetic domain walls (DWs) may play a pivotal role in the development of new non-volatile and down-scalable logic and memory devices. This thesis investigates current-induced magnetization dynamics and control mechanisms in the ideal ferromagnetic semiconductors Phosphorus-doped Gallium Manganese Arsenide (Ga,Mn)(As,P) and Gallium Manganese Arsenide (Ga,Mn)As. In spin-orbit (SO) coupled materials with broken inversion symmetry, unpolarized electric fields provide a means to control magnetization orientation via the inverse spin-galvanic effect (ISGE). The ISGE generates a non-equilibrium spin-accumulation which can exert a torque on a magnetization if the spins are generated in (or injected into) a ferromagnetic material. This so-called current-induced spin-orbit torque (SOT) is calculated for a broad range of experimental parameters and compared with previous measurements. The study also assess the viability of using SOTs to control DW motion in semiconductor micro-structures. Typically, DW mobility is divided into steady and precession motion regimes with different mobilities, separated by the so-called Walker breakdown (WB). By manipulating the magnetic anisotropy of (Ga,Mn)(As,P) using piezoelectric strain, these experiments investigate the potential of strain to shift the WB, establishing strain-modified DW mobility as tool for electrically controlled DW motion.Item Spin-polarized transport in magnetic nanostructures(2009-12) O'Gorman, Brian Curtin; Tsoi, Maxim; MacDonald, Allan; Markert, John; Shi, Li; Swift, JackTwo of the principal phenomena observed and exploited in the field of spintronics are giant magnetoresistance (GMR) and spin transfer torque (STT). With GMR, the resistance of a magnetic multilayer is affected by the relative orientation of its magnetic layers due to (electron) spin dependent scattering. For the STT effect, a spin-polarized electric current is used to alter the magnetic state of a ferromagnet. Together, GMR and STT are at the foundation of numerous technologies, and they hold promise for many more applications. To achieve the high current densities (~10¹² A/m²) that are necessary to observe STT effects, point contacts – constricted electrical pathways (~1–100 nm in diameter) between conducting materials – are often used because of their small cross-sectional areas. In this sense, we have explored STT in bilayer magnetic nanopillars, where an electric current was used to induce precession of a ferromagnetic layer. This precessional state was detected as an increase in resistance of the device, akin to GMR. Temperature dependent measurements of the onset of precession shed light on the activation mechanism, but raised further questions about its detailed theory. Point contacts can also be used as local sources or detectors of electrons. In this context, we have observed transverse electron focusing (TEF) in a single crystal of bismuth. TEF is a k-selective technique for studying electron scattering from within materials. Using lithographically fabricated point contacts, we have studied the temperature dependence of the relaxation time for ballistic electrons from 4.2 to 100 K. These measurements indicated a transition between electron-electron dominated scattering at low temperatures and electron-phonon scattering as the Debye temperature was approached. We present preliminary work toward a TEF experiment to measure spin dependent scattering from a non-magnet/magnet interface. We also investigated spin wave propagation in thin, magnetic waveguide structures. At the boundary between the waveguide and continuous magnetic film, spin wave rays were found to radiate into the film, or to reflect and form standing waves in the waveguide. A circular defect in the waveguide was observed to cause diffraction of spin waves, generating an interference pattern of higher modes of oscillation.Item Spin-transfer-torque effect in ferromagnets and antiferromagnets(2008-12) Wei, Zhen; Tsoi, MaximSpintronics in metallic multilayers, composed of ferromagnetic (F) and non-magnetic (N) metals, grew out of two complementary discoveries. The first, Giant Magnetoresistance (GMR), refers to a change in multilayer resistance when the relative orientation of magnetic moments in adjacent F-layers is altered by an applied magnetic field. The second, Spin-Transfer-Torque (STT), involves a change in the relative orientation of F-layer moments by an electrical current. This novel physical phenomenon offers unprecedented spatial and temporal control over the magnetic state of a ferromagnet and has tremendous potential in a broad range of technologies, including magnetic memory and recording. Because of its small size (<10nm), point contact is a very efficient probe of electrical transport properties in extremely small sample volumes yet inaccessible with other techniques. We have observed the point-contact excitations in magnetic multilayers at room temperature and extended