A ripple of tilted – the atoms’ tiny magnets – travelling through a magnet, like a stadium wave in which each spectator only stands up and sits down yet the wave runs all the way round. Because the ripple carries information without moving any electrons, it is one route to lower-power devices.
Going deeper
A magnon is one quantum of a spin wave: each spin precesses about the common direction slightly behind its neighbour. Because the wave carries spin without moving charge, it can travel through a magnetic insulator – no current, and no Joule heating along the way.
Waves in a magnet
The lowest-energy excitation of a magnet is not flipping a single spin, which costs a lot of exchange energy, but tilting many spins slightly so that the tilt travels as a wave. Quantised, that wave is a magnon, carrying one unit of spin angular momentum. Long-wavelength magnons cost little energy, which is why magnetisation falls with temperature as they are excited.
The dispersion – how energy rises with wavevector – is set by exchange, which dominates at short wavelength, by , which adds an energy cost even at zero wavevector, and by dipolar interactions at long wavelength. In an there are two branches with opposite spin, and their energies are typically much higher, in the terahertz range rather than the gigahertz range of ferromagnets.
What the spectrum says about a 2D magnet
The gap at zero wavevector is the quantity that matters most in two dimensions. Without the gap is zero, magnons of arbitrarily low energy are excited at any temperature, and long-range order is destroyed – the Mermin–Wagner argument. With an easy axis the gap opens, few magnons are excited at low temperature, and order survives.
Measuring the magnon spectrum of a is therefore a way to measure the anisotropy that makes the magnet possible. needs too much material, so 2D magnets are studied with and infrared spectroscopy, and magnons were observed directly in atomically thin CrI3, where the gap and the branches also revealed how the layers couple.
Magnonics
Because magnons carry spin without charge, they can transport information through , with no charge current to dissipate energy along the way. A charge current in a metal strip can inject spin into an adjacent magnetic insulator, magnons carry it across, and a second strip converts it back into a voltage – the basis of magnon . Wave behaviour brings interference, so logic gates based on phase have been proposed and demonstrated.
The limits are attenuation and speed: magnons decay over micrometres in the best materials and much less in most, and generating and detecting them efficiently remains the bottleneck. magnets add gate tunability and clean interfaces to the toolbox, but their low keep most experiments cold.
For specialists
A quantised spin-wave excitation of a magnetically ordered state. Its dispersion is set by exchange, anisotropy and dipolar terms, and in a 2D magnet the anisotropy gap at zero wavevector is what holds order against thermal fluctuations – so the magnon spectrum measures the same anisotropy that lets the material evade the .
Where this comes from
Magnon spintronicsChumak et al. · Nature Physics 11, 453 (2015)cited by 2,596