Magnetic excitations can travel through some materials in the same way that electrons can through an electrical conductor. These excitations, which are referred to as “magnons” in physics because they are analogous to electrons, could transfer information considerably more efficiently than electrical conductors.
A multinational research team has now achieved a significant step forward in the development of such components, which might be very energy-efficient and much smaller.
Currently, most electronic components are built on the transportation and management of electrical charges. Due to the electrical resistance, the flow of electric currents generates heat, which is a major disadvantage of this technology.
The amount of energy lost is enormous when you consider the massive number of electronic components in use around the world. Because they don’t emit nearly as much waste heat, using spin waves to convey and process information could be a more energy-efficient option.
These components could possibly be substantially smaller. As a result, scientists all over the world are hunting for materials that can transfer information using magnetic spin waves.
In this effort, an international research consortium with major participation from the Technical University of Munich (TUM) has made great progress. Their discovery of spin waves traveling in circular routes in certain magnetic materials could pave the way for quantum devices that rely on waves to carry data.
It is simply great to see that, after countless experiments at world-leading spectrometers and the clarification of major experimental and theoretical challenges during my time at Los Alamos, the microscopic detection of Landau quantization at the world’s unique beamline RESEDA at TUM’s FRM II in Garching closes a circle that began almost fifteen years ago with my first measurements at the Heinz Maier-Leibnitz Zentrum in Garching.
Marc Janoschek
Propagation of magnetic waves in materials
When you throw a stone into water, it disrupts the balance of the water molecules. They begin to oscillate, forming a circular wave. Magnetic moments in some materials can be manipulated to oscillate in a similar way.
The magnetic moment undergoes a gyroscopic motion in relation to its rest location during this procedure. The vibration of one instant is affected by the precession of its neighbor, and the wave propagates as a result.
Controlling features like wavelength and direction is critical for applications that use magnetic waves. Magnetic waves propagate in a straight path in ordinary ferromagnets with all magnetic moments pointing in the same direction.
The propagation of such waves is substantially different in a new class of magnetic materials, which are made up of a tight arrangement of magnetic vortex tubes, similar to a box of uncooked spaghetti. A team led by Christian Pfleiderer and Peter Böni at the Technical University of Munich used neutron experiments to discover this magnetic order nearly fifteen years ago.
These vortex tubes are called skyrmions because of their non-trivial topological features and in honor of British nuclear physicist Tony Skyrme’s theoretical-mathematical contributions.
Propagation of magnetic waves on a circular path
Neutrons are particularly well suited for the research of magnetic materials since they have a magnetic moment. They react to magnetic fields similarly to a compass needle. Because it gives the required resolution across very high length and time scales, neutron scattering proved to be the only technology capable of detecting spin waves on circular orbits.
Tobias Weber and his team at the Institut Laue Langevin (ILL) in Grenoble, France, have now demonstrated that magnetic waves perpendicular to such skyrmions do not propagate in a straight line, but rather in a circular path, using polarized neutron scattering.
The reason for this is that the direction of surrounding magnetic moments, and hence the axis around which precessional motion occurs, changes perpendicular to the magnetic vortex tube on a continual basis.
Similarly, as precessional motion propagates from one magnetic instant to the next, the propagation direction changes continuously. The intensity and direction of the magnetic moments’ tilt determine the radius and direction of the circular path of the propagation direction of the spin waves.
Quantization of circular orbits
“But there is even more to it,” says Markus Garst of the Karlsruhe Institute of Technology (KIT), who had developed the theoretical description of spin waves in skyrmions and their coupling to neutrons some time ago.
“There is a close analogy between the circular propagation of spin waves perpendicular to a skyrmion lattice and the motion of an electron perpendicular to a magnetic field caused by the Lorentz force.”
When circular orbits are closed at very low temperatures, their energy is quantized. Landau quantization is a phenomenon that was predicted about a century ago by Russian physicist Lev Landau, who is well known for his work on electrons. The influence of the skyrmions’ vortex-like feature on the spin waves can be elegantly represented as a fictional magnetic field, by analogy.
To put it another way, the seemingly complex interaction of spin waves with the skyrmion structure is actually rather simple and can be explained in the same way as electrons moving in a real magnetic field can.
Furthermore, the circular orbits are quantized in the propagation of spin waves perpendicular to skyrmions. As a result, the spin wave’s characteristic energy is quantized, allowing for whole new uses.
Furthermore, the circular orbit has a tiny twist, comparable to that of a Möbius strip. The twist can only be undone by cutting and reconnecting the strip, which is topologically non-trivial. All of this contributes to a very stable spin-wave motion.
Successful international cooperation
“The experimental determination of spin waves in skyrmion lattices required both a combination of world-leading neutron spectrometers and a massive advancement of the software to interpret the data,” explains TUM physicist Peter Böni.
The research team employed instruments of the Institut Laue-Langevin in France, the spallation source SINQ at the Swiss Paul Scherrer Institute, the UK’s ISIS neutron and muon source, and the Research Neutron Source Heim Maier-Leibnitz (FRM II) at the Technical University of Munich.
The Los Alamos National Laboratory in New Mexico and the Karlsruhe Institute of Technology in Germany continued to work on theory and data analysis.
Marc Janoschek, who now works at the Paul Scherrer Institute, emphasizes: “It is simply great to see that, after countless experiments at world-leading spectrometers and the clarification of major experimental and theoretical challenges during my time at Los Alamos, the microscopic detection of Landau quantization at the world’s unique beamline RESEDA at TUM’s FRM II in Garching closes a circle that began almost fifteen years ago with my first measurements at the Heinz Maier-Leibnitz Zentrum in Garching.”
The quantized motion of spin waves in circular orbits, on the other hand, represents a breakthrough not just in terms of fundamental study.
Christian Pfleiderer, managing director of the newly founded Center for QuantumEngineering at TUM, emphasizes: “The spontaneous motion of spin waves on circular orbits, whose radius and direction arise from the vortex-like structure of skyrmions, opens up a new perspective for realizing functional devices for information processing in quantum technologies, such as simple couplers between qubits in quantum computers.”
