Magnetism on the nanoscale

Johns Hopkins University, Baltimore MD

Investigators

Abstract

TECHNICAL SUMMARY This award supports theoretical and computational research and education on the dynamics of magnetization in nanoscale magnets. The PI will investigate the influence of topological defects on static and dynamic properties of nanoscale magnets by using analytical methods of field theory in combination with numerical simulations. Topological defects such as domain walls, vortices, and skyrmions, are highly stable textures interpolating between distinct ground states of a condensed-matter system. Because any memory unit relies on the existence of two or more ground states, the switching of a memory bit often involves the creation, propagation, and annihilation of topological defects. Understanding how topological defects behave in nanostructures is thus important for future technological applications. Topological defects can be characterized most concisely using the language of field theory. Rather than trying to capture the evolution of a system in complete detail, the field-theoretic approach provides a coarse-grained description. This approach has been enormously helpful in understanding flux vortices in type-II superconductors and superfluids, domain walls in polyacetylene, dislocations in hexatics, and hedgehogs in nematics. The main advantage of the field-theoretic approach in the current context is its ability to reduce the complex problem of many spins to a more tractable one: determining the evolution of a few elementary excitations directly involved in the switching process. The PI will focus on several magnetic systems with topological defects exemplified by the following: artificial spin ice, where the dynamics of magnetization is mediated by the propagation of defects carrying magnetic charge; composite domain walls in ferromagnetic nanowires; Bloch domain walls in thin ferromagnetic films with negative surface tension and unusual dynamics of transverse fluctuations; skyrmion crystals recently discovered in non-centrosymmetric ferromagnets. The PI will create interactive programs simulating various physical phenomena for educational tools. The programs will be implemented as Java applets. The attractive feature of Java-based simulations is their portability: such programs can be run on any computer equipped with a browser. The simulations will be paired with assignments to provide a genuine learning experience. NON-TECHNICAL SUMMARY This award supports theoretical and computational research and education on the dynamics of magnetization in nanoscale magnets, magnets about a ten-millionth of an inch in size. The development of new electronic technologies for ever-shrinking electric circuits and memory elements requires a solid grasp of physical laws governing the collective behavior of electrons on the nanoscale. Atomic-scale magnets in computer hard drives interact with one another via several distinct forces with different ranges. As a result, magnets of different sizes work in different ways. Switching the state of a magnetic bit from 0 to 1 involves simultaneous flipping of the direction of all the atomic-scale magnets, or dipoles, in a magnet with a few thousand atoms. The process is much more involved in magnets containing billions of dipoles and more. It usually involves the creation and propagation of topological defects, solitary stable spatial patterns in the directions of atomic-scale magnets that propagate throughout the magnet and in the process switch its direction. These defects can be of different types and shapes including domain walls, separating two regions with dipoles pointing in different directions, and vortices, where the directions of the dipoles form a swirling pattern. The PI will study the dynamics of switching in several diverse physical systems where the way the directions of the dipoles change in time is governed by the propagation of topological defects. These include ferromagnetic nanowires proposed for the 'racetrack' magnetic memory, artificial arrays of ferromagnetic islands known as spin ice, thin ferromagnetic films, and periodic lattices of topological defects in ferromagnets with twisting magnetization. The PI will create interactive programs simulating various physical phenomena for educational tools. The programs will be implemented as Java applets. The attractive feature of Java-based simulations is their portability: such programs can be run on any computer equipped with a browser. The simulations will be paired with assignments to provide a genuine learning experience.