the capabilities of our point-contact technique to include the sensitivity to wavelengths of the current-induced spin waves. Recently MacDonald and coworkers have predicted that similar to ferromagnetic multilayers, the magnetic state of an antiferromagnetic (AFM) system can affect its transport properties and result in antiferromagnetic analogue of giant magnetoresistance (GMR) = AGMR; while high enough electrical current density can affect the magnetic state of the system via spin-transfer-torque effect. We show that a high density dc current injected from a point contact into an exchange-biased spin valve (EBSV) can systematically change the exchange bias, increasing or decreasing it depending on the current direction. This is the first evidence for current-induced effects on magnetic moments in antiferromagnetic (FeMn or IrMn) metals. We searched for AGMR in multilayers containing different combinations of AFM=FeMn and F=CoFe layers. At low currents, no magnetoresistance (MR) was observed in any samples suggesting that no AGMR is present in these samples. In samples containing F-layers, high current densities sometimes produced a small positive MR – largest resistance at high fields. For a given contact resistance, this MR was usually larger for thicker F-layers, and for a given current, it was usually larger for larger contact resistances (smaller contacts). We tentatively attribute this positive MR to suppression at high currents of spin accumulation induced around and within the F-layers.Item Spintronics in ferromagnets and antiferromagnets from first principles(2007-08) Haney, Paul Michael, 1976-; MacDonald, Allan H.Item Study of a ferromagnetic semiconductor by the scanning Hall probe microscope(2008-08) Kweon, Seongsoo, 1967-; de Lozanne, Alejandro L.The primary goal of my dissertation was to build a Scanning Hall Probe Microscope (SHPM) for studying the domain structure of a ferromagnetic semiconductor (Ga[subscript 0.94]Mn[subscript 0.06]). This new semiconductor may be used in the emerging field of spintronics, where both the charge and spin of an electron are utilized. The first part of this dissertation introduces the scanning probe microscopy techniques that are used for our homemade SHPM performance test and images. In chapter 2, general spintronics and ferromagnetic semiconductor are introduced. A compact design of our LT-SHPM is introduced in chapter 3. A unique taper seal based on stainless steel and Cu for opening/closing the vacuum chamber is used for our homemade SHPM. In chapter 4, Hall probes are discussed. In this chapter, ESD (Electrostatic discharge) and its repair work are discussed. Finally, in Chapter 5, SHPM imaging results of Ga[subscript 0.94]Mn[subscript 0.06]As are discussed. We observed stripe domain patterns. We also observed the domain patterns as a function of magnetic field and temperature.Item Tuning the Thermal Properties of Magnetic Tunnel Junctions(2014-04-18) Amin, Vivek PravinDue to their ubiquitous presence in hard-disk drives and growing potential as commercially viable memory bits, Magnetic Tunnel Junctions (MTJs) continue to provide impetus for scientific study. The demand for smaller devices and efficient energy consumption mandates further investigation of their thermal properties and possible finite-size effects. Such considerations have prompted a renewed interest in the long-known Seebeck effect, in which a thermal gradient spanning a material induces a voltage. The strength of this induced voltage can change as a function of the device's magnetization configuration - known as the magneto-Seebeck effect or magnetothermopower - in analogy with the Giant (and Tunnel) Magnetoresistance. This thesis presents a theoretical study of this effect in MgO-based MTJs with spin-orbit coupling. We present theoretical calculations of the Tunneling Anisotropic Magneto-Seebeck effect using realistic band structures, and show that the thermal properties of MTJs are tunable via magnetic field. This phenomenon potentially enables the controlled manipulation of temperature gradients, the recycling of wasted heat, and thermal spin-logic. Our calculations employ the Landauer-Buttiker scattering formalism, in conjunction with realistic multi-band tight-binding models fitted to ab-initio calculations. We demonstrate that numerically-unstable transmission resonances, ordinarily described as hot-spots in the literature, more accurately resemble "walls" that weave through each device's two-dimensional Brillouin Zone. We discuss their physical relevance in modern day nanostructures, and argue that their selective removal (via ltering algorithms) aids convergence while preserving each system's essential magnetic-transport properties. Finally, we demonstrate that exploiting spin-orbit coupling in MTJs with a single ferromagnetic contact can actually enhance certain magnetic transport anisotropies, allowing for higher packing densities as well.