Tutorial NM07—Magnetism at Nanoscale

This tutorial will be comprised of two talks, which are of contemporary interest in the field of magnetism and magnet materials. The tutorial should benefit graduate students and other researchers entering the field of magnetism. The tutorial will be given by an experimentalist and a theorist.

8:00 am – 9:00 am

Part I: Axel Hoffmann
Manipulating Magnetic Skyrmions
Magnetic skyrmions are topologically distinct spin textures that are stabilized by the interplay between applied magnetic fields, magnetic anisotropies, as well as symmetric and antisymmetric exchange interactions. Due to their topology, magnetic skyrmions can be stable with quasi-particle like behavior, where they can be manipulated with very low electric currents. This makes them interesting for extreme low-power information technologies, where it is envisioned that data will be encoded in topological charges, instead of electronic charges as in conventional semiconducting devices.  In particular, recently there has been a lot of progress stabilizing magnetic skyrmions at room temperature in magnetic heterostructures. This talk will review specific aspects that relate to the manipulation of individual magnetic skyrmions, such as their electrical generation, motion and dynamical excitations.

9:00 am – 9:30 am   BREAK

9:30 am – 11:30 am

Part II: Ralph Skomski
Physics and Applications of Nanomagnets
With decreasing feature size, magnetic nanostructures exhibit new physics that exist neither on an atomic scale nor in the bulk. These physical phenomena can be exploited in a wide range of applications, such as magnetic recording, permanent magnetism, soft magnets, spin electronics, catalysis and drug delivery. Specific materials requirements vary among applications, but in most cases, one needs to control intrinsic and extrinsic magnetic properties at or above room temperature. Both length and timescales are important. Length scales are of direct importance, to achieve miniaturization, or indirectly, to create nanostructures that support desired physical properties. Atomic effects are often limited to a few interatomic distances, normally less than 2 nm. Most nanoscale phenomena are operative on length scales between 5 nm and 100 nm, and this range is of primary interest in the present context. Macroscopic phenomena, such as stray fields, long-range magnetic order and magnetic domains, also occur on a nanoscale but are most important on length scales above one 1 µm. Key atomic-scale interactions are interatomic exchange, which determines the magnetic ordering temperature, and spin-orbit coupling, which affects magnetic anisotropy, spin-structure and electron transport. Many nanostructural effects involve the interatomic exchange, which is often strongest for one interatomic distance, or about 0.25 nm. If the ferro- or antiferromagnetic nearest-neighbor exchange was the only consideration, then the (anti)ferromagnetic would extend throughout the nanostructure, that is, to infinity in wires and thin films. In reality, many phenomena counteract this long-range effect, such as quantum fluctuations in nanowires, thermal excitations, exchange beyond nearest neighbors, Dzyaloshinskii–Moriya interactions and magnetostatic interactions. Particularly intriguing effects are encountered near phase transitions, including quantum phase transitions. Zero-dimensional solids (clusters) and one-dimensional solids (wires) do not exhibit phase transitions, whereas thin films are a borderline case, exemplified by the Kosterlitz–Thouless transition and by the ferromagnetic order of nearly Heisenberg-like two-dimensional magnets. Timescales need to be tailored to the applications and reach from less than 1 ns for high frequency and fast-switching applications to many years in magnetically stored information and in permanent magnets. Atomic processes tend to be very fast, whereas micromagnetic phenomena, such as magnetization switching, are typically realized over flexible but much longer times. We will explicitly discuss these times by comparing the remanent magnetization of permanent magnets and recording media with the switching time in antiferromagnets, which has recently attracted attention. Some other recently appreciated nanoscale phenomena are the Berry phase, the topological Hall effect and the topological protection of quantum states.

Instructors

• Axel Hoffmann, Argonne National Laboratory
  
• Ralph Skomski, University of Nebraska–Lincoln