New Prospects in the Study of Two-Dimensional Magnetic Materials

Michael G. Cottam

Department of Physics and Astronomy, University of Western Ontario, London, Canada

The last six years have seen the exciting discovery of new magnetic materials that are truly two-dimensional (one atomic layer thick). They are known as van der Waals magnets and can exhibit either ferromagnetic or antiferromagnetic ordering of the spins at low temperatures, typically with a 2D honeycomb lattice, and are therefore often described as a magnetic analog of graphene. I shall review some of the basic characteristics of these materials and describe a few recent investigations for their spin dynamics (which may be thought of in terms of propagating spin waves or magnons). The talk will also include the spin-wave behavior in other magnetic nanostructures such a stripes (ribbons) or rings fabricated from ultra-thin films. These are essentially 2D materials with regards to their spin dynamics, since their thickness can be reduced to less than the so-called exchange correlation length (often around 5–10 nm for the magnetic metals of interest). In such systems, I will discuss how there can be a rich interplay between various competing magnetic interactions (such as quantum-mechanical exchange interactions, dipole-dipole interactions, Zeeman field energy, etc.). The lowered symmetry near surfaces and interfaces allows for novel forms of interactions that are absent (or negligible) in bulk materials. A reference will also be made to structures formed from arrays of magnetic nano-elements, where the inbuilt periodicity leads to special properties. These are the so-called magnonic crystals, where the magnons (or spin waves) can be selectively manipulated by analogy with photonic crystals designed for controlling photons (or light).

This work is supported by the Natural Sciences and Engineering Research Council of Canada and done in collaboration with Prof. Bushra Hussain, University of Michigan-Dearborn.