Magnetic domain walls can be manipulated at high-speeds and on nanometer spatial scales by applied magnetic fields and electric currents. Modern nanotechnology methods can be used to engineer and fabricate nanometer-scale magnetic wires. These model one-dimensional structures can serve as conduits for electric current and for guiding magnetic domain walls. This project utilizes magneto-optic techniques to probe the high-speed manipulation of magnetic domain walls in fabricated nanometer-scale magnetic structures. The goal of the work is to characterize and understand the (spin-torque) mechanisms that: 1) allow high-speed manipulation of magnetization on nanometer spatial scales by electric currents, and that: 2) govern energy loss and damping. The work is directly related to existing and emerging technology that relies on high-speed manipulation of magnetism on small spatial scales: magnetic meta materials, memory and logic structures, and imaging, radar and telecommunication technology. The research involves state-of-the-art materials synthesis and nanofabrication techniques, and addresses new phenomena that occur as a result of nanometer spatial constraints. It provides excellent education and training opportunities for the students and postdoctoral associates who work on the projects.
Electron spins in one-dimensional nanometer-scale structures of magnetic material (magnetic nanowires) form regions of uniform magnetization (domains) separated by a domain wall in which spin orientation reverses between the two opposing spin domains. The spin configuration in the nanostructure can be manipulated by the application of a magnetic field or an electric current. The ability to manipulate and probe electron spins (local magnetism) on nanometer scales at high speeds is technologically important: it provides the basis for magnetic cellular logic and related new device technology that could extend and improve existing microelectronic devices that digitally store and process information. This project explores high-speed manipulation of electron spins in magnetic nanostructures. The objective of the work is to determine and understand the mechanisms that allow electric current manipulation of spins and discover how material composition and geometrical constraints govern and limit the control of local magnetism on nanometer scales. The project provides a good venue for training the next generation of scientists, technologists, and teachers because it involves new phenomena and requires application of current state-of the-art research instruments and materials science/nanoscience technology.
INTRODUCTION AND BACKGROUND: A magnetic domain is a region of a magnetic material in which all of the electron spin magnetic moments are aligned. A magnetic domain wall is a thin transition region between two magnetic domains in which the alignment of spin moments changes between the uniform spin-distribution in adjacent domains. Magnetic domains and domain walls play important roles in technological applications of magnetic materials, therefore, it is important to understand the properties and behavior of domains and domain walls in magnetic materials, particularly in magnetic materials and magnetic structures that are synthesized and fabricated to perform specific tasks in electronic and magnetic based technology. TECHNOLOGICAL AND SCIENTIFIC FOCUS OF THIS PROJECT: An important class of synthesized magnetic materials in which domain walls play important roles consists of rectangular cross section nanometer-scale wires fabricated from Permalloy on oxide-coated silicon surfaces. The magnetic moments (magnetism) of spin-polarized electrons in these systems is confined to lie along the wire axis forming a single long narrow magnetic domain. It is possible to reverse the direction of the magnetism along the wire at a specific location forming one or more very narrow domain walls that span the width of the wire. The location of these domain walls can be manipulated at high speed by applied magnetic fields or electric currents. These phenomena form the basis for a number of promising technological applications. A general reciprocity rule requires a moving domain wall to generate a voltage. This so-called "Magnetic Josephson Effect" produces a universal voltage that is proportional to the electron spin polarization of current carrying magnetized electrons. This interesting effect has been predicted and characterized theoretically, but had not been detected or studied by experiments prior to our experiments. OBJECTIVES; This research project investigates magnetic-field and electric-current driven domain-wall dynamics in thin-film based magnetic microstructures fabricated on silicon oxide surfaces. The major goals of the project are to understand the physical mechanisms that permit high-speed manipulation of magnetism in magnetic materials on spatially constrained (nanometer) scales and to probe for new phenomena that arise from spatial constraints on magnetic systems. The experiments are carried out using magnetic wires having nanometer-scale rectangular cross sections fabricated by focused ion beam milling of thin Permalloy films. Corresponding numerical simulations are carried out using high-performance computing resources at UT Austin. The experiments and simulations probe the relationship between nanowire dimensions and the spin distributions and dynamics of electrons within the propagating domain walls. The research includes preliminary studies of magnetic nano particle capture and transport by stray fields produced by domain walls of the nanowire structures. These studies are relevant to high-speed manipulation of magnetic nano particles on a nanometer scale and have technological importance in several areas including biomedical applications. The research also probes the voltage produced by a moving domain wall in a nanowire magnetic conduit. RESULTS/KEY SCIENTIFIC OUTCOMES: Key scientific results for rectangular Permalloy nanowire conduits include: 1) identification of new stable coupled vortex domain-wall structures in field-driven domain-wall motion that propagate at high velocity, 2) development of a comprehensive domain-wall spin texture phase diagram for domain walls confined in rectangular cross-section thin-film based Permalloy nano wires, 3) observation and explanation of enhanced domain-wall velocity resulting from an applied bias magnetic field perpendicular to the propagation direction in a magnetic conduit, and 4) development of a more comprehensive understanding of the "magnetic Josephson effect". BROADER IMPACTS: The experiments and numerical simulations conducted under this NSF support have provided results that test the predictive accuracy of computer codes that are now capable of numerically simulating spin dynamics (magnetic response) of magnetic materials. These codes serve as the basis for advanced material and device structure engineering and development. This project also provided opportunities for education and professional training of research and development scientists that are essential for the sector of the U.S. economy that depends on technological innovation. Three graduate students (Jusang Yang, Kidam Mun and Todd Monson) and a postdoctoral associate Xubing Zhou worked on this project. Yang carried out experiments and numerical simulations of the domain-wall dynamics, and Zhou worked briefly on improving the Magnetic Josephson Effect voltage measurement. The voltage measurement could become a standard tool for measuring the spin polarization of magnetic conduction electrons in magnetic materials if the technique can be refined. Jusang Yang received his Ph.D. degree in 2012, and Todd Monson (Erskine's Ph.D. student working at Sandia National Laboratory on magnetic nano particles) received his Ph.D degree in 2011. Todd Monson is now employed as a staff scientist at Sandia in the nano materials division, and Jusang Yang is a postdoctoral associate in Erskine's group at UT Austin. The NSF support also allowed Erskine's research group to participated in outreach programs sponsored by the UT Austin Physics Department that are focused on recruiting high school students into scientific programs.